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[Illustration: JOHN TYNDALL]




       THE FORMS OF WATER IN CLOUDS AND RIVERS ICE AND GLACIERS


                                  BY

                     JOHN TYNDALL, LL.D., F. R. S.




                    _WITH TWENTY-FIVE ILLUSTRATIONS
                DRAWN AND ENGRAVED UNDER THE DIRECTION
                            OF THE AUTHOR_


                               NEW YORK
                        D. APPLETON AND COMPANY
                                 1899


                           Copyright, 1872,
                      By D. APPLETON AND COMPANY.

                       Electrotyped and Printed
                    at the Appleton Press, U. S. A.




AMERICAN PREFACE TO THE INTERNATIONAL SCIENTIFIC SERIES.


The rapid development of science in the present age, and the
increasing public interest in its results, make it desirable that the
most efficient measures should be adopted to elevate the character
of its popular literature. The tendency of careless and unscrupulous
book-makers to cater to public ignorance and love of the marvellous,
and to foist their crude productions upon those who are too little
instructed to judge of their real quality, has hitherto been so
strong as to cast discredit upon the idea of "popular science." It
is highly important to counteract this evil tendency by furnishing
the public with popular scientific books of a superior character.
The publication of the present volume is the first step in carrying
out a systematic enterprise of this kind. It initiates a series of
such works on a wide range of scientific subjects, to be prepared
by the leading thinkers of different countries, and known as the
"International Scientific Series."

It is designed to consist of compendious scientific treatises,
representing the latest advances of thought upon subjects of general
interest, theoretical and practical, to all classes of readers. The
familiar phenomena of surrounding Nature, in their physical and
chemical aspects, the knowledge of which has recently undergone
marked extension or revision, will be considered in their latest
interpretations. Biology, or the general science of life, which has
lately come into prominence, will be explained in its leading and
most important principles. The subject of mind, which, under the
inductive method and on the basis of its physical accompaniments
and conditions, is giving rise to a new psychology, will be treated
with the fulness to which it is entitled. The laws of man's social
development, or the natural history of society, which are now being
studied by the scientific method, will also receive a due share of
attention. While the books of this series are to deal with a wide
diversity of topics, it will be a leading object of the enterprise
to present the bearings of inquiry upon the higher questions of the
time, and to throw the latest light of science upon the phenomena of
human nature and the economy of human life.

As the first requisite of such a series of works is trustworthiness,
their preparation has been confided only to men of eminent ability,
and who are recognized authorities in their several departments.
As they are to address the non-scientific public, it is a further
requisite that they should be written in familiar and intelligible
language. It is not to be expected that the authors will all attain
to the same standard in this respect, but they are pledged to the
utmost simplicity of exposition that is possible consistently with
clear and accurate representation.

As science is now the supreme interest of civilization, and concerns
alike the people of every country, and as, moreover, it affords a
common ground upon which men of all races, tongues, faiths, and
nationalities, may work together in harmony, it seemed fitting that
an undertaking of this kind should be of comprehensive scope and
stand upon an international basis. With the growing sentiment of
sympathy and brotherhood among the most widely-separated students of
Nature, and the extensive facilities of business intercourse that now
exist, there appeared no reason why an international combination of
authors and publishers should not be effected that would be equally
favourable to their own private interests and advantageous to the
public. To gain this end and guarantee to authors better remuneration
for their work, is a distinctive purpose of the present enterprise.
But there was this difficulty in the way of any such arrangement,
that, while the rights of foreign authors are guarded by all other
civilized governments, they are not protected by the government of
the United States. To escape this difficulty, and secure American
coöperation, the first thing needed was to obtain the consent of an
American publishing-house to grant voluntarily to foreign authors
the justice which our government denies them. It was agreed by
Messrs. Appleton that they would pay the foreign contributors to
this series the full rates of copyright that are usually allowed
to American authors. When this was done, engagements were made
with distinguished scientists of England, France, Germany, and the
United States, to prepare works for the series, and with Henry S.
King & Co., of London, Germer Baillière, of Paris, and Messieurs
Brockhaus, of Leipsic, to publish them. Negotiations are pending for
the reproduction of the series in other countries, but the present
arrangements secure to the authors the benefits of the four leading
markets of the world.

It is a fact not without significance, that the proposal of this
enterprise was received with the most cordial favour by the eminent
scientific men who were solicited to aid in carrying it forward. Most
of them consented at once; but, while some were so heavily burdened
with work that they could enter into no immediate engagements, not
one of them declined to coöperate, and all promised to do so at the
earliest practicable opportunity. The feeling of the desirableness
of such an undertaking was strong and unanimous. The old dislike of
the cultivators of science to participate in the work of popular
teaching, seems very much to have passed away; and in England,
France, and Germany, alike it was freely acknowledged that _savants_
have an imperative duty to discharge in relation to the work of
general scientific education. As remarked by Prof. Virchow, of
Berlin, "the destiny of science is the service of humanity."

It was stipulated by the authors that they should have ample time
for the preparation of their books, and, as the arrangements were
recently made, only a few of the works are yet ready. Several,
however, are now in press, and will shortly appear.

Those interested in the series are under many obligations to Prof.
Tyndall for his kindness in consenting to furnish its commencing
volume. Being prepared in a short time, amid great pressure both
of laboratory and literary work, it contains somewhat less matter
than may be expected in the ensuing volumes. It treats of subjects
upon which he is perhaps the highest living authority; and it is an
admirable example of that vivid, stirring, impressive style for which
its author is so distinguished. Prof. Tyndall is not only a master in
the "scientific use of the imagination," but in kindling the action
of that faculty in his readers. He writes in pictures, so as to make
them see what he sees. In this volume he addresses himself directly
to his juvenile friends, groups them around him, takes them with him
to his favourite mountains, and thus adds a dramatic element and the
effect of personal sympathy to familiar colloquial exposition.

The "International Scientific Series" will form an elegant and
valuable library of popular science, fresh in treatment, attractive
in form, strong in character, moderate in price, and indispensable to
all who care for the acquisition of solid and serviceable knowledge;
and it is commended to American readers as a help in the important
work of sound public education.

                                                            E. L. Y.

  New York, _September, 1872_.




AUTHOR'S PREFACE.


After an absence of twelve years, I visited the Mer de Glace last
June. It exhibited in a striking degree that excess of consumption
over supply which, if continued, would eventually reduce the Swiss
glaciers to the mere spectres of their former selves. When I first
saw the Mer de Glace its ice-cliffs towered over Les Mottets, and an
arm of the Arveiron, issuing from the cliffs, plunged as a powerful
cascade down the rocks. The ice has now shrunk far behind them. A
huge moraine, left behind by the retreating glacier, will mark, for
some time to come, its recent magnitude. The vault of the Arveiron
has dwindled considerably. The way up to the Chapeau lies on the
top of a lateral moraine, reached a few years ago by the surface
of the glacier, the present surface lying far below. The visible
and continual breaking away of the moraines, left thus stranded on
the mountain flank, explains the absence of ancient ridges on the
mountains where the slopes are steep. The ice-cascades of the Géant
has suffered much from the general waste. Its crevasses are still
wild, but the ice-cliffs and séracs of former days are but poorly
represented to-day.

The great Aletsch and its neighbours exhibit similar evidences of
diminution. I found moreover this year that the two ancient moraines
mentioned in paragraph 364 are parts of the same great lateral
moraine which flanked the glacier for a long period, during which its
magnitude must have remained practically constant. The place occupied
by the ancient ice-river is rendered strikingly conspicuous by this
well-preserved boundary.

During my residence at the Bel Alp this year, a catastrophe occurred
which renders, for the time being, the description of the Märgelin
See given in § 50 inappropriate. In company with two young friends I
had descended the glacier and passed through the gorge of the Massa.
On our return to the Bel Alp we found the domestics of the hotel
leaning out of the windows and looking excitedly towards the glacier.
From it proceeded a sound which resembled the roar of a cataract.
The servants remarked that the Märgelin See must have broken loose.
This was the case. For a time, however, the water flowed beneath the
glacier; but at a point about midway between the Bel Alp and the
Æggischhorn, it broke forth on the Æggischhorn side, and formed a
torrent between the glacier and the slope of the mountain. In some
places this river was more than sixty yards wide, at others it was
contracted to less than one-fifth of this width. Broken cascades of
great height were formed here and there by successive ledges of ice,
the torrent leaping with indescribable fury from ledge to ledge, and
sending a smoke of spray into the air. At one place the bottom of
the torrent was deep soft sand, which, after the water had passed,
could be seen to have been tortured into huge funnels by the whirling
eddies overhead.

Soon after we reached the Bel Alp, on the occasion just referred to,
the front of the torrent appeared at the opposite side of the valley
carrying everything movable before it, and immediately afterwards
swept through the hollow that we had traversed a little earlier in
the day. When at the end of the glacier I was struck by the force and
volume of the Massa, and the grandeur of its vault, but I could not
then account for the huge blocks of ice which it incessantly carried
down. Doubtless the eruption above had been partial before the grand
rush set in. The Rhone was considerably swollen, crops were damaged
or ruined, and the driver of the diligence was sorely perplexed to
find himself in three feet of water, without any apparent reason,
on the public highway. Two or three days subsequently I learned
at the Æggischhorn that an engineer had been sent up to report on
the possibility of opening a channel, so as to prevent any future
accumulation of water in the Märgelin See. If this be done a useful
end will be gained, by the abolition, however, of one of the most
beautiful objects in Switzerland.

                                                         J. Tyndall.

  _September, 1872._




PREFACE TO THE FOURTH EDITION.


At a meeting of the Managers of the Royal Institution held on
December 12, 1825, "the Committee appointed to consider what lectures
should be delivered in the Institution in the next session," reported
"that they had consulted Mr. Faraday on the subject of engaging him
to take a part in the juvenile lectures proposed to be given during
the Christmas and Easter recesses, and they found his avocations were
such that it would be exceedingly inconvenient for him to engage in
such lectures."

At a general monthly meeting of the members of the Royal Institution,
held on December 4, 1826, the Managers reported "that they had
engaged Mr. Wallis to deliver a course of lectures on Astronomy,
adapted to a juvenile auditory, during the Christmas vacation."

In a report dated April 16, 1827, the Board of Visitors express
"their satisfaction at finding that the plan of juvenile courses of
lectures had been resorted to. They feel sure that the influence of
the Institution cannot be extended too far, and that the system of
instructing the younger portion of the community is one of the most
effective means which the Institution possesses for the diffusion of
science."

Faraday's holding aloof was but temporary, for at Christmas 1827 we
find him giving a "Course of Six Elementary Lectures on Chemistry,
adapted to a Juvenile Auditory."[A]

[A] There is no record to show that Mr. Wallis gave the Astronomical
lectures referred to, and our librarian believes that the _Christmas_
courses were opened by Faraday.

The Easter lectures were soon abandoned; but from the date here
referred to to the present time the Christmas lectures have been a
marked feature of the Royal Institution.

In 1871 it fell to my lot to give one of these courses. I had been
frequently invited to write on Glaciers in encyclopædias, journals,
and magazines, but had always declined to do so. I had also abstained
from making them the subject of a course of lectures, wishing to take
no advantage of my position here, and indeed to avoid writing a line
or uttering a sentence on the subject for which I could not be held
personally responsible. In view of the discussions which the subject
had provoked, I thought this the fairest course.

But, in 1871, the time (I imagined) had come when, without risk of
offence, I might tell our young people something about the labours
of those who had unravelled for their instruction the various
problems of the ice-world. My lamented friend and ever-helpful
counsellor, Dr. Bence Jones, thought the subject a good one, and
accordingly it was chosen. Strong in my sympathy with youth, and
remembering the damage done by defective exposition to my own young
mind, I sought, to the best of my ability, to confer upon these
lectures clearness, thoroughness, and life.

Wishing, moreover, to render them of permanent value, I wrote out
copious Notes of the course, and had them distributed among the boys
and girls. In preparing these Notes I aimed at nothing less than
presenting to my youthful audience, in a concentrated but perfectly
digestible form, every essential point embraced in the literature of
the glaciers, and some things in addition, which, derived as they
were from my own recent researches, no book previously published on
the subject contained.

But my theory of education agrees with that of Emerson, according
to which instruction is only half the battle, what he calls
_provocation_ being the other half. By this he means that power of
the teacher, through the force of his character and the vitality
of his thought, to bring out all the latent strength of his pupil,
and to invest with interest even the driest matters of detail. In
the present instance I was determined to shirk nothing essential,
however dry; and, to keep my mind alive to the requirements of my
pupil, I proposed a series of ideal ramblings, in which he should be
always at my side. Oddly enough, though I was here dealing with what
might be called the abstract idea of a boy, I realised his presence
so fully as to entertain for him, before our excursions ended, an
affection consciously warm and real.

The "Notes" here referred to were at first intended for the use of my
audience alone. At the urgent request of a friend I slightly expanded
them, and converted them into the little book here presented to the
reader.

The amount of attention bestowed upon the volume induces me to give
this brief history of its origin.

A German critic, whom I have no reason to regard as specially
favourable to me or it, makes the following remark on the style of
the book: "This passion [for the mountains] tempts him frequently
to reveal more of his Alpine wanderings than is necessary for
his demonstrations. The reader, however, will not find this a
disagreeable interruption of the course of thought; for the book
thereby gains wonderfully in vividness." This, I would say, was the
express aim of the breaks referred to. I desired to keep my companion
fresh as well as instructed, and these interruptions were so many
breathing-places where the intellectual tension was purposely
relaxed and the mind of the pupil braced to fresh action.

Of other criticisms, flattering and otherwise, I forbear to speak.
As regards some of them, indeed, it would be a reproach to that
manliness which I have sought to encourage in my pupil to return
blow for blow. If the reader be acquainted with them, this will let
him know how I regard them; and if he be not acquainted with them,
I would recommend him to ignore them, and to form his own judgment
of this book. No fair-minded person who reads it will dream that I,
in writing it, had a thought of acting otherwise than justly and
generously towards my predecessors, the last of whom, to the grief of
all who knew him, has recently passed away.

                                                       John Tyndall.

  _April, 1874._




CONTENTS.


  Cloud-banner of the Aiguille du Dru                   _Frontispiece_

                                                                  PAGE

  § 1, 2. Clouds, Rains, and Rivers                               1, 6

    3. The Waves of Light                                            8

    4. The Waves of Heat which produce the Vapour of our
         Atmosphere and melt our Glaciers                           11

    5. Experiments to prove the foregoing statements                14

    6. Oceanic Distillation                                         19

    7. Tropical Rains                                               23

    8. Mountain Condensers                                          27

    9. Architecture of Snow                                         29

   10. Atomic Poles                                                 32

   11. Architecture of Lake Ice                                     35

   12. The Source of the Arveiron. Ice Pinnacles, Towers, and
         Chasms of the Glacier des Bois. Passage to the Montanvert  38

   13. The Mer de Glace and its Sources. Our First Climb to the
         Cleft Station                                              43

   14. Ice-cascade and Snows of the Col du Géant                    46

   15. Questioning the Glaciers                                     48

   16. Branches and Medial Moraines of the Mer de Glace from the
         Cleft Station                                              51

   17. The Talèfre and the Jardin. Work among the Crevasses         52

   18. First Questions regarding Glacier Motion. Drifting of
         Bodies buried in a Crevasse                                54
   19. The Motion of Glaciers. Measurements by Hugi and Agassiz.
         Drifting of Huts on the Ice                                69

   20. Precise Measurements of Agassiz and Forbes. Motion of a
         Glacier proved to resemble the Motion of a River           60

   21. The Theodolite and its Use. Our own Measurements             62

   22. Motion of the Mer de Glace                                   66

   23. Unequal Motion of the two Sides of the Mer de Glace          70

   24. Suggestion of a new Likeness of Glacier Motion to River
         Motion. Conjecture tested                                  72

   25. New Law of Glacier Motion                                    76

   26. Motion of Axis of Mer de Glace                               78

   27. Motion of Tributary Glaciers                                 79

   28. Motion of Top and Bottom of Glacier                          80

   29. Lateral Compression of a Glacier                             81

   30. Longitudinal Compression of a Glacier                        84

   31. Sliding and Flowing. Hard Ice and Soft Ice                   86

   32. Winter on the Mer de Glace                                   88

   33. Winter Motion of the Mer de Glace                            93

   34. Motion of the Grindelwald and Aletsch Glacier                93

   35. Motion of Morteratsch Glacier                                95

   36. Birth of a Crevasse: Reflections                             98

   37. Icicles                                                      99

   38. The Bergschrund                                             102

   39. Transverse Crevasses                                        103

   40. Marginal Crevasses                                          105

   41. Longitudinal Crevasses                                      109

   42. Crevasses in relation to Curvature of Glacier               110

   43. Moraine-ridges, Glacier Tables, and Sand-Cones              112

   44. The Glacier Mills or Moulins                                116

   45. The Changes of Volume of Water by Heat and Cold             118

   46. Consequences flowing from the foregoing Properties of
         Water. Correction of Errors                               122

   47. The Molecular Mechanism of Water-Congelation                125

   48. The Dirt Bands of the Mer de Glace                          127

   49. Sea-ice and Icebergs                                        132

   50. The Æggischhorn, the Märgelin See and its Icebergs          136

   51. The Bel Alp                                                 139

   52. The Riffelberg and Görner Glacier                           140

   53. Ancient Glaciers of Switzerland                             145

   54. Erratic Blocks                                              147

   55. Ancient Glaciers of England, Ireland, Scotland, and Wales   150

   56. The Glacier Epoch                                           152

   57. Glacial Theories                                            155

   58. Dilatation and Sliding Theories                             155

   59. Plastic Theory                                              156

   60. Viscous Theory                                              161

   61. Regelation Theory                                           163

   62. Cause of Regelation                                         167

   63. Faraday's View of Regelation                                171

   64. The Blue Veins of Glaciers                                  176

   65. Relation of Structure to Pressure                           183

   66. Slate Cleavage and Glacier Lamination                       187

   67. Conclusion                                                  191

[Illustration: CLOUD-BANNER OF THE AIGUILLE DU DRU (par. 84 and 227).]




THE FORMS OF WATER

IN CLOUDS AND RIVERS, ICE AND GLACIERS.


§ 1. _Clouds, Rains, and Rivers._

1. Every occurrence in Nature is preceded by other occurrences which
are its causes, and succeeded by others which are its effects. The
human mind is not satisfied with observing and studying any natural
occurrence alone, but takes pleasure in connecting every natural fact
with what has gone before it, and with what is to come after it.

2. Thus, when we enter upon the study of rivers and glaciers, our
interest will be greatly augmented by taking into account not only
their actual appearances, but also their causes and effects.

3. Let us trace a river to its source. Beginning where it empties
itself into the sea, and following it backwards, we find it from
time to time joined by tributaries which swell its waters. The river
of course becomes smaller as these tributaries are passed. It shrinks
first to a brook, then to a stream; this again divides itself into a
number of smaller streamlets, ending in mere threads of water. These
constitute the source of the river, and are usually found among hills.

4. Thus the Severn has its source in the Welsh Mountains; the Thames
in the Cotswold Hills; the Danube in the hills of the Black Forest;
the Rhine and the Rhone in the Alps; the Ganges in the Himalaya
Mountains; the Euphrates near Mount Ararat; the Garonne in the
Pyrenees; the Elbe in the Giant Mountains of Bohemia; the Missouri in
the Rocky Mountains, and the Amazon in the Andes of Peru.

5. But it is quite plain that we have not yet reached the real
beginning of the rivers. Whence do the earliest streams derive their
water? A brief residence among the mountains would prove to you that
they are fed by rains. In dry weather you would find the streams
feeble, sometimes indeed quite dried up. In wet weather you would see
them foaming torrents. In general these streams lose themselves as
little threads of water upon the hill sides; but sometimes you may
trace a river to a definite spring. The river Albula in Switzerland,
for instance, rushes at its origin in considerable volume from a
mountain side. But you very soon assure yourself that such springs
are also fed by rain, which has percolated through the rocks or soil,
and which, through some orifice that it has found or formed, comes to
the light of day.

6. But we cannot end here. Whence comes the rain which forms the
mountain streams? Observation enables you to answer the question.
Rain does not come from a clear sky. It comes from clouds. But what
are clouds? Is there nothing you are acquainted with which they
resemble? You discover at once a likeness between them and the
condensed steam of a locomotive. At every puff of the engine a cloud
is projected into the air. Watch the cloud sharply: you notice that
it first forms at a little distance from the top of the funnel. Give
close attention and you will sometimes see a perfectly clear space
between the funnel and the cloud. Through that clear space the thing
which makes the cloud must pass. What, then, is this thing which
at one moment is transparent and invisible, and at the next moment
visible as a dense opaque cloud?

7. It is the _steam_ or _vapour of water_ from the boiler. Within
the boiler this steam is transparent and invisible; but to keep it
in this invisible state a heat would be required as great as that
within the boiler. When the vapour mingles with the cold air above
the hot funnel it ceases to be vapour. Every bit of steam shrinks
when chilled, to a much more minute particle of water. The liquid
particles thus produced form a kind of _water-dust_ of exceeding
fineness, which floats in the air, and is called _a cloud_.

8. Watch the cloud-banner from the funnel of a running locomotive;
you see it growing gradually less dense. It finally melts away
altogether, and if you continue your observations you will not fail
to notice that the speed of its disappearance depends upon the
character of the day. In humid weather the cloud hangs long and
lazily in the air; in dry weather it is rapidly licked up. What has
become of it? It has been reconverted into true invisible vapour.

9. The _drier_ the air, and the _hotter_ the air, the greater is the
amount of cloud which can be thus dissolved in it. When the cloud
first forms, its quantity is far greater than the air is able to
maintain in an invisible state. But as the cloud mixes gradually
with a larger mass of air it is more and more dissolved, and finally
passes altogether from the condition of a finely-divided liquid into
that of transparent vapour or gas.

10. Make the lid of a kettle air-tight, and permit the steam to issue
from the pipe; a cloud is precipitated in all respects similar to
that issuing from the funnel of the locomotive.

11. Permit the steam as it issues from the pipe to pass through the
flame of a spirit-lamp, the cloud is instantly dissolved by the heat,
and is not again precipitated. With a special boiler and a special
nozzle the experiment may be made more striking, but not more
instructive, than with the kettle.

12. Look to your bedroom windows when the weather is very cold
outside; they sometimes stream with water derived from the
condensation of the aqueous vapour from your own lungs. The windows
of railway carriages in winter show this condensation in a striking
manner. Pour cold water into a dry drinking-glass on a summer's day:
the outside surface of the glass becomes instantly dimmed by the
precipitation of moisture. On a warm day you notice no vapour in
front of your mouth, but on a cold day you form there a little cloud
derived from the condensation of the aqueous vapour from the lungs.

13. You may notice in a ball-room that as long as the door and
windows are kept closed, and the room remains hot, the air remains
clear; but when the doors or windows are opened a dimness is visible,
caused by the precipitation to fog of the aqueous vapour of the
ball-room. If the weather be intensely cold the entrance of fresh
air may even cause _snow_ to fall. This has been observed in Russian
ball-rooms; and also in the subterranean stables at Erzeroom, when
the doors are opened and the cold morning air is permitted to enter.

14. Even on the driest day this vapour is never absent from our
atmosphere. The vapour diffused through the air of this room may be
congealed to hoar-frost in your presence. This is done by filling
a vessel with a mixture of pounded ice and salt, which is colder
than the ice itself, and which, therefore, condenses and freezes the
aqueous vapour. The surface of the vessel is finally coated with a
frozen fur, so thick that it may be scraped away and formed into a
snow-ball.

15. To produce the cloud, in the case of the locomotive and the
kettle, _heat_ is necessary. By heating the water we first convert
it into steam, and then by chilling the steam we convert it into
cloud. Is there any fire in nature which produces the clouds of our
atmosphere? There is: the fire of the sun.

16. Thus, by tracing backward, without any break in the chain of
occurrences, our river from its end to its real beginnings, we come
at length to the sun.


§ 2.

17. There are, however, rivers which have sources somewhat different
from those just mentioned. They do not begin by driblets on a hill
side, nor can they be traced to a spring. Go, for example, to the
mouth of the river Rhone, and trace it backwards to Lyons, where it
turns to the east. Bending round by Chambery, you come at length
to the Lake of Geneva, from which the river rushes, and which you
might be disposed to regard as the source of the Rhone. But go to
the head of the lake, and you find that the Rhone there enters it,
that the lake is in fact a kind of expansion of the river. Follow
this upwards; you find it joined by smaller rivers from the mountains
right and left. Pass these, and push your journey higher still. You
come at length to a huge mass of ice the--end of a glacier--which
fills the Rhone valley, and from the bottom of the glacier the river
rushes. In the glacier of the Rhone you thus find the source of the
river Rhone.

18. But again we have not reached the real beginning of the river.
You soon convince yourself that this earliest water of the Rhone is
produced by the melting of the ice. You get upon the glacier and walk
upwards along it. After a time the ice disappears and you come upon
snow. If you are a competent mountaineer you may go to the very top
of this great snow-field, and if you cross the top and descend at the
other side you finally quit the snow, and get upon another glacier
called the Trift, from the end of which rushes a river smaller than
the Rhone.

19. You soon learn that the mountain snow feeds the glacier. By some
means or other the snow is converted into ice. But whence comes the
snow? Like the rain, it comes from the clouds, which, as before, can
be traced to vapour raised by the sun. Without solar fire we could
have no atmospheric vapour, without vapour no clouds, without clouds
no snow, and without snow no glaciers. Curious then as the conclusion
may be, the cold ice of the Alps has its origin in the heat of the
sun.


§ 3. _The Waves of Light._

20. But what is the sun? We know its size and its weight. We also
know that it is a globe of fire far hotter than any fire upon earth.
But we now enter upon another enquiry. We have to learn definitely
what is the meaning of solar light and solar heat; in what way they
make themselves known to our senses; by what means they get from the
sun to the earth, and how, when there, they produce the clouds of our
atmosphere, and thus originate our rivers and our glaciers.

21. If in a dark room you close your eyes and press the eyelid
with your finger-nail, a circle of light will be seen opposite to
the point pressed, while a sharp blow upon the eye produces the
impression of a flash of light. There is a nerve specially devoted to
the purposes of vision which comes from the brain to the back of the
eye, and there divide into fine filaments, which are woven together
to a kind of screen called the _retina_. The retina can be excited
in various ways so as to produce the consciousness of light; it may,
as we have seen, be excited by the rude mechanical action of a blow
imparted to the eye.

22. There is no spontaneous creation of light by the healthy eye.
To excite vision the retina must be affected by something coming
from without. What is that something? In some way or other luminous
bodies have the power of affecting the retina--but _how?_

23. It was long supposed that from such bodies issued, with
inconceivable rapidity, an inconceivably fine matter, which flew
through space, passed through the pores supposed to exist in the
humours of the eye, reached the retina behind, and by their shock
against the retina, aroused the sensation of light.

24. This theory, which was supported by the greatest men, among
others by Sir Isaac Newton, was found competent to explain a great
number of the phenomena of light, but it was not found competent
to explain _all_ the phenomena. As the skill and knowledge of
experimenters increased, large classes of facts were revealed which
could only be explained by assuming that light was produced, not by a
fine matter flying through space and hitting the retina, but by the
shock of minute waves against the retina.

25. Dip your finger into a basin of water, and cause it to quiver
rapidly to and fro. From the point of disturbance issue small ripples
which are carried forward by the water, and which finally strike the
basin. Here, in the vibrating finger, you have a source of agitation;
in the water you have a vehicle through which the finger's motion
is transmitted, and you have finally the side of the basin which
receives the shock of the little waves.

26. In like manner, according to the _wave theory_ of light, you
have a source of agitation in the vibrating atoms, or smallest
particles, of the luminous body; you have a vehicle of transmission
in a substance which is supposed to fill all space, and to be
diffused through the humours of the eye; and finally, you have the
retina, which receives the successive shocks of the waves. These
shocks are supposed to produce the sensation of light.

27. We are here dealing for the most part with suppositions and
assumptions merely. We have never seen the atoms of a luminous body,
nor their motions. We have never seen the medium which transmits
their motions, nor the waves of that medium. How, then, do we come to
assume their existence?

28. Before such an idea could have taken any real root in the human
mind, it must have been well disciplined and prepared by observations
and calculations of ordinary wave-motion. It was necessary to know
how both water-waves and sound-waves are formed and propagated. It
was above all things necessary to know how waves, passing through
the same medium, act upon each other. Thus disciplined, the mind was
prepared to detect any resemblance presenting itself between the
action of light and that of waves. Great classes of optical phenomena
accordingly appeared which could be accounted for in the most
complete and satisfactory manner by assuming them to be produced by
waves, and which could not be otherwise accounted for. It is because
of its competence to explain all the phenomena of light that the wave
theory now receives universal acceptance on the part of scientific
men.

Let me use an illustration. We infer from the flint implements
recently found in such profusion all over England and in other
countries that they were produced by men, and also that the Pyramids
of Egypt were built by men, because, as far as our experience goes,
nothing but men could form such implements or build such Pyramids.
In like manner, we infer from the phenomena of light the agency of
waves, because, as far as our experience goes, no other agency could
produce the phenomena.


§ 4. _The Waves of Heat which produce the Vapour of our Atmosphere
and melt our Glaciers._

29. Thus, in a general way, I have given you the conception and
the grounds of the conception, which regards light as the product
of wave-motion; but we must go farther than this, and follow the
conception into some of its details. We have all seen the waves of
water, and we know they are of different sizes different in length
and different in height. When, therefore, you are told that the
atoms of the sun, and of almost all other luminous bodies, vibrate
at different rates, and produce waves of different sizes, your
experience of water-waves will enable you to form a tolerably clear
notion of what is meant.

30. As observed above, we have never seen the light-waves, but we
judge of their presence, their position, and their magnitude, by
their effects. Their lengths have been thus determined, and found to
vary from about 1/30000th to 1/60000th of an inch.

31. But besides those which produce light, the sun sends forth
incessantly a multitude of waves which produce no light. The largest
waves which the sun sends forth are of this non-luminous character,
though they possess the highest heating power.

32. A common sunbeam contains waves of all kinds, but it is possible
to _sift_ or _filter_ the beam so as to intercept all its light, and
to allow its obscure heat to pass unimpeded. For substances have been
discovered which, while intensely opaque to the light-waves, are
almost perfectly transparent to the others. On the other hand, it is
possible, by the choice of proper substances, to intercept in a great
degree the pure heat-waves, and to allow the pure light-waves free
transmission. This last separation is, however, not so perfect as the
first.

33. We shall learn presently how to detach the one class of waves
from the other class, and to prove that waves competent to light a
fire, fuse metal, or burn the hand like a hot solid, may exist in a
perfectly dark place.

34. Supposing, then, that we withdraw, in the first instance the
large heat-waves, and allow the light-waves alone to pass. These
may be concentrated by suitable lenses and sent into water without
sensibly warming it. Let the light-waves now be withdrawn, and the
larger heat-waves concentrated in the same manner; they may be caused
to boil the water almost instantaneously.

35. This is the point to which I wished to lead you, and which
without due preparation could not be understood. You now perceive the
important part played by these large darkness-waves, if I may use
the term, in the work of evaporation. When they plunge into seas,
lakes, and rivers, they are intercepted close to the surface, and
they heat the water at the surface, thus causing it to evaporate;
the light-waves at the same time entering to great depths without
sensibly heating the water through which they pass. Not only,
therefore, is it the sun's fire which produces evaporation, but a
particular constituent of that fire, the existence of which you
probably were not aware of.

36. Further, it is these selfsame lightless waves which, falling upon
the glaciers of the Alps, melt the ice and produce all the rivers
flowing from the glaciers; for I shall prove to you presently that
the light-waves, even when concentrated to the uttermost, are unable
to melt the most delicate hoar-frost; much less would they be able to
produce the copious liquefaction observed upon the glaciers.

37. These large lightless waves of the sun, as well as the
heat-waves issuing from non-luminous hot bodies, are frequently
called obscure or invisible heat.

We have here an example of the manner in which phenomena, apparently
remote, are connected together in this wonderful system of things
that we call Nature. You cannot study a snow-flake profoundly without
being led back by it step by step to the constitution of the sun. It
is thus throughout Nature. All its parts are interdependent, and the
study of any one part completely would really involve the study of
all.


§ 5. _Experiments to prove the foregoing Statements._

38. Heat issuing from any source not visibly red cannot be
concentrated so as to produce the intense effects just referred to.
To produce these it is necessary to employ the obscure heat of a body
raised to the highest possible state of incandescence. The sun is
such a body, and its dark heat is therefore suitable for experiments
of this nature.

39. But in the atmosphere of London, and for experiments such as
ours, the heat-waves emitted by coke raised to intense whiteness by a
current of electricity are much more manageable than the sun's waves.
The electric light has also the advantage that its dark radiation
embraces a larger proportion of the total radiation than the dark
heat of the sun. In fact, the force or energy, if I may use the term,
of the dark waves of the electric light is fully seven times that of
its light-waves. The electric light, therefore, shall be employed in
our experimental demonstrations.

40. From this source a powerful beam is sent through the room,
revealing its track by the motes floating in the air of the room; for
were the motes entirely absent the beam would be unseen. It falls
upon a concave mirror (a glass one silvered behind will answer) and
is gathered up by the mirror into a cone of reflected rays; the
luminous apex of the cone, which is the _focus_ of the mirror, being
about fifteen inches distant from its reflecting surface. Let us mark
the focus accurately by a pointer.

41. And now let us place in the path of the beam a substance
perfectly opaque to light. This substance is iodine dissolved in a
liquid called bisulphide of carbon. The light at the focus instantly
vanishes when the dark solution is introduced. But the solution is
intensely transparent to the dark waves, and a focus of such waves
remains in the air of the room after the light has been abolished.
You may feel the heat of these waves with your hand; you may let them
fall upon a thermometer, and thus prove their presence; or, best of
all, you may cause them to produce a current of electricity, which
deflects a large magnetic needle. The magnitude of the deflection is
a measure of the heat.

42. Our object now is, by the use of a more powerful lamp, and
a better mirror (one silvered in front and with a shorter focal
distance), to intensify the action here rendered so sensible. As
before, the focus is rendered strikingly visible by the intense
illumination of the dust particles. We will first filter the beam so
as to intercept its dark waves, and then permit the purely luminous
waves to exert their utmost power on a small bundle of gun-cotton
placed at the focus.

43. No effect whatever is produced. The gun-cotton might remain there
for a week without ignition. Let us now permit the unfiltered beam
to act upon the cotton. It is instantly dissipated in an explosive
flash. This experiment proves that the light-waves are incompetent to
explode the cotton, while the waves of the full beam are competent to
do so; hence we may conclude that the dark waves are the real agents
in the explosion.

44. But this conclusion would be only probable; for it might be urged
that the _mixture_ of the dark waves and the light-waves is necessary
to produce the result. Let us then, by means of our opaque solution,
isolate our dark waves and converge them on the cotton. It explodes
as before.

45. Hence it is the dark waves, and they only, that are concerned in
the ignition of the cotton.

46. At the same dark focus sheets of platinum are raised to vivid
redness; zinc is burnt up; paper instantly blazes; magnesium wire is
ignited; charcoal within a receiver containing oxygen is set burning;
a diamond similarly placed is caused to glow like a star, being
afterward gradually dissipated. And all this while the _air_ at the
focus remains as cool as in any other part of the room.

47. To obtain the light-waves we employ a clear solution of alum in
water; to obtain the dark waves we employ the solution of iodine
above referred to. But as before stated (32), the alum is not so
perfect a filter as the iodine; for it transmits a portion of the
obscure heat.

48. Though the light-waves here prove their incompetence to ignite
gun-cotton, they are able to burn up black paper; or, indeed, to
explode the cotton when it is blackened. The white cotton does not
absorb the light, and without absorption we have no heating. The
blackened cotton absorbs, is heated, and explodes.

49. Instead of a solution of alum, we will employ for our next
experiment a cell of pure water, through which the light passes
without sensible absorption. At the focus is placed a test-tube also
containing water, the full force of the light being concentrated upon
it. The water is not sensibly warmed by the concentrated waves. We
now remove the cell of water; no change is visible in the beam, but
the water contained in the test-tube now boils.

50. The light-waves being thus proved ineffectual, and the full beam
effectual, we may infer that it is the dark waves that do the work of
heating. But we clench our inference by employing our opaque iodine
filter. Placing it on the path of the beam, the light is entirely
stopped, but the water boils exactly as it did when the full beam
fell upon it.

51. The truth of the statement made in paragraph (34) is thus
demonstrated.

52. And now with regard to the melting of ice. On the surface of
a flask containing a freezing mixture we obtain a thick fur of
hoar-frost. Sending the beam through a water-cell, its luminous waves
are concentrated upon the surface of the flask. Not a spicula of the
frost is dissolved. We now remove the water-cell, and in a moment a
patch of the frozen fur as large as half-a-crown is melted. Hence,
inasmuch as the full beam produces this effect, and the luminous part
of the beam does not produce it, we fix upon the dark portion the
melting of the frost.

53. As before, we clench this inference by concentrating the dark
waves alone upon the flask. The frost is dissipated exactly as it was
by the full beam.

54. These effects are rendered strikingly visible by darkening with
ink the freezing mixture within the flask. When the hoar-frost is
removed, the blackness of the surface from which it had been melted
comes out in strong contrast with the adjacent snowy whiteness. When
the flask itself, instead of the freezing mixture, is blackened, the
purely luminous waves being absorbed by the glass, warm it; the glass
reacts upon the frost, and melts it. Hence the wisdom of darkening,
instead of the flask itself, the mixture within the flask.

55. This experiment proves to demonstration the statement in
paragraph (36): that it is the dark waves of the sun that melt the
mountain snow and ice, and originate all the rivers derived from
glaciers.

There are writers who seem to regard science as an aggregate of
facts, and hence doubt its efficacy as an exercise of the reasoning
powers. But all that I have here taught you is the result of reason,
taking its stand, however, upon the sure basis of observation and
experiment. And this is the spirit in which our further studies are
to be pursued.


§ 6. _Oceanic Distillation._

56. The sun, you know, is never exactly overhead in England. But at
the equator, and within certain limits north and south of it, the sun
at certain periods of the year is directly overhead at noon. These
limits are called the Tropics of Cancer and of Capricorn. Upon the
belt comprised between these two circles the sun's rays fall with
their mightiest power; for here they shoot directly downward, and
heat both earth and sea more than when they strike slantingly.

57. When the vertical sunbeams strike the land they heat it, and the
air in contact with the hot soil becomes heated in turn. But when
heated the air expands, and when it expands it becomes lighter. This
lighter air rises, like wood plunged into water, through the heavier
air overhead.

58. When the sunbeams fall upon the sea the water is warmed, though
not so much as the land. The warmed water expands, becomes thereby
lighter, and therefore continues to float upon the top. This upper
layer of water warms to some extent the air in contact with it,
but it also sends up a quantity of aqueous vapour, which being far
lighter than air, helps the latter to rise. Thus both from the land
and from the sea we have ascending currents established by the action
of the sun.

59. When they reach a certain elevation in the atmosphere, these
currents divide and flow, part towards the north and part towards
the south; while from the north and the south a flow of heavier and
colder air sets in to supply the place of the ascending warm air.

60. Incessant circulation is thus established in the atmosphere. The
equatorial air and vapour flow above towards the north and south
poles, while the polar air flows below towards the equator. The two
currents of air thus established are called the upper and the lower
trade winds.

61. But before the air returns from the poles great changes have
occurred. For the air as it quitted the equatorial regions was laden
with aqueous vapour, which could not subsist in the cold polar
regions. It is there precipitated, falling sometimes as rain, or
more commonly as snow. The land near the pole is covered with this
snow, which gives birth to vast glaciers in a manner hereafter to be
explained.

62. It is necessary that you should have a perfectly clear view of
this process, for great mistakes have been made regarding the manner
in which glaciers are related to the heat of the sun.

63. It was supposed that if the sun's heat were diminished, greater
glaciers than those now existing would be produced. But the lessening
of the sun's heat would infallibly diminish the quantity of aqueous
vapour, and thus cut off the glaciers at their source. A brief
illustration will complete your knowledge here.

64. In the process of ordinary distillation, the liquid to be
distilled is heated and converted into vapour in one vessel, and
chilled and reconverted into liquid in another. What has just been
stated renders it plain that the earth and its atmosphere constitute
a vast distilling apparatus in which the equatorial ocean plays the
part of the boiler, and the chill regions of the poles the part of
the condenser. In this process of distillation _heat_ plays quite as
necessary a part as _cold_, and before Bishop Heber could speak of
"Greenland's icy mountains," the equatorial ocean had to be warmed by
the sun. We shall have more to say upon this question afterwards.


Illustrative Experiments.

65. I have said that when heated, air expands. If you wish to verify
this for yourself, proceed thus. Take an empty flask, stop it by a
cork; pass through the cork a narrow glass tube. By heating the tube
in a spirit-lamp you can bend it downwards, so that when the flask is
standing upright the open end of the narrow tube may dip into water.
Now cause the flame of your spirit-lamp to play against the flask.
The flame heats the glass, the glass heats the air; the air expands,
is driven through the narrow tube, and issues in a storm of bubbles
from the water.

66. Were the heated air unconfined, it would rise in the heavier
cold air. Allow a sunbeam or any other intense light to fall upon a
white wall or screen in a dark room. Bring a heated poker, a candle,
or a gas flame underneath the beam. An ascending current rises from
the heated body through the beam, and the action of the air upon the
light is such as to render the wreathing and waving of the current
strikingly visible upon the screen. When the air is hot enough, and
therefore light enough, if entrapped in a paper bag it carries the
bag upwards, and you have the Fire-balloon.

67. Fold two sheets of paper into two cones and suspend them with
their closed points upwards from the end of a delicate balance. See
that the cones balance each other. Then place for a moment the flame
of a spirit-lamp beneath the open base of one of them; the hot air
ascends from the lamp and instantly tosses upwards the cone above it.

68. Into an inverted glass shade introduce a little smoke. Let the
air come to rest, and then simply place your hand at the open mouth
of the shade. Mimic hurricanes are produced by the air warmed by the
hand, which are strikingly visible when the smoke is illuminated by a
strong light.

69. The heating of the tropical air by the sun is _indirect_. The
solar beams have scarcely any power to heat the air through which
they pass; but they heat the land and ocean, and these communicate
their heat to the air in contact with them. The air and vapour start
upwards charged with the heat thus communicated.


§ 7. _Tropical Rains._

70. But long before the air and vapour from the equator reach the
poles, precipitation occurs. Wherever a humid warm wind mixes with a
cold dry one, rain falls. Indeed the heaviest rains occur at those
places where the sun is vertically overhead. We must enquire a little
more closely into their origin.

71. Fill a bladder about two-thirds full of air at the sea level,
and take it to the summit of Mont Blanc. As you ascend, the bladder
becomes more and more distended; at the top of the mountain it is
fully distended, and has evidently to bear a pressure from within.
Returning to the sea level you find that the tightness disappears,
the bladder finally appearing as flaccid as at first.

72. The reason is plain. At the sea level the air within the
bladder has to bear the pressure of the whole atmosphere, being
thereby squeezed into a comparatively small volume. In ascending
the mountain, you leave more and more of the atmosphere behind; the
pressure becomes less and less, and by its expansive force the air
within the bladder swells as the outside pressure is diminished. At
the top of the mountain the expansion is quite sufficient to render
the bladder tight, the pressure within being then actually greater
than the pressure without. By means of an air-pump we can show the
expansion of a balloon partly filled with air, when the external
pressure has been in part removed.

73. But why do I dwell upon this? Simply to make plain to you that
the _unconfined air_, heated at the earth's surface, and ascending by
its lightness, must expand more and more the higher it rises in the
atmosphere.

74. And now I have to introduce to you a new fact, towards the
statement of which I have been working for some time. It is this:
_The ascending air is chilled by its expansion._ Indeed this chilling
is one source of the coldness of the higher atmospheric regions. And
now fix your eye upon those mixed currents of air and aqueous vapour
which rise from the warm tropical ocean. They start with plenty of
heat to preserve the vapour as vapour; but as they rise they come
into regions already chilled, and they are still further chilled by
their own expansion. The consequence might be foreseen. The load of
vapour is in great part precipitated, dense clouds are formed, their
particles coalesce to rain-drops, which descend daily in gushes so
profuse that the word "torrential" is used to express the copiousness
of the rainfall. I could show you this chilling by expansion, and
also the consequent precipitation of clouds.

75. Thus long before the air from the equator reaches the poles
its vapour is in great part removed from it, having redescended to
the earth as rain. Still a good quantity of the vapour is carried
forward, which yields hail, rain, and snow in northern and southern
lands.


Illustrative Experiments.

76. I have said that the air is chilled during its expansion. Prove
this, if you like, thus. With a condensing syringe, you can force air
into an iron box furnished with a stopcock, to which the syringe is
screwed. Do so till the density of the air within the box is doubled
or trebled. Immediately after this condensation, both the box and
the air within it are warm, and can be proved to be so by a proper
thermometer. Simply turn the cock and allow the compressed air to
stream into the atmosphere. The current, if allowed to strike the
thermometer, visibly chills it; and with other instruments the chill
may be made more evident still. Even the hand feels the chill of the
expanding air.

77. Throw a strong light, a concentrated sunbeam for example, across
the issuing current; if the compressed air be ordinary humid air, you
see the precipitation of a little cloud by the chill accompanying the
expansion. This cloud-formation may, however, be better illustrated
in the following way:--

78. In a darkened room send a strong beam of light through a glass
tube three feet long and three inches wide, stopped at its ends by
glass plates. Connect the tube by means of a stopcock with a vessel
of about one-fourth its capacity, from which the air has been removed
by an air-pump. The exhausted cylinder of the pump itself will answer
capitally. Fill the glass tube with humid air; then simply turn on
the stopcock which connects it with the exhausted vessel. Having
more room the air expands, cold accompanies the expansion, and, as a
consequence, a dense and brilliant cloud immediately fills the tube.
If the experiment be made for yourself alone you may see the cloud
in ordinary daylight; indeed, the brisk exhaustion of any receiver
filled with humid air is known to produce this condensation.

79. Other vapours than that of water may be thus precipitated,
some of them yielding clouds of intense brilliancy, and displaying
iridescences, such as are sometimes, but not frequently, seen in the
clouds floating over the Alps.

80. In science what is true for the small is true for the large. Thus
by combining the conditions observed on a large scale in nature we
obtain on a small scale the phenomena of atmospheric clouds.


§ 8. _Mountain Condensers._

81. To complete our view of the process of atmospheric precipitation
we must take into account the action of mountains. Imagine a
south-west wind blowing across the Atlantic towards Ireland. In
its passage it charges itself with aqueous vapour. In the south of
Ireland it encounters the mountains of Kerry: the highest of these
is Magillicuddy's Reeks, near Killarney. Now the lowest stratum of
this Atlantic wind is that which is most fully charged with vapour.
When it encounters the base of the Kerry mountains it is tilted up
and flows bodily over them. Its load of vapour is therefore carried
to a height, it expands on reaching the height, it is chilled in
consequence of the expansion, and comes down in copious showers
of rain. From this, in fact, arises the luxuriant vegetation of
Killarney; to this, indeed, the lakes owe their water supply. The
cold crests of the mountains also aid in the work of condensation.

82. Note the consequence. There is a town called Cahirciveen to
the south-west of Magillicuddy's Reeks, at which observations of
the rainfall have been made, and a good distance farther to the
north-east, right in the course of the south-west wind, there is
another town, called Portarlington, at which observations of rainfall
have also been made. But before the wind reaches the latter station
it has passed over the mountains of Kerry and left a great portion of
its moisture behind it. What is the result? At Cahirciveen, as shown
by Dr. Lloyd, the rainfall amounts to 59 inches in a year, while at
Portarlington it is only 21 inches.

83. Again, you may sometimes descend from the Alps when the fall of
rain and snow is heavy and incessant, into Italy, and find the sky
over the plains of Lombardy blue and cloudless, the wind at the same
time _blowing over the plain towards the Alps_. Below the wind is hot
enough to keep its vapour in a perfectly transparent state; but it
meets the mountains, is tilted up, expanded, and chilled. The cold of
the higher summits also helps the chill. The consequence is that the
vapour is precipitated as rain or snow, thus producing bad weather
upon the heights, while the plains below, flooded with the same air,
enjoy the aspect of the unclouded summer sun. Clouds blowing _from_
the Alps are also sometimes dissolved over the plains of Lombardy.

84. In connection with the formation of clouds by mountains, one
particularly instructive effect may be here noticed. You frequently
see a streamer of cloud many hundred yards in length drawn out from
an Alpine peak. Its steadiness appears perfect, though a strong
wind may be blowing at the same time over the mountain head. Why
is the cloud not blown away? It is blown away; its permanence is
only apparent. At one end it is incessantly dissolved, at the other
end it is incessantly renewed: supply and consumption being thus
equalized, the cloud appears as changeless as the mountain to which
it seems to cling. When the red sun of the evening shines upon these
cloud-streamers they resemble vast torches with their flames blown
through the air.


§ 9. _Architecture of Snow._

85. We now resemble persons who have climbed a difficult peak,
and thereby earned the enjoyment of a wide prospect. Having made
ourselves masters of the conditions necessary to the production of
mountain snow, we are able to take a comprehensive and intelligent
view of the phenomena of glaciers.

86. A few words are still necessary as to the formation of snow. The
molecules and atoms of all substances, when allowed free play, build
themselves into definite and, for the most part, beautiful forms
called crystals. Iron, copper, gold, silver, lead, sulphur, when
melted and permitted to cool gradually, all show this crystallizing
power. The metal bismuth shows it in a particularly striking manner,
and when properly fused and solidified, self-built crystals of great
size and beauty are formed of this metal.

87. If you dissolve saltpetre in water, and allow the solution to
evaporate slowly, you may obtain large crystals, for no portion of
the salt is converted into vapour. The water of our atmosphere is
fresh though it is derived from the salt sea. Sugar dissolved in
water, and permitted to evaporate, yields crystals of sugar-candy.
Alum readily crystallizes in the same way. Flints dissolved, as they
sometimes are in nature, and permitted to crystallize, yield the
prisms and pyramids of rock crystal. Chalk dissolved and crystallized
yields Iceland spar. The diamond is crystallized carbon. All our
precious stones, the ruby, sapphire, beryl, topaz, emerald, are all
examples of this crystallizing power.

88. You have heard of the force of gravitation, and you know that
it consists of an attraction of every particle of matter for every
other particle. You know that planets and moons are held in their
orbits by this attraction. But gravitation is a very simple affair
compared to the force, or rather forces, of crystallization. For here
the ultimate particles of matter, inconceivably small as they are,
show themselves possessed of attractive and repellent poles, by the
mutual action of which the shape and structure of the crystal are
determined. In the solid condition the attracting poles are rigidly
locked together; but if sufficient heat be applied the bond of union
is dissolved, and in the state of fusion the poles are pushed so far
asunder as to be practically out of each other's range. The natural
tendency of the molecules to build themselves together is thus
neutralized.

89. This is the case with water, which as a liquid is to all
appearance formless. When sufficiently cooled the molecules are
brought within the play of the crystallizing force, and they then
arrange themselves in forms of indescribable beauty. When snow
is produced in calm air, the icy particles build themselves into
beautiful stellar shapes, each star possessing six rays. There is no
deviation from this type, though in other respects the appearances
of the snow-stars are infinitely various. In the polar regions these
exquisite forms were observed by Dr. Scoresby, who gave numerous
drawings of them. I have observed them in mid-winter filling the air,
and loading the slopes of the Alps. But in England they are also to
be seen, and no words of mine could convey so vivid an impression of
their beauty as the annexed drawings of a few of them, executed at
Greenwich by Mr. Glaisher.

90. It is worth pausing to think what wonderful work is going on in
the atmosphere during the formation and descent of every snow-shower:
what building power is brought into play! and how imperfect seem the
productions of human minds and hands when compared with those formed
by the blind forces of nature!

91. But who ventures to call the forces of nature blind? In reality,
when we speak thus we are describing our own condition. The blindness
is ours; and what we really ought to say, and to confess, is that our
powers are absolutely unable to comprehend either the origin or the
end of the operations of nature.

92. But while we thus acknowledge our limits, there is also reason
for wonder at the extent to which science has mastered the system
of nature. From age to age, and from generation to generation, fact
has been added to fact, and law to law, the true method and order
of the Universe being thereby more and more revealed. In doing this
science has encountered and overthrown various forms of superstition
and deceit, of credulity and imposture. But the world continually
produces weak persons and wicked persons; and as long as they
continue to exist side by side, as they do in this our day, very
debasing beliefs will also continue to infest the world.


§ 10. _Atomic Poles._

93. "What did I mean when, a few moments ago (88), I spoke of
attracting and repellent poles?" Let me try to answer this question.
You know that astronomers and geographers speak of the earth's poles,
and you have also heard of magnetic poles, the poles of a magnet
being the points at which the attraction and repulsion of the magnet
are as it were concentrated.

[Illustration: SNOW CRYSTALS.]

94. Every magnet possesses, two such poles; and if iron filings be
scattered over a magnet, each particle becomes also endowed with
two poles. Suppose such particles devoid of weight and floating in
our atmosphere, what must occur when they come near each other?
Manifestly the repellent poles will retreat from each other, while
the attractive poles will approach and finally lock themselves
together. And supposing the particles, instead of a single pair, to
possess several pairs of poles arranged at definite points over their
surfaces; you can then picture them, in obedience to their mutual
attractions and repulsions, building themselves together to form
masses of definite shape and structure.

95. Imagine the molecules of water in calm cold air to be gifted
with poles of this description, which compel the particles to lay
themselves together in a definite order, and you have before your
mind's eye the unseen architecture which finally produces the
visible and beautiful crystals of the snow. Thus our first notions
and conceptions of poles are obtained from the sight of our eyes
in looking at the effects of magnetism; and we then transfer these
notions and conceptions to particles which no eye has ever seen. The
power by which we thus picture to ourselves effects beyond the range
of the senses is what philosophers call the Imagination, and in the
effort of the mind to seize upon the unseen architecture of crystals,
we have an example of the "scientific use" of this faculty. Without
imagination we might have _critical_ power, but not _creative_ power
in science.


§ 11. _Architecture of Lake Ice._

96. We have thus made ourselves acquainted with the beautiful
snow-flowers self-constructed by the molecules of water in calm cold
air. Do the molecules show this architectural power when ordinary
water is frozen? What, for example, is the structure of the ice over
which we skate in winter? Quite as wonderful as the flowers of the
snow. The observation is rare, if not new, but I have seen in water
slowly freezing six-rayed ice-stars formed, and floating free on the
surface. A six-rayed star, moreover, is typical of the construction
of all our lake ice. It is built up of such forms wonderfully
interlaced.

97. Take a slab of lake ice and place it in the path of a
concentrated sunbeam. Watch the track of the beam through the ice.
Part of the beam is stopped, part of it goes through; the former
produces internal liquefaction, the latter has no effect whatever
upon the ice. But the liquefaction is not uniformly diffused. From
separate spots of the ice little shining points are seen to sparkle
forth. Every one of those points is surrounded by a beautiful liquid
flower with six petals.

98. Ice and water are so optically alike that unless the light fall
properly upon these flowers you cannot see them. But what is the
central spot? A vacuum. Ice swims on water because, bulk for bulk,
it is lighter than water; so that when ice is melted it shrinks in
size. Can the liquid flowers then occupy the whole space of the ice
melted? Plainly no. A little empty space is formed with the flowers,
and this space, or rather its surface, shines in the sun with the
lustre of burnished silver.

99. In all cases the flowers are formed parallel to the surface of
freezing. They are formed when the sun shines upon the ice of every
lake; sometimes in myriads, and so small as to require a magnifying
glass to see them. They are always attainable, but their beauty is
often marred by internal defects of the ice. Even one portion of the
same piece of ice may show them exquisitely, while the second portion
shows them imperfectly.

100. Annexed is a very imperfect sketch of these beautiful figures.

101. Here we have a reversal of the process of crystallization. The
searching solar beam is delicate enough to take the molecules down
without deranging the order of their architecture. Try the experiment
for yourself with a pocket-lens on a sunny day. You will not find the
flowers confused; they all lie parallel to the surface of freezing.
In this exquisite way every bit of the ice over which our skaters
glide in winter is put together.

102. I said in (97) that a portion of the sunbeam was stopped by the
ice and liquefied it. What is this portion? The dark heat of the
sun. The great body of the light waves and even a portion of the dark
ones, pass through the ice without losing any of their heating power.
When properly concentrated on combustible bodies, even after having
passed through the ice, their burning power becomes manifest.

[Illustration: LIQUID FLOWERS IN LAKE ICE.]

103. And the ice itself may be employed to concentrate them. With an
ice-lens in the polar regions Dr. Scoresby has often concentrated the
sun's rays so as to make them burn wood, fire gunpowder, and melt
lead; thus proving that the heating power is retained by the rays,
even after they have passed through so cold a substance.

104. By rendering the rays of the electric lamp parallel, and then
sending them through a lens of ice, we obtain all the effects which
Dr. Scoresby obtained with the rays of the sun.


§ 12. _The Source of the Arveiron. Ice Pinnacles, Towers, and Chasms
of the Glacier des Bois. Passage to the Montanvert._

105. Our preparatory studies are for the present ended, and thus
informed, let us approach the Alps. Through the village of Chamouni,
in Savoy, a river rushes which is called the Arve. Let us trace this
river backwards from Chamouni. At a little distance from the village
the river forks; one of its branches still continues to be called
the Arve, the other is the Arveiron. Following this latter we come
to what is called the "source of the Arveiron"--a short hour's walk
from Chamouni. Here, as in the case of the Rhone already referred
to, you are fronted by a huge mass of ice, the end of a glacier, and
from an arch in the ice the Arveiron issues. Do not trust the arch in
summer. Its roof falls at intervals with a startling crash, and would
infallibly crush any person on whom it might fall.

106. We must now be observant. Looking about us here, we find in
front of the ice curious heaps and ridges of débris, which are more
or less concentric. These are the _terminal moraines_ of the glacier.
We shall examine them subsequently.

107. We now turn to the left, and ascend the slope beside the
glacier. As we ascend we get a better view, and find that the ice
here fills a narrow valley. We come upon another singular ridge,
not of fresh débris, like those lower down, but covered in part
with trees, and appearing to be literally as "old as the hills." It
tells a wonderful tale. We soon satisfy ourselves that the ridge is
an ancient moraine, and at once conclude that the glacier, at some
former period of its existence, was vastly larger than it is now.
This old moraine stretches right across the main valley, and abuts
against the mountains at the opposite side.

[Illustration: SOURCE OF THE ARVEIRON.]

108. Having passed the terminal portion of the glacier, which is
covered with stones and rubbish, we find ourselves beside a very
wonderful exhibition of ice. The glacier descends a steep gorge, and
in doing so is riven and broken in the most extraordinary manner.
Here are towers, and pinnacles, and fantastic shapes wrought out by
the action of the weather, which put one in mind of rude sculpture.
Annexed is a sketch of an ice-pinnacle. From deep chasms in the
glacier issues a delicate shimmer of blue light. At times we hear a
sound like thunder, which arises either from the falling of a tower
of ice, or from the tumble of a huge stone into a chasm. The glacier
maintains this wild and chaotic character for some time; and the best
iceman would find himself defeated in any attempt to get along it.

[Illustration: ICE-PINNACLE.]

109. We reach a place called the Chapeau, where, if we wish, we can
have refreshment in a little mountain hut. We then pass the _Mauvais
Pas_, a precipitous rock, on the face of which steps are hewn, and
the unpractised traveller is assisted by a rope. We pursue our
journey, partly along the mountain side, and partly along a ridge of
singularly artificial aspect a _lateral moraine_. We at length face a
house perched upon an eminence at the opposite side of the glacier.
This is the auberge of the Montanvert, well known to all visitors to
this portion of the Alps.

110. Here we cross the glacier. I should have told you that its lower
part, including the broken portion we have passed, is called the
Glacier des Bois; while the place that we are now about to cross is
the beginning of the Mer de Glace. You feel that this term is not
quite appropriate, for the glacier here is much more like a _river_
of ice than a sea. The valley which it fills is about half a mile
wide.

111. The ice may be riven where we enter upon it, but with the
necessary care there is no difficulty in crossing this portion
of the Mer de Glace. The clefts and chasms in the ice are called
_crevasses_; we shall make their acquaintance on a grander scale by
and by.

[Illustration: THE MER DE GLACE, SHOWING MONT TACUL AND THE GRANDE
JORASSE, WITH OUR CLEFT ABOVE TRÉLEPORTE TO THE RIGHT.]

112. Look up and down this side of the glacier. It is considerably
riven, but as we advance the crevasses will diminish, and we shall
find very few of them at the other side. Note this for future use.
The ice is at first dirty; but the dirt soon disappears, and you come
upon the clean crisp surface of the glacier. You have already noticed
that the clean ice is white, and that from a distance it resembles
snow rather than ice. This is caused by the breaking up of the
surface by the solar heat. When you pound transparent rock-salt into
powder it is as white as table-salt, and it is the minute fissuring
of the surface of the glacier by the sun's rays that causes it to
appear white. _Within_ the glacier the ice is transparent. After an
exhilarating passage we get upon the opposite lateral moraine, and
ascend the steep slope from it to the Montanvert Inn.


§ 13. _The Mer de Glace and its Sources. Our First Climb to the Cleft
Station._

113. Here the view before us is very grand. We look across the
glacier at the beautiful pyramid of the Aiguille du Dru (shown in our
frontispiece); and to the right at the Aiguille des Charmoz, with
its sharp pinnacles bent as if they were ductile. Looking straight
up the glacier the view is bounded by the great crests called La
Grande Jorasse, nearly 14,000 feet high. Our object now is to get
into the very heart of the mountains, and to pursue to its origin the
wonderful frozen river which we have just crossed.

114. Starting from the Montanvert with, the glacier below us to our
left, we soon reach some rocks resembling the Mauvais Pas; they are
called _les Fonts_. We cross them and reach _l'Angle_, where we quit
the land for the ice. We walk up the glacier, but before reaching
the promontory called Trélaporte, we take once more to the mountain
side; for though the path here has been forsaken on account of its
danger, for the sake of knowledge we are prepared to incur danger
to a reasonable extent. A little glacier reposes on the slope to
our right. We may see a huge boulder or two poised on the end of
the glacier, and, if fortunate, also see the boulder liberated and
plunging violently down the slope. Presence of mind is all that is
necessary to render our safety certain; but travellers do not always
show presence of mind, and hence the path which formerly led over
this slope has been forsaken. The whole slope is cumbered by masses
of rock which this little glacier has sent down. These I wished you
to see; by and by they shall be fully accounted for.

115. Above Trélaporte to the right you see a most singular cleft in
the rocks, in the middle of which stands an isolated pillar, hewn out
by the weather. Our next object is to get to the tower of rock to
the left of that cleft, for from that position we shall gain a most
commanding and instructive view of the Mer de Glace and its sources.

116. The cleft referred to, with its pillar, may be seen to the
right of the preceding engraving of the Mer de Glace. Below the cleft
is also seen the little glacier just referred to.

117. We may reach this cleft by a steep gully, visible from our
present position, and leading directly up to the cleft. But these
gullies, or couloirs, are very dangerous, being the pathways of
stones falling from the heights. We will therefore take the rocks
to the left of the gully, by close inspection ascertain their
assailable points, and there attack them. In the Alps as elsewhere
wonderful things may be done by looking steadfastly at difficulties,
and testing them wherever they appear assailable. We thus reach our
station, where the glory of the prospect, and the insight that we
gain as to the formation of the Mer de Glace, far more than repay us
for the labour of our ascent.

118. For we see the glacier below us, stretching its frozen tongue
downwards past the Montanvert. And we now find this single glacier
branching out into three others, some of them wider than itself.
Regard the branch to the right, the Glacier du Géant. It stretches
smoothly up for a long distance, then becomes disturbed, and then
changes to a great frozen cascade, down which the ice appears to
tumble in wild confusion. Above the cascade you see an expanse of
shining snow, occupying an area of some square miles.


§ 14. _Ice-cascade and Snows of the Col du Géant._

119. Instead of climbing to the height where we now stand, we might
have continued our walk upon the Mer de Glace, turned round the
promontory of Trélaporte, and walked right up the Glacier du Géant.
We should have found ice under our feet up to the bottom of the
cascade. It is not so compact as the ice lower down, but you would
not think of refusing to call it ice.

120. As we approach the fall, the smooth and unbroken character of
the glacier changes more and more. We encounter transverse ridges
succeeding each other with augmenting steepness. The ice becomes more
and more fissured and confused. We wind through tortuous ravines,
climb huge ice-mounds, and creep cautiously along crumbling crests,
with crevasses right and left. The confusion increases until further
advance along the centre of the glacier is impossible.

121. But with the aid of an axe to cut steps in the steeper ice-walls
and slopes we might, by swerving to either side of the glacier, work
our way to the top of the cascade. If we ascended to the right,
we should have to take care of the ice avalanches which sometimes
thunder down the slopes; if to the left, we should have to take care
of the stones let loose from the Aiguille Noire. After we had cleared
the cascade, we should have to beware for a time of the crevasses,
which for some distance above the fall yawn terribly. But by caution
we could get round them, and sometimes cross them by bridges of snow.
Here the skill and knowledge to be acquired only by long practice
come into play; and here also the use of the Alpine rope suggests
itself. For not only are the 'snow bridges often frail, but whole
crevasses are sometimes covered, the unhappy traveller being first
made aware of their existence by the snow breaking under his feet.
Many lives have thus been lost, and some quite recently.

122. Once upon the plateau above the ice-fall we find the surface
totally changed. Below the fall we walked upon ice; here we are
upon snow. After a gentle but long ascent we reach a depression of
the ridge which bounds the snow-field at the top, and now look over
Italy. We stand upon the famous Col du Géant.

123. They were no idle scamperers on the mountains that made these
wild recesses first known; it was not the desire for health which
now brings some, or the desire for grandeur and beauty which brings
others, or the wish to be able to say that they have climbed a
mountain or crossed a col, which I fear brings a good many more; it
was a desire for _knowledge_ that brought the first explorers here,
and on this col the celebrated De Saussure lived for seventeen days,
making scientific observations.


§ 15. _Questioning the Glaciers._

124. I would now ask you to consider for a moment the facts which
such an excursion places in our possession. The snow through which
we have in idea trudged is the snow of last winter and spring. Had
we placed last August a proper mark upon the surface of the snow, we
should find it this August at a certain depth beneath the surface. A
good deal has been melted by the summer sun, but a good deal of it
remains, and it will continue until the snows of the coming winter
fall and cover it. This again will be in part preserved till next
August, a good deal of it remaining until it is covered by the snow
of the subsequent winter. We thus arrive at the certain conclusion
that on the plateau of the Col du Géant _the quantity of snow that
falls annually exceeds the quantity melted_.

125. Had we come in the month of April or May, we should have found
the glacier below the ice-fall also covered with snow, which is now
entirely cleared away by the heat of summer. Nay, more, the ice there
is obviously melting, forming running brooks which cut channels in
the ice, and expand here and there into small blue-green lakes. Hence
you conclude with certainty that below the ice-fall _the quantity of
frozen material falling upon the glacier is less than the quantity
melted_.

126. And this forces upon us another conclusion: between the glacier
below the ice-fall and the plateau above it there must exist a line
where the quantity of snow which falls _is exactly equal_ to the
quantity annually melted. This is the _snow-line_. On some glaciers
it is quite distinct, and it would be distinct here were the ice less
broken and confused than it actually is.

127. The French term _névé_ is applied to the glacial region above
the snow-line, while the word _glacier_ is restricted to the ice
below it. Thus the snows of the Col du Géant constitute the névé of
the Glacier du Géant, and in part, the névé of the Mer de Glace.

128. But if every year thus leaves a residue of snow upon the plateau
of the Col du Géant, it necessarily follows that the plateau must get
annually higher, _provided the snow remain upon it_. Equally certain
is the conclusion that the whole length of the glacier below the
cascade must sink gradually lower, _if the waste of annual melting
be not made good_. Supposing two feet of snow a year to remain upon
the Col, this would raise it to a height far surpassing that of Mont
Blanc in five thousand years. Such accumulation must take place if
the snow remain upon the Col; but the accumulation does _not_ take
place, hence the snow does not remain on the Col. The question then
is, whither does it go?

[Illustration: SKETCH-PLAN, SHOWING THE MORAINES, _a_, _b_, _c_, _d_,
_e_, OF THE MER DE GLACE.]


§ 16. _Branches and Medial Moraines of the Mer de Glace from the
Cleft Station._

129. We shall grapple with this question immediately. Meanwhile
look at that ice-valley in front of us, stretching up between Mont
Tacul and the Aiguille de Léchaud, to the base of the great ridge
called the Grande Jorasse. This is called the Glacier de Léchaud. It
receives at its head the snows of the Jorasse and of Mont Mallet,
and joins the Glacier du Géant at the promontory of the Tacul. The
glaciers seem welded together where they join, but they continue
distinct. Between them you clearly trace a stripe of débris (_c_ on
the annexed sketch-plan); you trace a similar though smaller stripe
(_a_ on the sketch), from the junction of the Glacier du Géant with
the Glacier des Périades at the foot of the Aiguille Noire, which you
also follow along the Mer de Glace.

130. We also see another glacier, or a portion of it, to the left,
falling apparently in broken fragments through a narrow gorge
(Cascade du Talèfre on the sketch) and joining the Léchaud, and from
their point of junction also a stripe of débris (_d_) runs downwards
along the Mer de Glace. Beyond this again we notice another stripe
(_e_), which seems to begin at the bottom of the ice-fall, rising
as it were from the body of the glacier. Beyond all of these we can
notice the lateral moraine of the Mer de Glace.

131. These stripes are the _medial moraines_ of the Mer de Glace. We
shall learn more about them immediately.

132. And now, having informed our minds by these observations, let
our eyes wander over the whole glorious scene, the splintered peaks
and the hacked and jagged crests, the far-stretching snow-fields, the
smaller glaciers which nestle on the heights, the deep blue heaven
and the sailing clouds. Is it not worth some labour to gain command
of such a scene? But the delight it imparts is heightened by the
fact that we did not come expressly to see it; we came to instruct
ourselves about the glacier, and this high enjoyment is an incident
of our labour. You will find it thus through life; without honest
labour there can be no deep joy.


§ 17. _The Talèfre and the Jardin. Work among the Crevasses._

133. And now let us descend to the Mer de Glace, for I want to take
you across the glacier to that broken ice-fall the origin of which we
have not yet seen. We aim at the farther side of the glacier, and to
reach it we must cross those dark stripes of débris which we observed
from the heights. Looked at from above, these moraines seemed flat,
but now we find them to be ridges of stones and rubbish, from twenty
to thirty feet high.

134. We quit the ice at a place called the Couvercle, and wind round
this promontory, ascending all the time. We squeeze ourselves through
the _Égralets_, a kind of natural staircase in the rock, and soon
afterwards obtain a full view of the ice-fall, the origin of which we
wish to find. The ice upon the fall is much broken; we have pinnacles
and towers, some erect, some leaning, and some, if we are fortunate,
falling like those upon the Glacier des Bois; and we have chasms from
which issues a delicate blue light. With the ice-fall to our right
we continue to ascend, until at length we command a view of a huge
glacier basin, almost level, and on the middle of which stands a
solitary island, entirely surrounded by ice. We stand at the edge of
the _Glacier du Talèfre_, and connect it with the ice-fall we have
passed. The glacier is bounded by rocky ridges, hacked and torn at
the top into teeth and edges, and buttressed by snow fluted by the
descending stones.

135. We cross the basin to the central island, and find grass and
flowers at the place where we enter upon it. This is the celebrated
_Jardin_, of which you have often heard. The upper part of the Jardin
is bare rock. Close at hand is one of the noblest peaks in this
portion of the Alps, the Aiguille Verte. It is between thirteen and
fourteen thousand feet high, and down its sides, after freshly-fallen
snow, avalanches incessantly thunder. From one of its projections a
streak of moraine starts down the Talèfre; from the Jardin also a
similar streak of moraine issues. Both continue side by side to the
top of the ice-fall, where they are engulphed in the chasms. But at
the bottom of the fall they reappear, as if newly emerging from the
body of the glacier, and afterwards they continue along the Mer de
Glace.

136. Walk with me now alongside the moraine from the Jardin down
towards the ice-fall. For a time our work is easy, such fissures as
appear offering no impediment to our march. But the crevasses become
gradually wider and wilder, following each other at length so rapidly
as to leave merely walls of ice between them. Here perfect steadiness
of foot is necessary a slip would be death. We look towards the fall,
and observe the confusion of walls and blocks and chasms below us
increasing. At length prudence and reason cry "Halt!" We may swerve
to the right or to the left, and making our way along crests of ice,
with chasms on both hands, reach either the right lateral moraine or
the left lateral moraine of the glacier.


§ 18. _First Questions regarding Glacier Motion. Drifting of Bodies
buried in a Crevasse._

137. But what are these lateral moraines? As you and I go from day
to day along the glaciers, their origin is gradually made plain. We
see at intervals the stones and rubbish descending from the mountain
sides and arrested by the ice. All along the fringe of the glacier
the stones and rubbish fall, and it soon becomes evident that we
have here the source of the lateral moraines.

138. But how are the medial moraines to be accounted for? How does
the débris range itself upon the glacier in stripes some hundreds
of yards from its edge, leaving the space between them and the edge
clear of rubbish? Some have supposed the stones to have rolled
over the glacier from the sides, but the supposition will not bear
examination. Call to mind now our reasoning regarding the excess of
snow which falls above the snow-line, and our subsequent question,
How is the snow disposed of. Can it be that the entire mass is moving
slowly downwards? If so, the lateral moraines would be carried along
by the ice on which they rest, and when two branch glaciers unite
they would lay their adjacent lateral moraines together to form a
medial moraine upon the trunk glacier.

139. There is, in fact, no way that we can see of disposing of the
excess of snow above the snow-line; there is no way of making good
the constant waste of the ice below the snow-line; there is no way of
accounting for the medial moraines of the glacier, but by supposing
that from the highest snow-fields of the Col du Géant, the Léchaud,
and the Talèfre, to the extreme end of the Glacier des Bois, the
whole mass of frozen matter is moving downwards.

140. If you were older, it would give me pleasure to take you up
Mont Blanc. Starting from Chamouni, we should first pass through
woods and pastures, then up the steep hill-face with the Glacier des
Bossons to our right, to a rock known as the _Pierre Pointue_; thence
to a higher rock called the _Pierre l'Échelle_, because here a ladder
is usually placed to assist in crossing the chasms of the glacier.
At the Pierre l'Échelle we should strike the ice, and passing under
the Aiguille du Midi, which towers to the left, and which sometimes
sweeps a portion of the track with stone avalanches, we should cross
the Glacier des Bossons; amid heaped-up mounds and broken towers of
ice; up steep slopes; over chasms so deep that their bottoms are hid
in darkness.

141. We reach the rocks of the Grands Mulets, which form a kind
of barren islet in the icy sea; thence to the higher snow-fields,
crossing the _Petit Plateau_, which we should find cumbered by blocks
of ice. Looking to the right, we should see whence they came, for
rising here with threatening aspect high above us are the broken
ice-crags[B] of the Dôme du Goûté. The guides wish to pass this place
in silence, and it is just as well to humour them, however much you
may doubt the competence of the human voice to bring the ice-crags
down. From the Petit Plateau a steep snow-slope would carry us to the
Grand Plateau, and at day-dawn I know nothing in the whole Alps more
grand and solemn than this place.

[B] Named _séracs_ from their resemblance in shape and colour to an
inferior kind of curdy cheese called by this name at Chamouni.

142. One object of our ascent would be now attained; for here at the
head of the Grand Plateau, and at the foot of the final slope of Mont
Blanc, I should show you a great crevasse, into which three guides
were poured by an avalanche in the year 1820.

[Illustration: CREVASSE ON GRAND PLATEAU.]

143. Is this language correct? A crevasse hardly to be distinguished
from the present one undoubtedly existed here in 1820. But was it
the identical crevasse now existing? Is the ice riven here to-day
the same as that riven fifty-one years ago? By no means. How is this
proved? By the fact that more than forty years after their interment,
the remains of those three guides were found near the end of the
Glacier des Bossons, many miles below the existing crevasse.

144. The same observation proves to demonstration that it is the ice
near the _bottom_ of the higher névé that becomes the _surface-ice_
of the glacier near its end. The waste of the surface below the
snow-line brings the deeper portions of the ice more and more to the
light of day.

145. There are numerous obvious indications of the existence of
glacier motion, though it is too slow to catch the eye at once.
The crevasses change within certain limits from year to year, and
sometimes from month to month; and this could not be if the ice did
not move. Rocks and stones also are observed, which have been plainly
torn from the mountain sides. Blocks seen to fall from particular
points are afterwards observed lower down. On the moraines rocks
are found of a totally different mineralogical character from those
composing the mountains right and left; and in all such cases strata
of the same character are found bordering the glacier higher up.
Hence the conclusion that the foreign boulders have been _floated_
down by the ice. Further, the ends or "snouts" of many glaciers act
like ploughshares on the land in front of them, overturning with slow
but merciless energy huts and chalets that stand in their way. Facts
like these have been long known to the inhabitants of the High Alps,
who were thus made acquainted in a vague and general way with the
motion of the glaciers.


§ 19. _The Motion of Glaciers. Measurements by Hugi and Agassiz.
Drifting of Huts on the Ice._

146. But the growth of knowledge is from vagueness towards precision,
and exact determinations of the rate of glacier motion were soon
desired. With reference to such measurements one glacier in the
Bernese Oberland will remain forever memorable. From the little town
of Meyringen in Switzerland you proceed up the valley of Hasli, past
the celebrated waterfall of Handeck, where the river Aar plunges into
a chasm more than 200 feet deep. You approach the Grimsel Pass, but
instead of crossing it you turn to the right and follow the course of
the Aar upwards. Like the Rhone and the Arveiron, you find the Aar
issuing from a glacier.

147. Get upon the ice, or rather upon the deep moraine shingle which
covers the ice, and walk upwards. It is hard walking, but after
some time you get clear of the rubbish, and on to a wide glacier
with a great medial moraine running along its back. This moraine is
formed by the junction of two branch glaciers, the Lauteraar and the
Finsteraar, which unite at a promontory called the Abschwung to form
the trunk glacier of the Unteraar.

148. On this great medial moraine in 1827 an intrepid and
enthusiastic Swiss professor, Hugi, of Solothurm (French Soleure),
built a hut with a view to observations upon the glacier. His hut
moved, and he measured its motion. In the three years--from 1827 to
1830--it had moved 330 feet downwards. In 1836 it had moved 2,354
feet; and in 1841 M. Agassiz found it 4,712 feet below its first
position.

149. In 1840, M. Agassiz himself and some bold companions took
shelter under a great overhanging slab of rock on the same moraine,
to which they added side walls and other means of protection. And
because he and his comrades came from Neufchâtel, the hut was called
long afterwards the "Hôtel des Neuchâtelois." Two years subsequent
to its erection M. Agassiz found that the "hotel" had moved 486 feet
downwards.


§ 20. _Precise Measurements of Agassiz and Forbes. Motion of a
Glacier proved to resemble the Motion of a River._

150. We now approach an epoch in the scientific history of glaciers.
Had the first observers been practically acquainted with the
instruments of precision used in surveying, _accurate_ measurements
of the motion of glaciers would probably have been earlier executed.
We are now on the point of seeing such instruments introduced almost
simultaneously by M. Agassiz on the glacier of the Unteraar, and by
Professor Forbes on the Mer de Glace. Attempts had been made by M.
Escher de la Linth to determine the motion of a series of wooden
stakes driven into the Aletsch glacier, but the melting was so
rapid that the stakes soon fell. To remedy this, M. Agassiz in 1841
undertook the great labour of carrying boring tools to his "hotel,"
and piercing the Unteraar glacier at six different places to a depth
of ten feet, in a straight line across the glacier. Into the holes
six piles were so firmly driven that they remained in the glacier for
a year, and in 1842 the displacements of all six were determined.
They were found to be 160 feet, 225 feet, 269 feet, 245 feet, 210
feet, and 125 feet, respectively.

151. A great step is here gained. You notice that the middle numbers
are the largest. They correspond to the central portion of the
glacier. Hence, these measurements conclusively establish, not only
the fact of glacier motion, but that _the centre of a glacier, like
that of a river, moves more rapidly than the sides_.

152. With the aid of trained engineers M. Agassiz followed up these
measurements in subsequent years. His researches are recorded in a
work entitled "Système glaciaire," which is accompanied by a very
noble Atlas of the Glacier of the Unteraar, published in 1847.

153. These determinations were made by means of a theodolite, of
which I will give you some notion immediately. The same instrument
was employed the same year by the late Professor Forbes upon the
Mer de Glace. He established independently the greater central
motion. He showed, moreover, that it is not necessary to wait a
year, or even a week to determine the motion of a glacier; with a
correctly-adjusted theodolite he was able to determine the motion
of various points of the Mer de Glace from day to day. He affirmed,
and with truth, that the motion of the glacier might be determined
from hour to hour. We shall prove this farther on (162). Professor
Forbes also triangulated the Mer de Glace, and laid down an excellent
map of it. His first observations and his survey are recorded in a
celebrated book published in 1843, and entitled "Travels in the Alps."

154. These observations were also followed up in subsequent years,
the results being recorded in a series of detached letters and essays
of great interest. These were subsequently collected in a volume
entitled "Occasional Papers on the Theory of Glaciers," published in
1859. The labours of Agassiz and Forbes are the two chief sources of
our knowledge of glacier phenomena.


§ 21. _The Theodolite and its Use. Our own Measurements._

155. My object thus far is attained. I have given you proofs of
glacier motion, and a historic account of its measurement. And
now we must try to add a little to the knowledge of glaciers by
our own labours on the ice. Resolution must not be wanting at
the commencement of our work, nor steadfast patience during its
prosecution. Look then at this theodolite; it consists mainly of a
telescope and a graduated circle, the telescope capable of motion
up and down, and the circle, carrying the telescope along with it,
capable of motion right and left. When desired to make the motion
exceedingly fine and minute, suitable screws, called tangent screws,
are employed. The instrument is supported by three legs, movable, but
firm when properly planted.

156. Two spirit-levels are fixed at right angles to each other on
the circle just referred to. Practice enables one to take hold of
the legs of the instrument, and so to fix them that the circle shall
be nearly horizontal. By means of four levelling screws we render it
_accurately_ horizontal. Exactly under the centre of the instrument
is a small hook from which a plummet is suspended; the point of the
bob just touches a rock on which we make a mark; or if the earth be
soft underneath, we drive a stake into it exactly under the plummet.
By re-suspending the plummet at any future time we can find to a
hairbreadth the position occupied by the instrument to-day.

157. Look through the telescope; you see it crossed by two fibres
of the finest spider's thread. In actual work we first direct the
telescope across the glacier, until the intersection of the two
fibres accurately covers some well-defined point of rock or tree at
the other side of the valley. This, our fixed standard, we sketch
with its surroundings in a note-book, so as to be able immediately
to recognise it on our return to this place. Imagine a straight line
drawn from the centre of the telescope to this point, and that this
line is permitted to drop straight down upon the glacier, every point
of it falling as a stone would fall; along such a line we have now to
fix a series of stakes.

158. A trained assistant is already upon the glacier. He erects
his staff and stands behind it; the telescope is lowered without
swerving to the right or to the left; in mathematical language it
remains _in the same vertical plane_. The crossed fibres of the
telescope probably strike the ice a little away from the staff of the
assistant; by a wave of the arm he moves right or left; he may move
too much, so we wave him back again. After a trial or two he knows
whether he is near the proper point, and if so makes his motions
small. He soon exactly strikes the point covered by the intersection
of the fibres. A signal is made which tells him that he is right; he
pierces the ice with an auger and drives in a stake. He then goes
forward, and in precisely the same manner takes up another point.
After one or two stakes have been driven in, the assistant is able to
take up the other points very rapidly. Any requisite number of stakes
may thus be fixed in a straight line across the glacier.

159. Next morning we measure the motion of all the stakes. The
theodolite is mounted in its former position and carefully levelled.
The telescope is directed first upon the standard point at the
opposite side of the valley, being moved by a tangent screw until the
intersection of the spider's threads accurately covers the point.
The telescope is then lowered to the first stake, beside which our
trained assistant is already standing. He is provided with a staff
with feet and inches marked on it. A glance shows us the stake has
moved down. By our signals the assistant recovers the point from
which we started yesterday, and then determines the distance from
this point to the stake. It is, say, 6 inches; through this distance,
therefore, the stake has moved.

160. We are careful to note the hour and minute at which each stake
is driven in, and the hour and the minute when its distance from its
first position is measured; this enables us to calculate the accurate
_daily motion_ of the point in question. The distances through which
all the other points have moved are determined in precisely the same
way.

161. Thus we shall proceed to work, first making clear to our minds
what is to be done, and then making sure that it shall be accurately
done. To give our work reality, I will here record the actual
measurements executed, and the actual thoughts suggested, on the
Mer de Glace in 1857. The only unreality that I would ask you to
allow, is that you and I are supposed to be making the observations
together. The labour of measuring was undertaken for the most part
by Mr. Hirst.


§ 22. _Motion of the Mer de Glace._

162. On July 14, then, we find ourselves at the end of the Glacier
des Bois, not far from the source of the Arveiron. We direct our
telescope across the glacier, and fix the intersection of its
spider's threads accurately upon the edge of a pinnacle of ice. We
leave the instrument untouched, looking through it from hour to
hour. The edge of ice moves slowly, but plainly, past the fibres,
and at the end of three hours we assure ourselves that the motion
has amounted to several inches. While standing near the vault of the
Arveiron, and talking about going into it, its roof gives way, and
falls with the sound of thunder. It is not, therefore, without reason
that I warned you against entering these vaults in summer.

163. We ascend to the Montanvert Inn, fix on it as a residence, and
then descend to the lateral moraine of the glacier a little below the
inn. Here we erect our theodolite, and mark its exact position by
a plummet. We must first make sure that our line is perpendicular,
or nearly so, to the axis or middle line of the glacier. Our
instructed assistant lays down a long staff in the direction of
the axis, assuring himself, by looking up and down, that it is the
true direction. With another staff in his hand, pointed towards
our theodolite, he shifts his position until the second staff is
perpendicular to the first. Here he gives us a signal. We direct our
telescope upon him, and then gradually raising its end in a vertical
plane we find, and note by sketching, a standard point at the other
side of the glacier. This point known, and our plummet mark known, we
can on any future day find our line. (To render the measurements more
intelligible, I append on the next page an outline diagram of the Mer
de Glace, and of its tributaries.)

164. Along the line just described ten stakes were set on July 17,
1857. Their displacements were measured on the following day. Two of
them had fallen, but here are the distances passed over by the eight
remaining ones in twenty-four hours.

DAILY MOTION OF THE MER DE GLACE.

First Line: A A' upon the Sketch.

        East                 West
  Stake   1  2  3  4  5  7  9 10
  Inches 12 17 23 26 25 26 27 33

165. You have already assured yourself by actual contact that the
body of the glacier is real ice, and you may have read that glaciers
move; but the actual observation of the motion of a body apparently
so rigid is strangely interesting. And not only does the ice move
bodily, but one part of it moves past another; the rate of motion
augmenting gradually from 12 inches a day at the side to 33 inches a
day at a distance from the side. This quicker movement of the central
ice of glaciers had been already observed by Agassiz and Forbes; we
verify their results, and now proceed to something new. Crossing the
Glacier du Géant, which occupies more than half the valley, we find
that our line of stakes is not yet at an end. The 10th stake stands
on the part of the ice which comes from the Talèfre.

[Illustration: OUTLINE PLAN, SHOWING THE MEASURED LINES OF THE MER DE
GLACE AND ITS TRIBUTARIES.]

166. Now the motion of the sides is slow, because of the friction of
the ice against its boundaries; but then one would think that midway
between the boundaries, where the friction of the sides is least, the
motion ought to be greatest. This is clearly not the case; for though
the 10th stake is nearer than the 9th to the eastern or _Chapeau_
side of the valley, the 10th stake surpasses the 9th by 6 inches a
day.

167. Here we have something to think of; but before a natural
philosopher can think with comfort he must be perfectly sure of his
facts. The foregoing line ran across the glacier a little below the
Montanvert. We will run another line across a little way above the
hotel. On July 18 we set out this line, and to multiply our chances
of discovery we place along it 31 stakes. On the subsequent day five
of these were found unfit for use; but here are the distances passed
over by the remaining six-and-twenty in 24 hours.

Second Line: B B' upon the Sketch.

         West
  Stake   2  3  4  5  6  7  8  9 10 11 12 13
  Inches 11 12 15 15 16 17 18 19 20 20 21 21
  Stake  15 16 17 18 19 20 21 22 23 24 25 26
  Inches 23 23 23 21 23 21 25 22 22 23 25 26
                                         East

168. Look at these numbers. The first broad fact they reveal is the
advance in the rate of motion from first to last. There are however
some irregularities; from 23 inches at the 17th stake we fall to 21
inches at the 18th; from 23 inches at the 19th we fall to 21 inches
at the 20th; from 25 inches at the 21st we fall to 22 inches at the
22nd and 23rd; but notwithstanding these small ups and downs, the
general advance of the rate of motion is manifest. Now there may have
been some slight displacement of the stakes by melting, sufficient to
account for these small deviations from uniformity in the increase
of the motion. But another solution is also possible. We shall
afterwards learn that the glacier is retarded not only by its sides
but by its bed; that the upper portions of the ice slide over the
lower ones. Now if the bed of the Mer de Glace should have eminences
here and there rising sufficiently near to the surface to retard the
motion of the surface, they might produce the small irregularities
noticed above.

169. We note particularly, while upon the ice, that the 26th stake,
like the 10th stake in our last line, stands much nearer to the
eastern than to the western side of the glacier; the measurements,
therefore, offer a further proof that the centre of this portion of
the glacier is not the place of swiftest motion.


§ 23. _Unequal Motion of the two Sides of the Mer de Glace._

170. But in neither the first line nor the second were we able to
push our measurements quite across the glacier. Why? In attempting
to do one thing we are often taught another, and thus in science, if
we are only steadfast in our work, our very defeats are converted
into means of instruction. We at first planted our theodolite on the
lateral moraine of the Mer de Glace, expecting to be able to command
the glacier from side to side. But we are now undeceived; the centre
of the glacier proves to be higher than its sides, and from our last
two positions the view of the ice near the opposite side of the
glacier was intercepted by the elevation at the centre. The mountain
slopes, in fact, are warm in summer, and they melt the ice nearest to
them, thus causing a fall from the centre to the sides.

171. But yonder on the heights at the other side of the glacier we
see a likely place for our theodolite. We cross the glacier and plant
our instrument in a position from which we sweep the glacier from
side to side. Our first line was below the Montanvert, our second
line above it; this third line is exactly opposite the Montanvert;
in fact, the mark on which we have fixed the fibre-cross of the
theodolite is a corner of one of the windows of the little inn. Along
this line we fix twelve stakes on July 20. On the 21st one of them
had fallen; but the velocities of the remaining eleven in 24 hours
were found to be as follows:--

Third Line: C C' upon the Sketch.

          East                         West
  Stake     1  2  3  4  5  6  7  8  9 10 11
  Inches   20 23 29 30 34 28 25 25 25 18  9

172. Both the first stake and the eleventh in this series stood
near the sides of the glacier. On the eastern side the motion is
20 inches, while on the western side it is only 9. It rises on
the eastern side from 20 to 34 inches at the 5th stake, which we,
standing upon the glacier, can see to be much nearer to the eastern
than to the western side. _The united evidence of these three lines
places the fact beyond doubt, that opposite the Montanvert, and for
some distance above it and below it, the whole eastern side of the
glacier is moving more quickly than the western side._


§ 24. _Suggestion of a new Likeness of Glacier Motion to River
Motion. Conjecture tested._

173. Here we have cause for reflection, and facts are comparatively
worthless if they do not provoke this exercise of the mind. It
is because facts of nature are not isolated but connected, that
science, to follow them, must also form a connected whole. The mind
of the natural philosopher must, as it were, be a web of _thought_
corresponding in all its fibres with the web of _fact_ in nature.

174. Let us, then, ascend to a point which commands a good view
of this portion of the Mer de Glace. The ice-river we see is not
straight but curved, and its curvature is _from_ the Montanvert;
that is to say, its convex side is east, and its concave side is
west (look to the sketch). You have already pondered the fact that a
glacier, _in some respects_, moves like a river. How would a river
move through a curved channel? This is known. Were the ice of the
Mer de Glace displaced by water, the point of swiftest motion at the
Montanvert would not be the centre, but a point east of the centre.
Can it be then that this "water-rock," as ice is sometimes called,
acts in this respect also like water?

175. This is a thought suggested on the spot; it may or it may not
be true, but the means of testing it are at hand. Looking up the
glacier, we see that at _les Ponts_ it also bends, but that there
its convex curvature is towards the western side of the valley (look
again to the sketch). If our surmise be true, the point of swiftest
motion opposite _les Ponts_ ought to lie west of the axis of the
glacier.

176. Let us test this conjecture. On July 25 we fix in a line across
this portion of the glacier seventeen stakes; every one of them has
remained firm, and on the 26th we find the motion for 24 hours to be
as follows:--

Fourth Line: D D' upon the Sketch.

        East                                    West
  Stake   1 2  3  4  5  8  7  8  9 10 11 12 13 14 15
  Inches  7 8 13 15 16 19 20 21 21 23 23 21 22 17 15

177. Inspected by the naked eye alone, the stakes 10 and 11, where
the glacier reaches its greatest motion, are seen to be considerably
to the west of the axis of the glacier. Thus far we have a perfect
verification of the _guess_ which prompted us to make these
measurements. You will here observe that the "guesses" of science are
not the work of chance, but of thoughtful pondering over antecedent
facts. The guess is the "induction" facts, to be ratified or exploded
by the test of subsequent experiment.

178. And though even now we have exceedingly strong reason for
holding that the point of maximum velocity obeys the law of liquid
motion, the strength of our conclusion will be doubled if we can
show that the point shifts back to the eastern side of the axis at
another place of flexure. Fortunately such a place exists opposite
Trélaporte. Here the convex curvature of the valley turns again to
the east. Across this portion of the glacier a line was set out on
July 28, and from measurements on the 31st, the rate of motion per 24
hours was determined.

Fifth Line: E E' upon the Sketch.

          West                                     East
  Stake     1  2  3  4  5  6  7  8  9 10 11 12 13 14 15
  Inches   11 14 13 15 15 16 17 19 20 19 20 18 16 15 10

179. Here, again, the mere estimate of distances by the eye would
show us that the three stakes which moved fastest, viz. the 9th,
10th, and 11th, were all to the east of the middle line of the
glacier. The demonstration that the point of swiftest motion wanders
to and fro across the axis, as the flexure of the valley changes, is,
therefore,--shall I say complete?

180. Not yet. For if surer means are open to us we must not rest
content with estimates by the eye. We have with us a surveying chain:
let us shake it out and measure these lines, noting the distance of
every stake from the side of the glacier. This is no easy work among
the crevasses, but I confide it confidently to Mr. Hirst and you.
We can afterwards compare a number of stakes on the eastern side
with the same number of stakes taken at the same distances from the
western side. For example, a pair of stakes, one ten yards from the
eastern side and the other ten yards from the western side; another
pair, one fifty yards from the eastern side and the other fifty yards
from the western side, and so on, can be compared together. For the
sake of easy reference, let us call the points thus compared in
pairs, _equivalent points_.

181. There were five pairs of such points upon our fourth line, D D',
and here are their velocities:

  Eastern points; motion in inches    13 15 16 18 20
  Western   "       "     "    "      15 17 22 23 23

In every case here the stake at the western side moved more rapidly
than the equivalent stake at the eastern side.

182. Applying the same analysis to our fifth line, E E', we have the
following series of velocities of three pairs of equivalent points:--

  Eastern points; motion in inches    15 18 19
  Western   "       "     "    "      13 15 17

183. Here the three points on the eastern side move more rapidly than
the equivalent points on the western side.

184. It is thus proved:--

1. _That opposite the Montanvert the eastern half of the Mer de Glace
moves more rapidly than the western half._

2. _That opposite_ les Fonts _the western half of the glacier moves
more rapidly than the eastern half._

3. _That opposite Trélaporte the eastern half of the glacier again
moves more rapidly than the western half._

4. _That these changes in the place of greatest motion are determined
by the flexures of the valley through which the Mer de Glace moves._


§ 25. _New Law of Glacier Motion._

185. Let us express these facts in another way. Supposing the points
of swiftest motion for a very great number of lines crossing the
Mer de Glace to be determined; the line joining all those points
together is what mathematicians would call the _locus_ of the point
of swiftest motion.

186. At Trélaporte this line would lie east of the centre; at the
_Ponts_ it would lie west of the centre; hence in passing from
Trélaporte to the _Ponts_ it would cross the centre. But at the
Montanvert it would again lie east of the centre; hence between the
_Ponts_ and the Montanvert the centre must be crossed a second time.
If there were further sinuosities upon the Mer de Glace there would
be further crossings of the axis of the glacier.

187. The points on the axis which mark the transition from eastern to
western bending, and the reverse, may be called _points of contrary
flexure_.

188. Now what is true of the Mer de Glace is true of all other
glaciers moving through sinuous valleys; so that the facts
established in the Mer de Glace may be expanded into the following
general law of glacier motion:--

_When a glacier moves through a sinuous valley, the locus of the
points of maximum motion does not coincide with the centre of the
glacier, but, on the contrary, always lies on the convex side of
the central line. The locus is therefore a curved line more deeply
sinuous than the valley itself, and crosses the axis of the glacier
at each point of contrary flexure._

189. The dotted line on the Outline Plan (page 68) represents the
locus of the point of maximum motion, the firm line marking the
centre of the glacier.

190. Substituting the word _river_ for _glacier_, this law is
also true. The motion of the water is ruled by precisely the same
conditions as the motion of the ice.

191. Let us now apply our law to the explanation of a difficulty.
Turning to the careful measurements executed by M. Agassiz on
the glacier of the Unteraar, we notice in the discussion of these
measurements a section of the "Système glaciaire" devoted to the
"Migrations of the Centre." It is here shown that the middle of the
Unteraar glacier is not always the point of swiftest motion. This
fact has hitherto remained without explanation; but a glance at the
Unteraar valley, or at the map of the valley, shows the enigma to be
an illustration of the law which we have just established on the Mer
de Glace.


§ 26. _Motion of Axis of Mer de Glace._

192. We have now measured the rate of motion of five different lines
across the trunk of the Mer de Glace. Do they all move alike? No.
Like a river, a glacier at different places moves at different rates.
Comparing together the points of maximum motion of all five lines, we
have this result:

MOTION OF MER DE GLACE.

  At Trélaporte          20 inches a day.
  At _les Ponts_         23    "      "
  Above the Montanvert   26    "      "
  At the Montanvert      34    "      "
  Below the Montanvert   33[C] "      "

[C] This is probably under the mark. I think it likely that the
swiftest motion of this portion of the Mer de Glace in 1857 amounted
to a yard in twenty-four hours.

193. There is thus an increase of rapidity as we descend the glacier
from Trélaporte to the Montanvert; the maximum, motion at the
Montanvert being fourteen inches a day greater than at Trélaporte.


§ 27. _Motion of Tributary Glaciers._

194. So much for the trunk glacier; let us now investigate the
branches, permitting, as we have hitherto done, reflection on known
facts to precede our attempts to discover unknown ones.

195. As we stood upon our "cleft station," whence we had so capital
a view of the Mer de Glace, we were struck by the fact that some of
the tributaries of the glacier were wider than the glacier itself.
Supposing water to be substituted for the ice, how do you suppose
it would behave? You would doubtless conclude that the motion down
the broad and slightly-inclined valleys of the Géant and the Léchaud
would be comparatively slow, but that the water would force itself
with increased rapidity through the "narrows" of Trélaporte. Let us
test this notion as applied to the ice.

196. Planting our theodolite in the shadow of Mont Tacul, and
choosing a suitable point at the opposite side of the Glacier du
Géant, we fix on July 29 a series of ten stakes across the glacier.
The motion of this line in twenty-four hours was as follows:--

MOTION OF GLACIER DU GÉANT.

Sixth Line: H H' upon Sketch.

  Stake     1  2  3  4  5  6  7  8 9 10
  Inches   11 10 12 13 12 13 11 10 9  5

197. Our conjecture is fully verified. The maximum motion here is
seven inches a day less than that of the Mer de Glace at Trélaporte
(192).

198. And now for the Léchaud branch. On August 1 we fix ten stakes
across this glacier above the point where it is joined by the
Talèfre. Measured on August 3, and reduced to twenty-four hours, the
motion was found to be--

MOTION OF GLACIER DE LÉCHAUD.

Seventh Line: K K' upon Sketch.

  Stake  1 2  3 4 5 6 7 8 9 10
  Inches 5 8 10 9 9 8 6 9 7  6

199. Here our conjecture is still further verified, the rate of
motion being even less than that of the Glacier du Géant.


§ 28. _Motion of Top and Bottom of Glacier._

200. We have here the most ample and varied evidence that the sides
of a glacier, like those of a river, are retarded by friction against
its boundaries. But the likeness does not end here. The motion of a
river is retarded by the friction against its bed. Two observers,
viz. Prof. Forbes and M. Charles Martins, concur in showing the same
to be the case with a glacier. The observations of both have been
objected to; hence it is all the more incumbent on us to seek for
decisive evidence.

201. At the Tacul (near the point _a_ upon the sketch plan, p. 83) a
wall of ice about 150 feet high has already attracted our attention.
Bending round to join the Léchaud the Glacier du Géant is here drawn
away from the mountain side, and exposes a fine section. We try to
measure it top, bottom, and middle, and are defeated twice over. We
try it a third time and succeed. A stake is fixed at the summit of
the ice-precipice, another at 4 feet from the bottom, and a third at
35 feet above the bottom. These lower stakes are fixed at some risk
of boulders falling upon us from above; but by skill and caution
we succeed in measuring the motions of all three. For 24 hours the
motions are:--

  Top stake      6  inches.
  Middle stake   4½   "
  Bottom stake   2⅔   "

202. The retarding influence of the bed of the glacier is reduced to
demonstration by these measurements. The bottom does not move with
half the velocity of the surface.


§ 29. _Lateral Compression of a Glacier._

203. Furnished with the knowledge which these labours and
measurements have given us, let us once more climb to our station
beside the Cleft under the Aiguille de Charmoz. At our first visit
we saw the medial moraines of the glacier, but we knew nothing about
their cause. We now know that they mark upon the trunk its tributary
glaciers. Cast your eye, then, first upon the Glacier du Géant;
realise its width in its own valley, and see how much it is narrowed
at Trélaporte. The broad ice-stream of the Léchaud is still more
surprising, being squeezed upon the Mer de Glace to a narrow white
band between its bounding moraines. The Talèfre undergoes similar
compression. Let us now descend, shake out our chain, measure, and
express in numbers the width of the tributaries, and the actual
amount of compression suffered at Trélaporte.

204. We find the width of the Glacier du Géant to be 5,155 links, or
1,134 yards.

205. The width of the Glacier de Léchaud we find to be 3,725 links,
or 825 yards.

206. The width of the Talèfre we find to be 2,900 links, or 638 yards.

207. The sum of the widths of the three branch glaciers is therefore
2,597 yards.

208. At Trélaporte these three branches are forced through a gorge
893 yards wide, or one-third of their previous width, at the rate of
twenty inches a day.

209. If we limit our view to the Glacier de Léchaud, the facts are
still more astonishing. Previous to its junction with the Talèfre,
this glacier has a width of 825 yards; in passing through the jaws of
the granite vice at Trélaporte, its width is reduced to eighty-eight
yards, or in round numbers to one-tenth of its previous width. (Look
to the sketch on the next page.)

[Illustration: SKETCH-PLAN SHOWING THE MORAINES, _a_, _b_, _c_, _d_,
_e_, OF THE MER DE GLACE.]

210. Are we to understand by this that the ice of the Léchaud is
squeezed to one-tenth of its former _volume_? By no means. It is
mainly a change of _form_, not of volume, that occurs at Trélaporte.
Previous to its compression, the glacier resembles a plate of ice
_lying flat_ upon its bed. After its compression, it resembles a
plate _fixed upon its edge_. The squeezing, doubtless, has deepened
the ice.


§ 30. _Longitudinal Compression of a Glacier._

211. The ice is forced through the gorge at Trélaporte by a pressure
from behind; in fact the Glacier du Géant, immediately above
Trélaporte, represents a piston or a plug which drives the ice
through the gorge. What effect must this pressure have upon the plug
itself? Reasoning alone renders it probable that the pressure will
shorten the plug; that the lower part of the Glacier du Géant will to
some extent yield to the pressure from behind.

212. Let us test this notion. About three-quarters of a mile above
the Tacul, and on the mountain slope to the left as we ascend, we
observe a patch of verdure. Thither we climb; there we plant our
theodolite, and set out across the Glacier du Géant, a line, which we
will call line No. 1 (F F' upon sketch, p. 68).

213. About a quarter of a mile lower down we find a practicable
couloir on the mountain side; we ascend it, reach a suitable
platform, plant our instrument, and set out a second line, No. 2 (G
G' upon sketch). We must hasten our work here, for along this couloir
stones are discharged from a small glacier which rests upon the slope
of Mont Tacul.

214. Still lower down by another quarter of a mile, which brings us
near the Tacul, we set out a third line, No. 3 (H H' upon sketch),
across the glacier.

215. The daily motion of the centres of these three lines is as
follows:--

             Inches     Distances asunder
  No. 1       50·55 }
                    }      545 yards.
  No. 2       15·43 }
                    }      487   "
  No. 3       12·75 }

216. The first line here moves five inches a day more than the
second; and the second nearly three inches a day more than the third.
The reasoning is therefore confirmed. The ice-plug, which is in round
numbers one thousand yards long, is shortened by the pressure exerted
on its front at the rate of about eight inches a day.

217. A river descending the Valley du Géant would behave in
substantially the same fashion. It would have its motion on
approaching Trélaporte diminished, and it would pour through the
defile with a velocity greater than that of the water behind.


§ 31. _Sliding and Flowing. Hard Ice and Soft Ice._

218. We have thus far confined ourselves to the measurement and
discussion of glacier motion; but in our excursions we have noticed
many things besides. Here and there, where the ice has retreated
from the mountain side, we have seen the rocks fluted, scored, and
polished; thus proving that the ice had slidden over them and ground
them down. At the source of the Arveiron we noticed the water rushing
from beneath the glacier charged with fine matter. All glacier rivers
are similarly charged. The Rhone carries its load of matter into the
Lake of Geneva; the rush of the river is here arrested, the matter
subsides, and the Rhone quits the lake clear and blue. The Lake of
Geneva, and many other Swiss lakes, are in part filled up with this
matter, and will, in all probability, finally be obliterated by it.

219. One portion of the motion of a glacier is due to this bodily
sliding of the mass over its bed.

220. We have seen in our journeys over the glacier streams formed by
the melting of the ice, and escaping through cracks and _crevasses_
to the bed of the glacier. The fine matter ground down is thus washed
away; the bed is kept lubricated, and the sliding of the ice rendered
more easy than it would otherwise be.

221. As a skater also you know how much ice is weakened by a thaw.
Before it actually melts it becomes rotten and unsafe. Test such ice
with your penknife: you can dig the blade readily into it, or cut the
ice with ease. Try good sound ice in the same way: you find it much
more resistant. The one, indeed, resembles soft chalk; the other hard
stone.

222. Now the Mer de Glace in summer is in this thawing condition. Its
ice is rendered soft and yielding by the sun; its motion is thereby
facilitated. We have seen that not only does the glacier slide over
its bed, but that the upper layers slide over the under ones, and
that the centre slides past the sides. The softer and more yielding
the ice is, the more free will be this motion, and the more readily
also will it be forced through a defile like Trélaporte.

223. But in winter the thaw ceases; the quantity of water reaching
the bed of the glacier is diminished or entirely cut off. The
ice also, to a certain depth at least, is frozen hard. These
considerations would justify the opinion that in winter the glacier,
if it moves at all, must move more slowly than in summer. At all
events, the summer measurements give no clue to the winter motion.

224. This point merits examination. I will not, however, ask you to
visit the Alps in mid-winter; but, if you allow me, I will be your
deputy to the mountains, and report to you faithfully the aspect of
the region and the behaviour of the ice.


§ 32. _Winter on the Mer de Glace._

225. The winter chosen is an inclement one. There is snow in London,
snow in Paris, snow in Geneva; snow near Chamouni so deep that the
road fences are entirely effaced. On Christmas night--nearly at
mid-night--1859, your deputy reaches Chamouni.

226. The snow fell heavily on December 26; but on the 27th, during a
lull in the storm, we turn out. There are with me four good guides
and a porter. They tie planks to their feet to prevent them from
sinking in the snow; I neglect this precaution and sink often to the
waist. Four or five times during our ascent the slope cracks with an
explosive sound, and the snow threatens to come down in avalanches.[D]

[D] Four years later, viz. in the spring of 1863, a mighty climber
and noble guide and companion of mine, named Johann Joseph Bennen,
was lost, through the cracking and subsequent slipping of snow on
such a slope.

The freshly-fallen snow was in that particular condition which causes
its granules to adhere, and hence every flake falling on the trees
had been retained there. The laden pines presented beautiful and
often fantastic forms.

227. After five hours and a half of arduous work the Montanvert was
attained. We unlocked the forsaken auberge, round which the snow
was reared in buttresses. I have already spoken of the complex play
of crystallising forces. The frost figures on the window-panes of
the auberge were wonderful: mimic shrubs and ferns wrought by the
building power while hampered by the adhesion between the glass and
the film in which it worked. The appearance of the glacier was very
impressive; all sounds were stilled. The cascades which in summer
fill the air with their music were silent, hanging from the ledges
of the rocks in fluted columns of ice. The surface of the glacier
was obviously higher than it had been in summer; suggesting the
thought that while the winter cold maintained the lower end of the
glacier jammed between its boundaries, the upper portions still moved
downwards and thickened the ice. The peak of the Aiguille du Dru
shook out a cloud-banner, the origin and nature of which have been
already explained (84). (See _Frontispiece_.)

[Illustration: SNOW-LADEN PINE-TREE.]

228. On the morning of the 28th this banner was strikingly large and
grand, and reddened by the light of the rising sun, it glowed like a
flame. Roses of cloud also clustered round the crests of the Grande
Jorasse and hung upon the pinnacles of Charmoz. Four men, well roped
together, descended to the glacier. I had trained one of them in
1857, and he was now to fix the stakes. The storm had so distributed
the snow as to leave alternate lengths of the glacier bare and
thickly covered. Where much snow lay great caution was required, for
hidden crevasses were underneath. The men sounded with their staffs
at every step. Once while looking at the party through my telescope
the leader suddenly disappeared; the roof of a crevasse had given
way beneath him; but the other three men promptly gathered round and
lifted him out of the fissure. The true line was soon picked up by
the theodolite; one by one the stakes were fixed until a series of
eleven of them stood across the glacier.

229. To get higher up the valley was impracticable; the snow was
too deep, and the aspect of the weather too threatening; so the
theodolite was planted amid the pines a Little way below the
Montanvert, whence through a vista I could see across the glacier.
The men were wrapped at intervals by whirling snow-wreaths which
quite hid them, and we had to take advantage of the lulls in the
wind. Fitfully it came up the valley, darkening the air, catching the
snow upon the glacier, and tossing it throughout its entire length
into high and violently agitated clouds, separated from each other
by cloudless spaces corresponding to the naked portions of the ice.
In the midst of this turmoil the men continued to work. Bravely and
steadfastly stake after stake was set, until at length a series of
ten of them was fixed across the glacier.

230. Many of the stakes were fixed in the snow. They were four feet
in length, and were driven in to a depth of about three feet. But
that night, while listening to the wild onset of the storm, I thought
it possible that the stakes and the snow which held them might be
carried bodily away before the morning. The wind, however, lulled.
We rose with the dawn, but the air was thick with descending snow.
It was all composed of those exquisite six-petaled flowers, or
six-rayed stars, which have been already figured and described (§
9). The weather brightening, the theodolite was planted at the end
of the first line. The men descended, and, trained by their previous
experience, rapidly executed the measurements. The first line was
completed before 11 A. M. Again the snow began to fall, filling
all the air. Spangles innumerable were showered upon the heights.
Contrary to expectation, the men could be seen and directed through
the shower.

231. To reach the position occupied by the theodolite at the end of
our second line, I had to wade breast-deep through snow which seemed
as dry and soft as flour. The toil of the men upon the glacier in
breaking through the snow was prodigious. But they did not flinch,
and after a time the leader stood behind the farthest stake, and
cried, _Nous avons fini_. I was surprised to hear him so distinctly,
for falling snow had been thought very deadening to sound. The work
was finished, and I struck my theodolite with a feeling of a general
who had won a small battle.

232. We put the house in order, packed up, and shot by glissade down
the steep slopes of _La Filia_ to the vault of the Arveiron. We found
the river feeble, but not dried up. Many weeks must have elapsed
since any water had been sent down from the surface of the glacier.
But at the setting in of winter the fissures were in a great measure
charged with water; and the Arveiron of to-day was probably due to
the gradual _drainage_ of the glacier. There was now no danger of
entering the vault, for the ice seemed as firm as marble. In the
cavern we were bathed by blue light. The strange beauty of the place
suggested magic, and put me in mind of stories about fairy caves
which I had read when a boy. At the source of the Arveiron our winter
visit to the Mer de Glace ends; next morning your deputy was on his
way to London.


§ 33. _Winter Motion of the Mer de Glace._

233. Here are the measurements executed in the winter of 1859:--

Line No. I.

  Stake  1  2  3  4  5  6  7  8  9 10 11
  Inches 7 11 14 13 14 14 16 16 12 12  7

Line No. II.

  Stake  1  2  3  4  5  6  7  8  9 10
  Inches 8 10 14 16 16 16 18 17 15 14

234. Thus the winter motion of the Mer de Glace near the Montanvert
is, in round numbers, half the summer motion.

235. As in summer, the eastern side of the glacier at this place
moved quicker than the western.


§ 34. Motion of the Grindelwald and Aletsch Glaciers.

236. As regards the question of motion, to no other glacier have we
devoted ourselves with such thoroughness as to the Mer de Glace; we
are, however, able to add a few measurements of other celebrated
glaciers. Rear the village of Grindelwald in the Bernese Oberland,
there are two great ice-streams called respectively the Upper and the
Lower Grindelwald glaciers, the second of which is frequently visited
by travellers in the Alps. Across it on August 6, 1860, a series of
twelve stakes was fixed by Mr. Vaughan Hawkins and myself. Measured
on the 8th and reduced to its daily rate, the motion of these stakes
was as follows:--

MOTION OF LOWER GRINDELWALD GLACIER.

  Stake   1  2  3  4  5  6  7  8  9 10 11 12
  Inches 18 19 20 21 21 21 22 20 19 18 17 14

237. The theodolite was here planted a little below the footway
leading to the higher glacier region, and at about a mile above
the end of the glacier. The measurement was rendered difficult by
crevasses.

238. The largest glacier in Switzerland is the Great Aletsch, to
which further reference shall subsequently be made. Across it on
August 14, 1860, a series of thirty-four stakes was planted by Mr.
Hawkins and me. Measured on the 16th and reduced to their daily rate,
the velocities were found to be as follows:--

MOTION OF GREAT ALETSCH GLACIER.

          East
  Stake    1 2 3 4  5  6  7  8  9 10 11 12
  Inches   2 3 4 6  8 11 13 14 16 17 17 19
  Stake   13 14 15 16 17 18 19 20 21 22 23
  Inches  19 18 18 17 19 19 19 19 17 17 15
  Stake   24 25 26 27 28 29 30 31 32 33 34
  Inches  16 17 17 17 17 17 17 17 16 12 12
                                      West

239. The maximum motion here is nineteen inches a day. Probably the
eastern side of the glacier is shallow, the retardation of the bed
making the motion of the eastern stakes inconsiderable. The width of
the glacier here is 9,030 links, or about a mile and a furlong. The
theodolite was planted high among the rocks on the western flank of
the mountain, about half a mile above the Märgelin See.


§ 35. _Motion of Morteratsch Glacier._

240. Far to the east of the Oberland and in that interesting part
of Switzerland known as the Ober Engadin, stands a noble group of
mountains, less in height than those of the Oberland, but still
of commanding elevation. The group derives its name from its most
dominant peak, the Piz Bernina. To reach the place we travel by
railway from Basel to Zürich, and from Zürich to Chur (French Coire),
whence we pass by diligence over either the Albula pass or the Julier
pass to the village of Pontresina. Here we are in the immediate
neighbourhood of the Bernina mountains.

241. From Pontresina we may walk or drive along a good coach road
over the Bernina pass into Italy. At about an hour above the village
you would look from the road into the heart of the mountains, the
line of vision passing through a valley, in which is couched a
glacier of considerable size. Along its back you would trace a medial
moraine, and you could hardly fail to notice how the moraine, from a
mere narrow streak at first, widens gradually as it descends, until
finally it quite covers the lower end of the glacier. Nor is this an
effect of perspective; for were you to stand upon the mountain slopes
which nourish the glacier, you would see thence also the widening
of the streak of rubbish, though the perspective here would tend to
narrow the moraine as it retreats downwards.

242. The ice-stream here referred to is the Morteratsch glacier, the
end of which is a short hour's walk from the village of Pontresina.
We have now to determine its rate of motion and to account for the
widening of its medial moraine.

243. In the summer of 1864 Mr. Hirst and myself set out three lines
of stakes across the glacier. The first line crossed the ice high up;
the second a good distance lower down, and the third lower still.
Even the third line, however, was at a considerable distance above
the actual snout of the glacier. The daily motion of these three
lines was as follows:--

First Line.

  Stake      1  2  3  4  5  6  7  8  9 10 11
  Inches     8 12 13 13 14 13 12 12 10  7  5

Second Line.

  Stake      1 2 3 4  5  6  7  8  9 10 11
  Inches     1 4 6 8 10 11 11 11 11 11 11

Third Line.

  Stake      1 2 3 4 5 6 7 8 9 10 11
  Inches     1 2 4 5 6 6 7 7 5  5  4

244. Compare these lines together. You notice the velocity of the
first is greater than that of the second, and the velocity of the
second greater than that of the third.

245. The lines were permitted to move down wards for 100 hours,
at the end of which time the spaces passed over by the points of
swiftest motion of the three lines were as follows:

Maximum Motion in 100 Hours.

  First line  56 inches.
  Second line 45   "
  Third line  30   "

246. Here then is a demonstration that the upper portions of the
Morteratsch glacier are advancing on the lower ones. _In 1871 the
motion of a point on the middle of the glacier near its snout was
found to be less than two inches a day!_

247. What, then, is the consequence of this swifter march of
the upper glacier? Obviously to squeeze this medial moraine
longitudinally, and to cause it to spread out laterally. We have here
distinctly revealed the cause of the widening of the medial moraine.

248. It has been a question much discussed, whether a glacier is
competent to scoop out or deepen a valley through which it moves, and
this very Morteratsch glacier has been cited to prove that such is
not the case. Observers went to the snout of the glacier, and finding
it sensibly quiescent, they concluded that no scooping occurred. But
those who contended for the power of glaciers to excavate valleys
never stated, or meant to state, that it was the snout of the
glacier which did the work. In the Morteratsch glacier the work of
excavation, which certainly goes on to a greater or less extent,
must be far more effectual high up the valley than at the end of the
glacier.


§ 36. _Birth of a Crevasse: Reflections._

249. Preserving the notion that we are working together, we will
now enter upon a new field of enquiry. We have wrapped up our
chain, and are turning homewards after a hard day's work upon the
Glacier du Géant, when under our feet, as if coming from the body
of the glacier, an explosion is heard. Somewhat startled, we look
enquiringly over the ice. The sound is repeated, several shots
being fired in quick succession. They seem sometimes to our right,
sometimes to our left, giving the impression that the glacier is
breaking all round us. Still nothing is to be seen.

250. We closely scan the ice, and after an hour's strict search
we discover the cause of the reports. They announce the birth of
a crevasse. Through a pool upon the glacier we notice air bubbles
ascending, and find the bottom of the pool crossed by a narrow crack,
from which the bubbles issue. Eight and left from this pool we trace
the young fissure through long distances. It is sometimes almost
too feeble to be seen, and at no place is it wide enough to admit a
knife-blade.

251. It is difficult to believe that the formidable fissures among
which you and I have so often trodden with awe, could commence in
this small way. Such, however, is the case. The great and gaping
chasms on and above the ice-falls of the Géant and the Talèfre
begin as narrow cracks, which open gradually to crevasses. We are
thus taught in an instructive and impressive way that appearances
suggestive of very violent action may really be produced by processes
so slow as to require refined observations to detect them. In the
production of natural phenomena two things always come into play, the
_intensity_ of the acting force, and the _time_ during which it acts.
Make the intensity great, and the time small, and you have sudden
convulsion; but precisely the same apparent effect may be produced
by making the intensity small, and the time great. This truth is
strikingly illustrated by the Alpine ice-falls and crevasses;
and many geological phenomena, which at first sight suggest
violent convulsion, may be really produced in the selfsame almost
imperceptible way.


§ 37. _Icicles._

252. The crevasses are grandest on the higher névés, where they
sometimes appear as long yawning fissures, and sometimes as chasms
of irregular outline. A delicate blue light shimmers from them, but
this is gradually lost in the darkness of their profounder portions.
Over the edges of the chasms, and mostly over the southern edges,
hangs a coping of snow, and from this depend like stalactites rows
of transparent icicles, 10, 20, 30 feet long. These pendent spears
constitute one of the most beautiful features of the higher crevasses.

253. How are they produced? Evidently by the thawing of the snow. But
why, when once thawed, should the water freeze again to solid spears?
You have seen icicles pendent from a house-eave, which have been
manifestly produced by the thawing of the snow upon the roof. If we
understand these, we shall also understand the vaster stalactites of
the Alpine crevasses.

254. Gathering up such knowledge as we possess, and reflecting upon
it patiently, let us found upon it, if we can, a theory of icicles.

255. First, then, you are to know that the _air_ of our atmosphere
is hardly heated at all by the rays of the sun, whether visible or
invisible. The air is highly transparent to all kinds of rays, and
it is only the scanty fraction to which it is _not_ transparent that
expend their force in warming it.

256. Not so, however, with the snow on which the sunbeams fall. It
absorbs the solar heat, and on a sunny day you may see the summits of
the high Alps glistening with the water of liquefaction. The _air_
above and around the mountains may at the same time be many degrees
below the freezing point in temperature.

257. You have only to pass from sunshine into shade to prove this. A
single step suffices to carry you from a place where the thermometer
stands high to one where it stands low; the change being due, not
to any difference in the temperature of the _air_, but simply to the
withdrawal of the thermometer from the direct action of the solar
rays. Nay, without shifting the thermometer at all, by interposing a
suitable screen, which cuts off the sun's rays, the coldness of the
air may be demonstrated.

258. Look now to the snow upon your house roof. The sun plays upon
it, and melts it; the water trickles to the eave and then drops down.
If the eave face the sun the water remains water; but if the eave
do not face the sun, the drop, before it quits its parent snow, _is
already in shadow_. Now the shaded space, as we have learnt, may be
below the freezing temperature. If so the drop, instead of falling,
congeals, and the rudiment of an icicle is formed. Other drops and
driblets succeed, which trickle over the rudiment, congeal upon it in
part and _thicken_ it at the root. But a portion of the water reaches
the free end of the icicle, hangs from it, and is there congealed
before it escapes. The icicle is thus _lengthened_. In the Alps,
where the liquefaction is copious and the cold of the shaded crevasse
intense, the icicles, though produced in the same way, naturally grow
to a greater size. The drainage of the snow after the sun's power is
withdrawn also produces icicles.

259. It is interesting and important that you should be able to
explain the formation of an icicle; but it is far more important
that you should realise the way in which the various threads of what
we call Nature are woven together. You cannot fully understand an
icicle without first knowing that solar beams powerful enough to
fuse the snows and blister the human skin, nay, it might be added,
powerful enough, when concentrated, to burn up the human body itself,
may pass through the air, and still leave it at an icy temperature.


§ 38. _The Bergschrund._

260. Having cleared away this difficulty, let us turn once more to
the crevasses, taking them in the order of their formation. First
then above the névé we have the final Alpine peaks and crests,
against which the snow is often reared as a steep buttress. We have
already learned that both névés and glaciers are moving slowly
downwards; but it usually happens that the attachment of the highest
portion of the buttress to the rocks is great enough to enable it to
hold on while the lower portion breaks away. A very characteristic
crevasse is thus formed, called in the German-speaking portion of the
Alps a _Bergschrund_. It often surrounds a peak like a fosse, as if
to defend it against the assaults of climbers.

261. Look more closely into its formation. Imagine the snow as yet
unbroken. Its higher portions cling to the rocks, and move downwards
with extreme slowness. But its lower portions, whether from their
greater depth and weight, or their less perfect attachment, are
compelled to move more quickly. _A pull_ is therefore exerted,
tending to separate the lower from the upper snow. For a time this
pull is resisted by the cohesion of the névé; but this at length
gives way, and a crack is formed exactly across the line in which
the pull is exerted. In other words, _a crevasse is formed at right
angles to the line of tension_.


§ 39. _Transverse Crevasses._

262. Both on the névé and on the glacier the origin of the crevasses
is the same. Through some cause or other the ice is thrown into a
state of strain, and as it cannot _stretch_ it _breaks_ across the
line of tension. Take for example, the ice-fall of the Géant, or
of the Talèfre, above which you know the crevasses yawn terribly.
Imagine the névé and the glacier entirely peeled away, so as to
expose the surface over which they move. From the Col du Géant we
should see this surface falling gently to the place now occupied by
the brow of the cascade. Here the surface would fall steeply down to
the bed of the present Glacier du Géant, where the slope would become
gentle once more.

263. Think of the névé moving over such a surface. It descends from
the Col till it reaches the brow just referred to. It crosses the
brow, and must bend down to keep upon its bed. Realise clearly what
must occur. The surface of the névé is evidently thrown into a
state of strain; it breaks and forms a crevasse. Each fresh portion
of the névé as it passes the brow is similarly broken, and thus a
succession of crevasses is sent down the fall. Between every two
chasms is a great transverse ridge. Through local strains upon the
fall those ridges are also frequently broken across, towers of
ice--_séracs_--being the result. Down the fall both ridges and séracs
are borne, the dislocation being augmented during the descent.

264. What must occur at the foot of the fall? Here the slope suddenly
lessens in steepness. It is plain that the crevasses must not only
cease to open here, but that they must in whole or in part close
up. At the summit of the fall, the bending was such as to make the
surface convex; at the bottom of the fall the bending renders the
surface concave. In the one case we have _strain_, in the other
_pressure_. In the one case, therefore, we have the _opening_, and
in the other the _closing_ of crevasses. This reasoning corresponds
exactly with the facts of observation.

265. Lay bare your arm and stretch it straight. Make two ink dots
half an inch or an inch apart, exactly opposite the elbow. Bend your
arm, the dots approach each other, and are finally brought together.
Let the two dots represent the two sides of a crevasse at the bottom
of an ice-fall; the bending of the arm resembles the bending of the
ice, and the closing up of the dots resembles the closing of the
fissures.

266. The same remarks apply to various portions of the Mer de Glace.
At certain places the inclination changes from a gentler to a steeper
slope, and on crossing the brow between both the glacier breaks its
back. _Transverse crevasses_ are thus formed. There is such a change
of inclination opposite to the Angle, and a still greater but similar
change at the head of the Glacier des Bois. The consequence is that
the Mer de Glace at the former point is impassable, and at the latter
the rending and dislocation are such as we have seen and described.
Below the Angle, and at the bottom of the Glacier des Bois, the
steepness relaxes, the crevasses heal up, and the glacier becomes
once more continuous and compact.


§ 40. _Marginal Crevasses._

267. Supposing, then, that we had no changes of inclination, should
we have no crevasses? We should certainly have less of them, but they
would not wholly disappear. For other circumstances exist to throw
the ice into a state of strain, and to determine its fracture. The
principal of these is the more rapid movement of the centre of the
glacier.

268. Helped by the labours of an eminent man, now dead, the late Mr.
Wm. Hopkins, of Cambridge, let us master the explanation of this
point together. But the pleasure of mastering it would be enhanced
if we could see beforehand the perplexing and delusive appearances
accounted for by the explanation. Could my wishes be followed out, I
would at this point of our researches carry you off with me to Basel,
thence to Thun, thence to Interlaken, thence to Grindelwald, where
you would find yourself in the actual presence of the Wetterhorn and
the Eiger, with all the greatest peaks of the Bernese Oberland, the
Finsteraarhorn, the Schreckhorn, the Monch, the Jungfrau, at hand.
At Grindelwald, as we have already learnt, there are two well-known
glaciers--the Ober Grindelwald and the Unter Grindelwald glaciers--on
the latter of which our observations should commence.

269. Dropping down from the village to the bottom of the valley, we
should breast the opposite mountain, and with the great limestone
precipices of the Wetterhorn to our left, we should get upon a path
which commands a view of the glacier. Here we should see beautiful
examples of the opening of crevasses at the summit of a brow, and
their closing at the bottom. But the chief point of interest would
be the crevasses formed at the _side_ of this glacier--the _marginal
crevasses_, as they may be called.

270. We should find the side copiously fissured, even at those places
where the centre is compact; and we should particularly notice that
the fissures would neither run in the direction of the glacier, nor
straight across it, but that they would be _oblique_ to it, enclosing
an angle of about 45 degrees with the sides. Starting from the side
of the glacier the crevasses would be seen to point _upwards_; that
is to say, the ends of the fissures abutting against the bounding
mountain would appear to be _dragged down_. Were you less instructed
than you now are, I might lay a wager that the aspect of these
fissures would cause you to conclude that the centre of the glacier
is left behind by the quicker motion of the sides.

271. This indeed was the conclusion drawn by M. Agassiz from this
very appearance, before he had measured the motion of the sides and
centre of the glacier of the Unteraar. Intimately versed with the
treatment of mechanical problems, Mr. Hopkins immediately deduced
the obliquity of the lateral crevasses from the quicker flow of the
centre. Standing beside the glacier with pencil and note-book in
hand, I would at once make the matter clear to you thus.

[Illustration]

272. Let A C, in the annexed figure, be one side of the glacier, and
B D the other; and let the direction of motion be that indicated
by the arrow. Let S T be a transverse slice of the glacier, taken
straight across it, say to-day. A few days or weeks hence this
slice will have been carried down, and because the centre moves more
quickly than the sides it will not remain straight, but will bend
into the form S' T'.

273. Supposing T _i_ to be a small square of the original slice near
the side of the glacier. In its new position the square will be
distorted to the lozenge-shaped figure T' _i'_. Fix your attention
upon the diagonal T _i_ of the square; in the lowest position this
diagonal, _if the ice could stretch_, would be lengthened to T' _i'_.
But the ice does not stretch; it breaks, and we have a crevasse
formed at right angles to T' _i'_. The mere inspection of the diagram
will assure you that the crevasse will point obliquely _upwards_.

274. Along the whole side of the glacier the quicker movement of the
centre produces a similar state of strain; and the consequence is
that the sides are copiously cut by those oblique crevasses, even at
places where the centre is free from them.

275. It is curious to see at other places the transverse fissures
of the centre uniting with those at the sides, so as to form great
curved crevasses which stretch across the glacier from side to
side. The convexity of the curve is turned _upwards_, as mechanical
principles declare it ought to be. (See sketch on opposite page.)
But if you were ignorant of those principles, you would never infer
from the aspect of these curves the quicker motion of the centre. In
landslips, and in the motion of partially indurated mud, you may
sometimes notice appearances similar to those exhibited by the ice.

[Illustration: SKETCH OF CURVED CREVASSES: THE GLACIER MOVES FROM
LEFT TO RIGHT.]


§ 41. _Longitudinal Crevasses._

276. We have thus unravelled the origin of both transverse and
marginal crevasses. But where a glacier issues from a steep and
narrow defile upon a comparatively level plain which allows it room
to expand laterally, its motion is in part arrested, and the level
portion has to bear the thrust of the steeper portions behind. Here
the line of thrust is in the direction of the glacier, while the
direction at right angles to this is one of tension. Across this
latter the glacier breaks, and _longitudinal crevasses_ are formed.

277. Examples of this kind of crevasse are furnished by the lower
part of the Glacier of the Rhone, when looked down upon from the
Grimsel Pass, or from any commanding point on the flanking mountains.


§ 42. _Crevasses in relation to Curvature of Glacier._

278. One point in addition remains to be discussed, and your present
knowledge will enable you to master it in a moment. You remember at
an early period of OUT researches that we crossed the Mer de Glace
from the Chapeau side to the Montanvert side. I then desired you to
notice that the Chapeau side of the glacier was more fissured than
either the centre or the Montanvert side (75). Why should this be so?
Knowing as we now do that the Chapeau side of the glacier moves more
quickly than the other; that the point of maximum motion does not
lie on the centre but far east of it, we are prepared to answer this
question in a perfectly satisfactory manner.

279. Let A B and C D, in the diagram opposite, represent the two
curved sides of the Mer de Glace at the Montanvert, and let _m n_ be
a straight line across the glacier. Let _o_ be the point of maximum
motion. The mechanical state of the two sides of the glacier may be
thus made plain. Supposing the line _m n_ to be a straight elastic
string with its ends fixed; let it be grasped firmly at the point
o by the finger and thumb, and drawn to _o'_, keeping the distance
between _o'_ and the side C D constant. Here the length, _n o_ of
the string would have stretched to _n o'_, and the length _m o_ to
_m o'_ and you see plainly that the stretching of the short line, in
comparison with its length, is greater than that of the long line in
comparison with its length. In other words, the strain upon _n o'_ is
greater than that upon _m o'_; so that if one of them were to break
under the strain, it would be the short one.

[Illustration: _Montanvert_]

280. These two lines represent the conditions of strain upon the
two sides of the glacier. The sides are held back, and the centre
tries to move on, a strain being thus set up between the centre and
sides. But the displacement of the point of maximum motion through
the curvature of the valley makes the strain upon the eastern ice
greater than that upon the western. The eastern side of the glacier
is therefore more crevassed than the western.

281. Here indeed resides the difficulty of getting along the eastern
side of the Mer de Glace: a difficulty which was one reason for our
crossing the glacier opposite to the Montanvert. There are two convex
sweeps on the eastern side to one on the western side, hence on the
whole the eastern side of the Mer de Glace is most riven.


§ 43. _Moraine-ridges, Glacier Tables, and Sand-Cones._

282. When you and I first crossed the Mer de Glace from Trélaporte
to the Couvercle, we found that the stripes of rocks and rubbish
which constituted the medial moraines were ridges raised above the
general level of the glacier to a height at some places of twenty or
thirty feet. On examining these ridges we found the rubbish to be
superficial, and that it rested upon a great spine of ice which ran
along the back of the glacier. By what means has this ridge of ice
been raised?

283. Most boys have read the story of Dr. Franklin's placing bits of
cloth of various colours upon snow on a sunny day. The bits of cloth
sank in the snow, the dark ones most.

284. Consider this experiment. The sun's rays first of all fall
upon the upper surface of the cloth and warm it. The heat is then
conducted through the cloth to the under surface, and the under
surface passes it on to the snow, which is finally liquefied by the
heat. It is quite manifest that the quantity of snow melted will
altogether depend upon the amount of heat sent from the upper to the
under surface of the cloth.

285. Now cloth is what is called a bad conductor. It does not permit
heat to travel freely through it. But where it has merely to pass
through the thickness of a single bit of cloth, a good quantity of
the heat gets through. But if you double or treble or quintuple the
thickness of the cloth; or, what is easier, if you put several pieces
one upon the other, you come at length to a point where no sensible
amount of heat could get through from the upper to the under surface.

286. What must occur if such a thick piece, or such a series of
pieces of cloth, were placed upon snow on which a strong sun is
falling? The snow round the cloth is melted, but that underneath the
cloth is protected. If the action continue long enough the inevitable
result will be, that the level of the snow all round the cloth will
sink, and the cloth will be left behind perched upon an eminence of
snow.

287. If you understand this, you have already mastered the cause of
the moraine-ridges. They are not produced by any swelling of the ice
upwards. But the ice underneath the rocks and rubbish being protected
from the sun, the glacier right and left melts away and leaves a
ridge behind.

288. Various other appearances upon the glacier are accounted for
in the same way. Here upon the Mer de Glace we have flat slabs of
rock sometimes lifted up on pillars of ice. These are the so-called
_Glacier Tables_. They are produced, not by the growth of a stalk of
ice out of the glacier, but by the melting of the glacier all round
the ice protected by the stone. Here is a sketch of one of the Tables
of the Mer de Glace.

[Illustration]

289. Notice moreover that a glacier table is hardly ever set square
upon its pillar. It generally leans to one side, and repeated
observation teaches you that it so leans as to enable you always to
draw the north and south line upon the glacier. For the sun being
south of the zenith at noon pours its rays against the southern end
of the table, while the northern end remains in shadow. The southern
end, therefore, being most warmed does not protect the ice underneath
it so effectually as the northern end. The table becomes inclined,
and ends by sliding bodily off its pedestal.

290. In the figure opposite we have what maybe called an ideal
Table. The oblique lines represent the direction of the sunbeams,
and the consequent tilting of the table here shown resembles that
observed upon the glaciers.

291. A pebble will not rise thus: like Franklin's single bit of
cloth, a dark-coloured pebble sinks in the ice. A spot of black mould
will not rest upon the surface, but will sink; and various parts of
the Glacier du Géant are honeycombed by the sinking of such spots of
dirt into the ice.

[Illustration]

292. But when the dirt is of a thickness sufficient to protect the
ice the case is different. Sand is often washed away by a stream from
the mountains, or from the moraines, and strewn over certain spaces
of the glacier. A most curious action follows: the sanded surface
rises, the part on which the sand lies thickest rising highest.
Little peaks and eminences jut forth, and when the distribution
of the sand is favourable, and the action sufficiently prolonged,
you have little mountains formed, sometimes singly, and sometimes
grouped so as to mimic the Alps themselves. The _Sand-Cones_ of the
Mer de Glace are not striking; but on the Görner, the Aletsch, the
Morteratsch, and other glaciers, they form singly and in groups,
reaching sometimes a height of ten or twenty feet.


§ 44. _The Glacier Mills or Moulins._

293. You and I have learned by long experience the character of the
Mer de Glace. We have marched over it daily, with a definite object
in view, but we have not closed our eyes to other objects. It is
from side glimpses of things which are not at the moment occupying
our attention that fresh subjects of enquiry arise in scientific
investigation.

294. Thus in marching over the ice near Trélaporte we were often
struck by a sound resembling low rumbling thunder. We subsequently
sought out the origin of this sound, and found it.

295. A large area of this portion of the glacier is unbroken.
Driblets of water have room to form rills; rills to unite and form
streams; streams to combine to form rushing brooks, which sometimes
cut deep channels in the ice. Sooner or later these streams reach
a strained portion of the glacier, where a crack is formed across
the stream. A way is thus opened for the water to the bottom of the
glacier. By long action the stream hollows out a shaft, the crack
thus becoming the starting-point of a funnel of unseen depth, into
which the water leaps with the sound of thunder.

296. This funnel and its cataract form a glacier Mill or _Moulin_.

297. Let me grasp your hand firmly while you stand upon the edge of
this shaft and look into it. The hole, with its pure blue shimmer,
is beautiful, but it is terrible. Incautious persons have fallen
into these shafts, a second or two of bewilderment being followed by
sudden death. But caution upon the glaciers and mountains ought, by
habit, to be made a second nature to explorers like you and me.

298. The crack into which the stream first descended to form the
moulin, moves down with the glacier. A succeeding portion of the ice
reaches the place where the breaking strain is exerted. A new crack
is then formed above the moulin, which is thenceforth forsaken by the
stream, and moves downward as an empty shaft. Here upon the Mer de
Glace, in advance of the _Grand Moulin_, we see no less than six of
these forsaken holes. Some of them we sound to a depth of 90 feet.

299. But you and I both wish to determine, if possible, the entire
depth of the Mer de Glace. The Grand Moulin offers a chance of doing
this which we must not neglect. Our first effort to sound the moulin
fails through the breaking of our cord by the impetuous plunge of the
water. A lump of grease in the hollow of a weight enables a mariner
to judge of a sea bottom. We employ such a weight, but cannot reach
the bed of the glacier. A depth of 163 feet is the utmost reached by
our plummet.

300. From July 28 to August 8 we have watched the progress of the
Grand Moulin. On the former date the position of the Moulin was
fixed. On the 31st it had moved down 50 inches; a little more than
a day afterwards it had moved 74 inches. On August 8 it had moved
198 inches, which gives an average of about 18 inches in twenty-four
hours. No doubt next summer upon the Mer de Glace a Grand Moulin will
be found thundering near Trélaporte; but like the crevasse of the
Grand Plateau, already referred to (§ 16), it will not be our Moulin.
This, or rather the ice which it penetrated, is now probably more
than a mile lower down than it was in 1857.


§ 45. _The Changes of Volume of Water by Heat and Cold._

301. We have noticed upon the glacier shafts and pits filled with
water of the most delicate blue. In some cases these have been the
shafts of extinct moulins closed at the bottom. A theory has been
advanced to account for them, which, though it may be untenable,
opens out considerations regarding the properties of water that
ought to be familiar to enquirers like you and me.

302. In our dissection of lake ice by a beam of heat (§ 11) we
noticed little vacuous spots at the centres of the liquid flowers
formed by the beam. These spots we referred to the fact that when ice
is melted the water produced is less in volume than the ice, and that
hence the water of the flower was not able to occupy the whole space
covered by the flower.

303. Let us more fully illustrate this subject. Stop a small flask
water-tight with a cork, and through the cork introduce a narrow
glass tube also water-tight. It is easy to fill the flask with water
so that the liquid shall stand at a certain height in the glass tube.

304. Let us now warm the flask with the flame of a spirit-lamp. On
first applying the flame you notice a momentary sinking of the liquid
in the glass tube. This is due to the momentary expansion of the
flask by heat; it becomes suddenly larger when the flame is first
applied.

305. But the expansion of the water soon overtakes that of the flask
and surpasses it. We immediately see the rise of the liquid column
in the glass tube, exactly as mercury rises in the tube of a warmed
thermometer.

306. Our glass tube is ten inches long, and at starting the water
stood in it at a height of five inches. We will apply the spirit-lamp
flame until the water rises quite to the top of the tube and trickles
over. This experiment suffices to show the expansion of the water by
heat.

307. We now take a common finger-glass and put into it a little
pounded ice and salt. On this we place the flask, and then build
round it the freezing mixture. The liquid column retreats down the
tube, proving the contraction of the liquid by cold. We allow the
shrinking to continue for some minutes, noticing that the downward
retreat of the liquid becomes gradually slower, and that it finally
ceases altogether.

308. Keep your eye upon the liquid column; it remains quiescent for
a fraction of a minute, and then moves once more. But its motion is
now _upwards_ instead of downwards. _The freezing mixture now acts
exactly like the flame._

309. It would not be difficult to pass a thermometer through the cork
into the flask, and it would tell us the exact temperature at which
the liquid ceased to contract and began to expand. At that moment we
should find the temperature of the liquid a shade over 39° Fahr.

310. At this temperature, then, water attains _its maximum density_.

311. Seven degrees below this temperature, or at 32° Fahr., the
liquid begins to turn into solid crystals of ice, which you know
swims upon water because it is bulkier for a given weight. In fact,
this halt of the approaching molecules at the temperature of 39°,
is but the preparation for the subsequent act of crystallisation,
in which the expansion by cold culminates. Up to the point of
solidification the increase of volume is slow and gradual; while in
the act of solidification it is sudden, and of overwhelming strength.

312. By this force of expansion the Florentine Academicians long
ago burst a sphere of copper nearly three quarters of an inch in
thickness. By the same force the celebrated astronomer Huyghens burst
in 1667 iron cannons a finger breadth thick. Such experiments have
been frequently made since. Major Williams during a severe Quebec
winter filled a mortar with water, and closed it by driving into
its muzzle a plug of wood. Exposed to a temperature 50° Fahr. below
the freezing point of water, the metal resisted the strain, but the
plug gave way, being projected to a distance of 400 feet. At Warsaw
howitzer shells have been thus exploded; and you and I have shivered
thick bombshells to fragments, by placing them for half an hour in a
freezing mixture.

313. The theory of the shafts and pits referred to at the beginning
of this section is this: The water at the surface of the shaft is
warmed by the sun, say to a temperature of 39° Fahr. The water at
the bottom, in contact with the ice, must be at 32° or near it. The
heavier water is therefore at the top; it will descend to the bottom,
melt the ice there, and thus deepen the shaft.

314. The circulation here referred to undoubtedly goes on, and
some curious effects are due to it; but not, I think, the one here
ascribed to it. The _deepening_ of a shaft implies a quicker melting
of its bottom than of the surface of the glacier. It is not easy to
see how the fact of the solar heat being first absorbed by water,
and then conveyed by it to the bottom of the shaft, should make the
melting of the bottom more rapid than that of the ice which receives
the direct impact of the solar rays. The surface of the glacier must
sink _at least_ as rapidly as the bottom of the pit, so that the
circulation, though actually existing, cannot produce the effect
ascribed to it.


§ 46. _Consequences flowing from the foregoing Properties of Water.
Correction of Errors._

315. I was not much above your age when the property of water ceasing
to contract by cold at a temperature of 39° Fahr. was made known to
me, and I still remember the impression it made upon me. For I was
asked to consider what would occur in case this solitary exception to
an otherwise universal law ceased to exist.

316. I was asked to reflect upon the condition of a lake stored with
fish and offering its surface to very cold air. It was made clear
to me that the water on being first chilled would shrink in volume
and become heavier, that it would therefore sink and have its place
supplied by the warmer and lighter water from the deeper portions of
the lake.

317. It was pointed out to me that without the law referred to
this process of circulation would go on until the whole water of
the lake had been lowered to the freezing temperature. Congelation
would then begin, and would continue as long as any water remained
to be solidified. One consequence of this would be to destroy every
living thing contained in the lake. Other calamities were added,
all of which were said to be prevented by the perfectly exceptional
arrangement, that after a certain time the _colder_ water becomes the
_lighter_, floats on the surface of the lake, is there congealed,
thus throwing a protecting roof over the life below.

318. Count Rumford, one of the most solid of scientific men, writes
in the following strain about this question:--"It does not appear to
me that there is anything which human sagacity can fathom, within the
wide-extended bounds of the visible creation, which affords a more
striking or more palpable proof of the wisdom of the Creator, and
of the special care He has taken in the general arrangement of the
universe, to preserve animal life, than this wonderful contrivance.

319. "Let me beg the attention of my readers while I endeavour to
investigate this most interesting subject; and let me at the same
time bespeak his candour and indulgence. I feel the danger to which a
mortal exposes himself who has the temerity to explain the designs
of Infinite Wisdom. The enterprise is adventurous, but it surely
cannot be improper.

320. "Had not Providence interfered on this occasion in a manner
which may well be considered as _miraculous_, all the fresh water
within the polar circle must inevitably have been frozen to a very
great depth in winter, and every plant and tree destroyed."

321. Through many pages of his book Count Rumford continues in this
strain to expound the ways and intentions of the Almighty, and he
does not hesitate to apply very harsh words to those who cannot
share his notions. He calls them hardened and degraded. We are here
warned of the fact, which is too often forgotten, that the pleasure
or comfort of a belief, or the warmth or exaltation of feeling which
it produces, is no guarantee of its truth. For the whole of Count
Rumford's delight and enthusiasm in connexion with this subject, and
the whole of his ire against those who did not share his opinions,
were founded upon an erroneous notion.

322. Water is _not_ a solitary exception to an otherwise general law.
There are other molecules than those of this liquid which require
more room in the solid crystalline condition than in the adjacent
molten condition. Iron is a case in point. Solid iron floats upon
molten iron exactly as ice floats upon water. Bismuth is a still
more impressive case, and we could shiver a bomb as certainly by the
solidification of bismuth as by that of water. There is no fish, to
be taken care of here, still the "contrivance" is the same.

323. I am reluctant to mention them in the same breath with Count
Rumford, but I am told that in our own day there are people who
profess to find the comforts of a religion in a superstition lower
than any that has hitherto degraded the civilized human mind. So that
the _happiness_ of a faith and the _truth_ of a faith are two totally
different things.

324. Life and the conditions of life are in necessary harmony. This
is a truism, for without the suitable conditions life could not
exist. But both life and its conditions set forth the operations of
inscrutable Power. We know not its origin; we know not its end. And
the presumption, if not the degradation, rests with those who place
upon the throne of the universe a magnified image of themselves, and
make its doings a mere colossal imitation of their own.


§ 47. _The Molecular Mechanism of Water-Congelation._

325. But let us return to our science. How are we to picture this act
of expansion on the part of freezing water? By what operation do the
molecules demand with such irresistible emphasis more room in the
solid than in the adjacent liquid condition? In all cases of this
kind we must derive our conceptions from the world of the senses, and
transfer them afterwards to a world transcending the range of the
senses.

326. You have not forgotten our conversation regarding "atomic
poles" (§ 10), and how the notion of polar force came to be applied
to crystals. With this fresh in your memory, you will have no great
difficulty in understanding how expansion of volume may accompany the
act of crystallisation.

327. I place a number of magnets before you. They, as matter, are
affected by gravity, and, if perfectly free, they would move towards
each other in obedience to the attraction of gravity.

328. But they are not only matter, but _magnetic_ matter. They not
only act upon each other by the simple force of gravity, but by the
polar force of magnetism. Imagine them placed at a distance from each
other, and perfectly free to move. Gravity first makes itself felt
and draws them together. For a time the magnetic force issuing from
the poles is insensible; but when a certain nearness is attained, the
polar force comes into play. The mutually attracting points close up,
the mutually repellent points retreat, and it is easy to see that
this action may produce an arrangement of the magnets which requires
more room. Suppose them surrounded by a box which exactly encloses
them at the moment the polar force first comes into play. It is easy
to see that in arranging themselves subsequently the repelled corners
and ends of the magnets may be caused to press against the sides of
the box, and even to burst it, if the forces be sufficiently strong.

329. Here then we have a conception which may be applied to the
molecules of water. They, like the magnets, are acted upon by two
distinct forces. For a time while the liquid is being cooled they
approach each other, in obedience to their general attraction for
each other. But at a certain point new forces, some attractive,
some repulsive, _emanating from special points_ of the molecules,
come into play. The attracted points close up, the repelled points
retreat. Thus the molecules turn and rearrange themselves, demanding,
as they do so, more space, and overcoming all ordinary resistance by
the energy of their demand. This, in general terms, is an explanation
of the expansion of water in solidifying: it would be easy to
construct an apparatus for its illustration.


§ 48. _The Dirt Bands of the Mer de Glace._

330. Pass from bright sunshine into a moderately lighted room; for a
time all appears so dark that the objects in the room are not to be
clearly distinguished. Hit violently by the waves of light (§ 3) the
optic nerve is numbed, and requires time to recover its sensitiveness.

331. It is for this reason that I choose the present hour for a
special observation on the Mer de Glace. The sun has sunk behind the
ridge of Charmoz, and the surface of the glacier is in sober shade.
The main portion of our day's work is finished, but we have still
sufficient energy to climb the slopes adjacent to the Montanvert to
a height of a thousand feet or thereabouts above the ice.

[Illustration]

332. We now look fairly down upon the glacier, and see it less
foreshortened than from the Montanvert. We notice the diet
overspreading its eastern side, due to the crowding together of
its medial moraines. We see the comparatively clean surface of the
Glacier du Géant; but we notice upon this surface an appearance
which we have not hitherto seen. It is crossed by a series of grey
bent bands, which follow each other in succession, from Trélaporte
downwards. We count eighteen of these from our present position. (See
sketch, page 128.)

[Illustration]

333. These are the _Dirt Bands_ of the Mer de Glace; they were first
observed by Professor Forbes in 1842.

[Illustration]

334. They extend down the glacier further than we can see; and if we
cross the valley of Chamouni, and climb the mountains at the opposite
side, to a point near the little auberge, called La Flégère, we shall
command a view of the end of the glacier and observe the completion
of the series of bands. We notice that they are confined throughout
to the portion of the glacier derived from the Col du Géant. (See
sketch, page 129.)

335. We must trace them to their source. You know how noble and
complete a view is obtained of the glacier and Col du Géant from the
Cleft Station above Trélaporte. Thither we must once more climb;
and thence we can see the succession of bands stretching downwards
to the Montanvert, and upwards to the base of the ice-cascade upon
the Glacier du Géant. The cascade is evidently concerned in their
formation. (See sketch opposite.)

336. And how? Simply enough. The glacier, as we know, is broken
transversely at the summit of the ice-fall, and descends the
declivity in a series of great transverse ridges. At the base of the
fall, the chasms are closed, but the ridges in part remain forming
protuberances, which run like vast wrinkles across the glacier. These
protuberances are more and more bent because of the quicker motion
of the centre, and the depressions between them form receptacles for
the fine mud and débris washed by the little rills from the adjacent
slopes.

337. The protuberances sink gradually through the wasting action of
the sun, so that long before Trélaporte is reached they have wholly
disappeared. Not so the dirt of which they were the collectors: it
continues to occupy, in transverse bands, the flat surface of the
glacier. At Trélaporte, moreover, where the valley becomes narrow,
the bands are much sharpened, obtaining there the character which
they afterwards preserve throughout the Mer de Glace. Other glaciers
with cascades also exhibit similar bands.


§ 49. _Sea Ice and Icebergs._

338. We are now equipped intellectually for a campaign into another
territory. Water becomes heavier and more difficult to freeze when
salt is dissolved in it. Sea water is therefore heavier than fresh,
and the Greenland Ocean requires to freeze it a temperature 3½
degrees lower than fresh water. When concentrated till its specific
gravity reaches 1.1045, sea water requires for its congelation a
temperature 18⅓ degrees lower than the ordinary freezing-point.[E]

[E] Scoresby.

339. But even when the water is saturated with salt, the
crystallising force studiously rejects the salt, and devotes itself
to the congelation of the water alone. Hence the ice of sea water,
when melted, produces fresh water. The only saline particles existing
in such ice are those entangled 'mechanically in its pores. They have
no part or lot in the structure of the crystal.

340. This _exclusiveness_, if I may use the term, of the water
molecules; this entire rejection of all foreign elements from the
edifices which they build, is enforced to a surprising degree.
Sulphuric acid has so strong an affinity for water that it is one
of the most powerful agents known to the chemist for the removal of
humidity from air. Still, as shown by Faraday, when a mixture of
sulphuric acid and water is frozen, the crystal formed is perfectly
sweet and free from acidity. The water alone has lent itself to the
crystallising force.

341. Every winter in the Arctic regions the sea freezes, roofing
itself with ice of enormous thickness and vast extent. By the summer
heat, and the tossing of the waves, this is broken up; the fragments
are drifted by winds and borne by currents. They clash, they crush
each other, they pile themselves into heaps, thus constituting the
chief danger encountered by mariners in the polar seas.

342. But among the drifting masses of flat sea-ice, vaster masses
sail, which spring from a totally different source. These are the
_Icebergs_ of the Arctic seas. They rise sometimes to an elevation of
hundreds of feet above the water, while the weight of ice submerged
is about seven times that seen above.

343. The first observers of striking natural phenomena generally
allow wonder and imagination more than their due place. But to
exclude all error arising from this cause, I will refer to the
journal of a cool and intrepid Arctic navigator, Sir Leopold
McClintock. He describes an iceberg 250 feet high, which was aground
in 500 feet of water. This would make the entire height of the berg
750 feet, not an unusual altitude for the greater icebergs.

344. From Baffin's Bay these mighty masses come sailing down through
Davis' Straits into the broad Atlantic. A vast amount of heat is
demanded for the simple liquefaction of ice (§ 48); and the melting
of icebergs is on this account so slow, that when large they
sometimes maintain themselves till they have been drifted 2000 miles
from their place of birth.

345. What is their origin? The Arctic glaciers. From the mountains
in the interior the indurated snows slide into the valleys and fill
them with ice. The glaciers thus formed move like the Swiss ones,
incessantly downward. But the Arctic glaciers reach the sea, enter
it, often ploughing up its bottom into submarine moraines. Undermined
by the lapping of the waves, and unable to resist the strain imposed
by their own weight, they break across, and discharge vast masses
into the ocean. Some of these run aground on the adjacent shores,
and often maintain themselves for years. Others escape southward, to
be finally dissolved in the warm waters of the Atlantic. The first
engraving on the opposite page is copied from a photograph taken by
Mr. Bradford during a recent expedition to the Northern seas. The
second represents a mass of ice upon the Glacier des Bossons. Their
likeness suggests their common origin.

[Illustration]

[Illustration]


§ 50. _The Æggischhorn, the Märgelin See and its Icebergs._

346. I am, however, unwilling that you should quit Switzerland
without seeing such icebergs as it can show, and indeed there are
other still nobler glaciers than the Mer de Glace with which you
ought to be acquainted. In tracing the Rhone to its source, you have
already ascended the valley of the Rhone. Let us visit it again
together; halt at the little town of Viesch, and go from it straight
up to the excellent hostelry on the slope of the Æggischhorn. This
we shall make our head-quarters while we explore that monarch of
European ice-streams,--the great Aletsch glacier.

347. Including the longest of its branches, this noble ice-river is
about twenty miles long, while at the middle of its trunk it measures
nearly a mile and a quarter from side to side. The grandest mountains
of the Bernese Oberland, the Jungfrau, the Monch, the Trugberg, the
Aletschhorn, the Breithorn, the Gletscherhorn, and many another noble
peak and ridge, are the collectors of its névés. From three great
valleys formed in the heart of the mountains these névés are poured,
uniting together to form the trunk of the Aletsch at a place named
by a witty mountaineer, the "Place de la Concorde of Nature." If the
phrase be meant to convey the ideas of tranquil grandeur, beauty of
form, and purity of hue, it is well bestowed.

348. Our hotel is not upon the peak of the Æggischhorn, but a brisk
morning walk soon places us upon the top. Thence we see the glacier
like a broad river stretching upwards to the roots of the Jungfrau,
and downwards past the Bel Alp towards its end. Prolonging the vision
downwards, we strike the noblest mountain group in all the Alps,--the
Dom and its attendant peaks, the Matterhorn and the Weisshorn. The
scene indeed is one of impressive grandeur, a multitude of peaks and
crests here unnamed contributing to its glory.

349. But low down to our right, and surrounded by the sheltering
mountains, is an object the beauty of which startles those who are
unprepared for it. Yonder we see the naked side of the glacier,
exposing glistening ice-cliffs sixty or seventy feet high. It would
seem as if the Aletsch here were engaged in the vain attempt to
thrust an arm through a lateral valley. It once did so; but the arm
is now incessantly broken off close to the body of the glacier, a
great space formerly covered by the ice being occupied by its water
of liquefaction. A lake of the loveliest blue is thus formed, which
reaches quite to the base of the ice-cliffs, saps them, as the Arctic
waves sap the Greenland glaciers, and receives from them the broken
masses which it has undermined. As we look down upon the lake, small
icebergs sail over the tranquil surface, each resembling a snowy swan
accompanied by its shadow.

350. This is the beautiful little lake of Märgelin, or, as the Swiss
here call it, the Märgelin See. You see that splash, and immediately
afterwards hear the sound of the plunging ice. The glacier has broken
before our eyes, and dropped an iceberg into the lake. All over the
lake the water is set in commotion, thus illustrating on a small
scale the swamping waves produced by the descent of vast islands of
ice from the Arctic glaciers. Look to the end of the lake. It is
cumbered with the remnants of icebergs now aground, which have been
in part wafted thither by the wind, but in part slowly borne by the
water which moves gently in this direction.

351. Imagine us below upon the margin of the lake, as I happened to
be on one occasion. There is one large and lonely iceberg about the
middle. Suddenly a sound like that of a cataract is heard; we look
towards the iceberg and see water teeming from its sides. Whence
comes the water? the berg has become top-heavy through the melting
underneath; it is in the act of performing a somersault, and in
rolling over carries with it a vast quantity of water, which rushes
like a waterfall down its sides. And notice that the iceberg, which
a moment ago was snowy-white, now exhibits the delicate blue colour
characteristic of compact ice. It will soon, however, be rendered
white again by the action of the sun. The vaster icebergs of the
Northern seas sometimes roll over in the same fashion. A week may be
spent with delight and profit at the Æggischhorn.


§ 51. _The Bel Alp._

352. From the Æggischhorn I might lead you along the mountain ridge
by the Betten See, the fish of which we have already tasted, to the
Rieder Alp, and thence across the Aletsch to the Bel Alp. This is a
fine mountain ramble, but you and I prefer making the glacier our
highway downwards. Easy at some places, it is by no means child's
play at others to unravel its crevasses. But the steady constancy and
close observation which we have hitherto found availing in difficult
places do not forsake us here. We clear the fissures; and, after four
hours of exhilarating work, we find ourselves upon the slope leading
up to the Bel Alp hotel.

353. This is one of the finest halting-places in the Alps. Stretching
before us up to the Æggischhorn and Märgelin See is the long last
reach of the Aletsch, with its great medial moraine running along its
back. At hand is the wild gorge of the Massa, in which the snout of
the glacier lies couched like the head of a serpent. The beautiful
system of the Oberaletsch glaciers is within easy reach. Above us
is a peak called the Sparrenhorn, accessible to the most moderate
climber, and on the summit of which little more than an hour's
exertion will place you and me. Below us now is the Oberaletsch
glacier, exhibiting the most perfect of medial moraines. Near us
is the great mass of the Aletschhorn, clasped by its névés, and
culminating in brown rock. It is supported by other peaks almost as
noble as itself. The Nesthorn is at hand; while sweeping round to the
west we strike the glorious triad already referred to, the Weisshorn,
the Matterhorn, and the Dom. Take one glance at the crevasses of the
glacier immediately below us. It tumbles at its end down a steep
incline, and is greatly riven. But the crevasses open before the
steep part is reached, and you notice the coalescence of marginal and
transverse crevasses, producing a system of curved fissures with the
convexities of the curves pointing upwards. The mechanical reason of
this is now known to you. The glacier-tables are also numerous and
fine. I should like to linger with you here for a week, exploring the
existing glaciers, and tracing out the evidences of others that have
passed away.


§ 52. _The Riffelberg and Görner Glacier._

354. And though our measurements and observations on the Mer de Glace
are more or less representative of all that can be made or solved
elsewhere, I am unwilling to leave you unacquainted with the great
system of glaciers which stream from the northern slopes of Monte
Rosa and the adjacent mountains. From the Bel Alp we can descend to
Brieg, and thence drive to Visp; but you and I prefer the breezy
heights, so we sweep round the promontory of the Nessel, until we
stand over the Rhone valley, in front of Visp. From this village an
hour's walking carries us to Stalden, where the valley divides into
two branches: the one leading through Saas over the Monte Moro, and
the other through St. Nicholas to Zermatt. The latter is our route.

355. We reach Zermatt, but do not halt here. On the mountain ridge,
4,000 feet above the valley, we discern the Riffelberg hotel. This
we reach. Right in front of us is the pinnacle of the Matterhorn,
upon the top of which it must appear incredible to you that a human
foot could ever tread. Constancy and skill, however, accomplished
this, but in the first instance at a terrible price. In the little
churchyard of Zermatt we have seen the graves of two of the greatest
mountaineers that Savoy and England have produced: and who, with two
gallant young companions, fell from the Matterhorn in 1865.

356. At the Riffelberg we are within an hour's walk of the famous
Görner Grat, which commands so grand a view of the glaciers of Monte
Rosa. But yonder huge knob of perfectly bare rock, which is called
the Riffelhorn, must be our station. What the Cleft Station is to
the Mer de Glace, the Riffelhorn is to the Görner glacier and its
tributaries. From its lower side the rock, easy as it may seem, is
inaccessible. Here, indeed, in 1865, a fifth good man met his end,
and he also lies beside his fellow countrymen in the churchyard of
Zermatt. Passing a little tarn, or lake, called the Riffel See, we
assail the Riffelhorn on its upper side. It is capital rock-practice
to reach the summit; and from it we command a most extraordinary
scene.

357. The huge and many-peaked mass of Monte Rosa faces us, and we
scan its snows from bottom to top. To the right is the mighty ridge
of the Lyskamm, also laden with snow; and between both lies the
Western Glacier of Monte Rosa. This glacier meets another from the
vast snow-fields of the Cima di Jazzi; they join to form the Görner
glacier, and from their place of junction stretches the customary
medial moraine. On this side of the Lyskamm rise two beautifully
snowy eminences, the Twins Castor and Pollux; then come the brown
crags of the Breithorn, then the Little Matterhorn, and then the
broad snow-field of the Théodule, out of which springs the Great
Matterhorn, and which you and I will cross subsequently into Italy.

358. The valleys and depressions between these mountains are filled
with glaciers. Down the flanks of the Twin Castor comes the Glacier
des Jumeaux, from Pollux comes the Schwartze glacier, from the
Breithorn the Trifti glacier, then come the Little Matterhorn glacier
and the Théodule glacier, each, as it welds itself to the trunk,
carrying with it its medial moraine. We can count nine such moraines
from our present position. And to a still more surprising degree
than on the Mer de Glace, we notice the power of the ice to yield to
pressure; the broad névés being squeezed on the trunk of the Görner
into white stripes, which become ever narrower between the bounding
moraines, and finally disappear under their own shingle.

[Illustration: THE GÖRNER GLACIER, WITH MONTE ROSA IN THE DISTANCE,
AND THE RIFFELHORN TO THE LEFT.]

359. On the two main tributaries we also notice moraines which seem
in each case to rise from the body of the glacier, appearing in the
middle of the ice without any apparent origin higher up. These at
their sources, are sub-glacial moraines, which have been rubbed away
from rocky promontories entirely covered with ice. They lie hidden
for a time in the body of the glacier, and appear at the surface
where the ice above them has been melted away by the sun.

360. This is the place to mention a notion long entertained by the
inhabitants of the high Alps, that glaciers possess the power of
thrusting out all impurities from them. On the Mer de Glace you and
I have noticed large patches of clay and black mud which evidently
came from the body of the glacier, and we can therefore understand
how natural was this notion of extrusion to people unaccustomed to
close observation. But the power of the glacier in this respect is
in reality the power of the sun, which fuses the ice above concealed
impurities, and, like the bodies of the guides on the Glacier des
Bossons (143), brings them to the light of day.

361. On no other glacier will you find more objects of interest than
on the Görner. Sand-cones, glacier-tables, deep ice-gorges cut by
streams and bridged fantastically by boulders, moulins, sometimes
arched ice-caverns of extraordinary size and beauty. On the lower
part of the glacier we notice the partial disappearance of the medial
moraine in the crevasses, and its reappearance at the foot of the
incline. For many years this glacier was steadily advancing on the
meadow in front of it, ploughing up the soil and overturning the
chalets in its way. It now shares in the general retreat exhibited
during the last fifteen years among the glaciers of the Alps. As
usual, a river, the Visp, rushes from a vault at the extremity of the
Görner glacier.


§ 53. _Ancient Glaciers of Switzerland._

362. You have not lost the memory of the old Moraine, which
interested us so much in our first ascent from the source of the
Arveiron; for it opened our minds to the fact that at one period of
its history the Mer de Glace attained far greater dimensions than it
now exhibits. Our experience since that time has enabled us to pursue
these evidences of ice action to an extent of which we had then no
notion.

363. Close to the existing glacier, for example, we have repeatedly
seen the mountain side laid bare by the retreat of the ice. This is
especially conspicuous just now, because for the last fifteen or
sixteen years the glaciers of the Alps have been steadily shrinking;
so that it is no uncommon thing to see the marginal rocks laid bare
for a height of fifty, sixty, eighty, or even one hundred feet above
the present glacier. On the rocks thus exposed we see the evident
marks of the sliding; and our eyes and minds have been so educated in
the observation of these appearances that we are now able to detect,
with certainty, icemarks, or moraines, ancient or modern, wherever
they appear.

364. But the elevations at which we have found such evidence might
well shake belief in the conclusions to which they point. Beside
the Massa Gorge, at 1,000 feet above the present Aletsch, we found
a great old moraine. Descending the meadows between the Bel Alp
and Flatten, we found another, now clothed with grass, and bearing
a village on its back. But I wish to carry you to a region which
exhibits these evidences on a still grander and more impressive
scale. We have already taken a brief flight to the valley of
Hasli and the Glacier of the Aar. Let us make that glacier our
starting-point. Walking from it downwards towards the Grimsel, we
pass everywhere over rocks singularly rounded, and fluted, and
scarred. These appearances are manifestly the work of the glacier in
recent times. But we approach the Grimsel, and at the turning of the
valley stand before the precipitous granite flank of the mountain.
The traces of the ancient ice are here as plain as they are amazing.
The rocks are so hard that not only the fluting and polishing, but
even the fine scratches which date back unnamable thousands of years
are as evident as if they had been made yesterday. We may trace these
evidences to a height of two thousand feet above the present valley
bed. It is indubitable that an ice-river of this astounding depth
once flowed through the vale of Hasli.

365. Yonder is the summit of the Siedelhorn; and if we gain it, the
Unteraar glacier will lie like a map below us. From this commanding
point we plainly see marked upon the mountain sides the height to
which the ancient ice extended. The ice-ground part of the mountains
is clearly distinguished from the splintered crests which in those
distant days rose above the surface of the glacier, and which must
have then appeared as island peaks and crests in the midst of an
ocean of ice.

366. We now scamper down the Siedelhorn, get once more into the
valley of Hasli, along which we follow for more than twenty miles
the traces of the ice. Fluted precipices, polished slabs, and
beautifully-rounded granite domes. Right and left upon the mountain
flanks, at great elevations, the evidences appear. We follow the
footsteps of the glacier to the Lake of Brientz; and if we prolonged
our enquiries, we should learn that all the lake beds of this region,
at the time now referred to, bore the burden of immense masses of ice.

367. Instead of the vale of Hasli, we might take the valley of the
Rhone. The traces of a mighty glacier, which formerly filled it,
may be followed all the way to Martigny, which is 60 miles distant
from the present ice. At Martigny the Rhone glacier was reinforced
by another from Mont Blanc, and the welded masses moved onward,
planing the mountains right and left, to the Lake of Geneva, the
basin of which they entirely filled. Other evidences prove that the
glacier did not end here, but pushed across the low country until it
encountered the limestone barrier of the Jura Mountains.


§ 54. _Erratic Blocks._

368. What are these other evidences? We have seen mighty rocks poised
on the moraines of the Mer de Glace, and we now know that, unless
they are split and shattered by the frost, these rocks will, at
some distant day, be landed bodily by the Glacier des Bois in the
valley of Chamouni. You have already learned that these boulders
often reveal the mineralogical nature of the mountains among which
the glacier has passed; that specimens are thus brought down of a
character totally different from the rocks among which they are
finally landed; this is strikingly the case with the _erratic blocks_
stranded along the Jura.

369. For the Jura itself, as already stated, is limestone; there is
no trace of native granite to be found amongst these hills. Still
along the breast of the mountain above the town of Neufchâtel, and at
about 800 feet above the lake of Neufchâtel, we find stranded a belt
of granite boulders from Mont Blanc. And when we clear the soil away
from the adjacent mountain side, we find upon the limestone rocks the
scarrings of the ancient glacier which brought the boulders here.

370. The most famous of these rocks, called the Pierre à Bôt,
measures 50 feet in length, 40 in height, and 20 in width.
Multiplying these three numbers together, we obtain 40,000 cubic feet
as the volume of the boulder.

371. But this is small compared with some of the rocks which
constitute the freight of even recent glaciers. Let us visit another
of them. We have already been to Stalden, where the valley divides
into two branches, the right branch running to St. Nicholas and
Zermatt, and the left one to Saas and the Monte Moro. Three hours
above Saas we come upon the end of the Allelein glacier, not filling
the main valley, but thrown athwart it so as to stop its drainage
like a dam. Above this ice-dam we have the Mattmark Lake, and at the
head of the lake a small inn well known to travellers over the Monte
Moro.

372. Close to this inn is the greatest boulder that we have ever
seen. It measures 240,000 cubic feet. Looking across the valley we
notice a glacier with its present end half a mile from the boulder.
The stone, I believe, is serpentine, and were you and I to explore
the Schwartzberg glacier to its upper fastnesses, we should find
among them the birthplace of this gigantic stone. Four-and-twenty
years ago, when the glacier reached the place now occupied by the
boulder, it landed there its mighty freight, and then retreated.
There is a second ice-borne rock at hand which would be considered
vast were it not dwarfed by the aspect of its huger neighbour.

373. Evidence of this kind might be multiplied to any extent. In
fact, at this moment, distinguished men, like Professor Favre of
Geneva, are determining from the distribution of the erratic blocks
the extent of the ancient glaciers of Switzerland. It was, however,
an engineer named Venetz that first brought these evidences to light,
and announced to an incredulous world the vast extension of the
ancient ice. M. Agassiz afterwards developed and wonderfully expanded
the discovery. Perhaps the most interesting observation regarding
ancient glaciers is that of Dr. Hooker, who, during a recent visit
to Palestine, found the celebrated Cedars of Lebanon growing upon
ancient moraines.


§ 55. _Ancient Glaciers of England, Ireland, Scotland, and Wales._

374. At the time the ice attained this extraordinary development in
the Alps, many other portions of Europe, where no glaciers now exist,
were covered with them. In the Highlands of Scotland, among the
mountains of England, Ireland, and Wales, the ancient glaciers have
written their story as plainly as in the Alps themselves. I should
like to wander with you through Borrodale in Cumberland, or through
the valleys near Bethgellert in Wales. Under all the beauty of the
present scenery we should discover the memorials of a time when the
whole region was locked in the embrace of ice. Professor Ramsay is
especially distinguished by his writings on the ancient glaciers of
Wales.

375. We have made the acquaintance of the Reeks of Magillicuddy as
the great condensers of Atlantic vapour. At the time now referred to,
this moisture did not fall as soft and fructifying rain, but as snow,
which formed the nutriment of great glaciers. A chain of lakes now
constitutes the chief attraction of Killarney, the Lower, the Middle,
and the Upper Lake. Let us suppose ourselves rowing towards the head
of the Upper Lake with the Purple Mountain to our left. Remembering
our travels in the Alps, you would infallibly call my attention to
the planing of the rocks, and declare the action to be unmistakably
that of glaciers. With our attention thus sharpened, we land at
the head of the lake, and walk up the Black Valley to the base of
Magillicuddy's Reeks. Your conclusion would be, that this valley
tells a tale as wonderful as that of Hasli.

376. We reach our boat and row homewards along the Upper Lake. Its
islands now possess a new interest for us. Some of them are bare,
others are covered wholly or in part with luxuriant vegetation; but
both the naked and clothed islands are glaciated. The weathering of
ages has not altered their forms: there are the Cannon Rock, the
Giant's Coffin, the Man of War, all sculptured as if the chisel had
passed over them in our own lifetime. These lakes, now fringed with
tender woodland beauty, were all occupied by the ancient ice. It has
disappeared, and seeds from other regions have been wafted thither
to sow the trees, the shrubs, the ferns, and the grasses which now
beautify Killarney. Man himself, they say, has made his appearance in
the world since that time of ice; but of the real period and manner
of man's introduction little is professed to be known since, to make
them square with science, new meanings have been found for the
beautiful myths and stories of the Bible.

377. It is the nature and tendency of the human mind to look backward
and forward; to endeavour to restore the past and predict the future.
Thus endowed, from data patiently and painfully won, we recover in
idea a state of things which existed thousands, it may be millions,
of years before the history of the human race began.


§ 56. _The Glacial Epoch._

378. This period of ice-extension has been named the _Glacial Epoch_.
In accounting for it great minds have fallen into grave errors, as we
shall presently see.

379. The substance on which we have thus far been working exists in
three different states: as a solid in ice; as a liquid in water; as
a gas in vapour. To cause it to pass from one of these states to the
next following one, _heat_ is necessary.

380. Dig a hole in the ice of the Mer de Glace in summer, and place
a thermometer in the hole; it will stand at 32° Fahr. Dip your
thermometer into one of the glacier streams; it will still mark 32°.
_The water is therefore as cold as ice._

381. Hence the whole of the heat poured by the sun upon the glacier,
and which has been absorbed by the glacier, is expended in simply
liquefying the ice, and not in rendering either ice or water a single
degree warmer.

382. Expose water to a fire; it becomes hotter for a time. It boils,
and from that moment it ceases to get hotter. After it has begun to
boil, all the heat communicated by the fire is carried away by the
steam, _though the steam itself is not the least fraction of a degree
hotter than the water_.

383. In fact, simply to liquefy ice a large quantity of heat
is necessary, and to vaporize water a still larger quantity is
necessary. And inasmuch as this heat does not render the water warmer
than the ice, nor the steam warmer than the water, it was at one time
supposed to be _hidden_ in the water and in the steam. And it was
therefore called _latent_ heat.

384. Let us ask how much heat must the sun expend in order to convert
a pound weight of the tropical ocean into vapour? This problem has
been accurately solved by experiment. It would require in round
numbers 1,000 times the amount of heat necessary to raise one pound
of water one degree in temperature.

385. But the quantity of heat which would raise the temperature of a
pound of water one degree would raise the temperature of a pound of
iron _ten_ degrees. This has been also proved by experiment. Hence
to convert one pound of the tropical ocean into vapour the sun must
expend 10,000 times as much heat as would raise one pound of iron one
degree in temperature.

386. This quantity of heat would raise the temperature of 5 lbs. of
iron 2,000 degrees, which is the fusing point of cast iron; at this
temperature the metal would not only be _white hot_, but would be
passing into the molten condition.

387. Consider the conclusions at which we have now arrived. For every
pound of tropical vapour, or for every pound of Alpine ice produced
by the congelation of that vapour, an amount of heat has been
expended by the sun sufficient to raise 5 lbs. of cast iron to its
melting-point.

388. It would not be difficult to calculate approximately the weight
of the Mer de Glace and its tributaries--to say, for example, that
they contained so many millions of millions of tons of ice and snow.
Let the place of the ice be taken by a mass of white-hot iron of
quintuple the weight; with such a picture before your mind you get
some notion of the enormous amount of heat paid out by the sun to
produce the present glacier.

389. You must think over this, until it is as clear as sunshine.
For you must never henceforth fall into the error already referred
to, and which has entangled so many. So natural was the association
of ice and cold, that even celebrated men assumed that all that is
needed to produce a great extension of our glaciers is a diminution
of the sun's temperature. Had they gone through the foregoing
reflections and calculations, they would probably have demanded more
heat instead of less for the production of a "glacial epoch." What
they really needed were _condensers_ sufficiently powerful to congeal
the vapour generated by the heat of the sun.


§ 57. _Glacier Theories._

390. You have not forgotten, and hardly ever can forget, our climbs
to the Cleft Station. Thoughts were then suggested which we have not
yet discussed. We saw the branch glaciers coming down from their
névés, welding themselves together, pushing through Trélaporte, and
afterwards moving through the sinuous valley of the Mer de Glace.
These appearances alone, without taking into account subsequent
observations, were sufficient to suggest the idea that glacier
ice, however hard and brittle it may appear, is really a viscous
substance, resembling treacle, or honey, or tar, or lava.


§ 58. _Dilatation and Sliding Theories._

391. Still this was not the notion expressed by the majority of
writers upon glaciers. Scheuchzer of Zürich, a great naturalist,
visited the glaciers in 1705, and propounded a theory of their
motion. Water, he knew, expands in freezing, and the force of
expansion is so great, that thick bombshells filled with water, and
permitted to freeze, are, as we know (312), shattered to pieces by
the ice within. Scheuchzer supposed that the water in the fissures of
the glaciers, freezing there and expanding with resistless force, was
the power which urged the glacier downwards. He added to this theory
other notions of a less scientific kind.

392. Many years subsequently, De Charpentier of Bex renewed and
developed this theory with such ability and completeness, that it was
long known as Charpentier's Theory of Dilatation. M. Agassiz for a
time espoused this theory, and it was also more or less distinctly
held by other writers. The glacier, in fact, was considered to be a
magazine of cold, capable of freezing all water percolating through
it. The theory was abandoned when this notion of glacier cold was
proved by M. Agassiz to be untenable.

393. In 1760, Altmann and Grüner propounded the view that glaciers
moved by sliding over their beds. Nearly forty years subsequently,
this notion was revived by De Saussure, and it has therefore been
called "De Saussure's Theory," or the "Sliding Theory" of glacier
motion.

394. There was, however, but little reason to connect the name of
De Saussure with this or any other theory of glaciers. Incessantly
occupied in observations of another kind, this celebrated man devoted
very little time or thought to the question of glacier motion. What
he has written upon the subject reads less like the elaboration of a
theory than the expression of an opinion.


§ 59. _Plastic Theory._

395. By none of these writers is the property of viscosity or
plasticity ascribed to glacier ice; the appearances of many glaciers
are, however, so suggestive of this idea that we may be sure it
would have found more frequent expression, were it not in such
apparent contradiction with our every-day experience of ice.

396. Still the idea found its advocates. In a little book, published
in 1773, and entitled "Picturesque Journey to the Glaciers of Savoy,"
Bordier of Geneva wrote thus:--"It is now time to look at all these
objects with the eyes of reason; to study, in the first place, the
position and the progression of glaciers, and to seek the solution of
their principal phenomena. At the first aspect of the ice-mountains
an observation presents itself, which appears sufficient to explain
all. It is that the entire mass of ice is connected together, and
presses from above downwards after the manner of fluids. Let us then
regard the ice, not as a mass entirely rigid and immobile, but as a
heap of coagulated matter, or as softened wax, flexible and ductile
to a certain point."[F] Here probably for the first time the quality
of plasticity is ascribed to the ice of glaciers.

[F] I am indebted to my distinguished friend Prof. Studer of Berne
for directing my attention to Bordier's book, and to my friends at
the British Museum for the great trouble they have taken to find it
for me.

397. To us, familiar with the aspect of the glaciers, it must seem
strange that this idea once expressed did not at once receive
recognition and development. But in those early days explorers were
few, and the "Picturesque Journey" probably but little known, so
that the notion of plasticity lay dormant for more than half a
century. But Bordier was at length succeeded by a man of far greater
scientific grasp and insight than himself. This was Rendu, a Catholic
priest and canon when he wrote, and afterwards Bishop of Annecy. In
1841 Rendu laid before the Royal Academy of Sciences of Savoy his
"Theory of the Glaciers of Savoy," a contribution for ever memorable
in relation to this subject.[G]

[G] "Memoirs of the Academy," vol. x.

398. Rendu seized the idea of glacier plasticity with great power and
clearness, and followed it resolutely to its consequences. It is not
known that he had ever seen the work of Bordier; probably not, as he
never mentions it. Let me quote for you some of Rendu's expressions,
which, however, fail to give an adequate idea of his insight and
precision of thought:--"Between the Mer de Glace and a river there
is a resemblance so complete that it is impossible to find in the
glacier a circumstance which does not exist in the river. In currents
of water the motion is not uniform either throughout their width or
throughout their depth. The friction of the bottom and of the sides,
with the action of local hindrances, causes the motion to vary, and
only towards the middle of the surface do we obtain the full motion."

399. This reads like a prediction of what has since been established
by measurement. Looking at the glacier of Mont Dolent, which
resembles a sheaf in form, wide at both ends and narrow in the
middle, and reflecting that the upper wide part had become narrow,
and the narrow middle part again wide, Rendu observes, "There is a
multitude of facts which seem to necessitate the belief that glacier
ice enjoys a kind of ductility which enables it to mould itself to
its locality, to thin out, to swell, and to contract as if it were a
soft paste."

400. To fully test his conclusions, Rendu required the accurate
measurement of glacier motion. Had he added to his other endowments
the practical skill of a land-surveyor, he would now be regarded as
the prince of glacialists. As it was he was obliged to be content
with imperfect measurements. In one of his excursions he examined the
guides regarding the successive positions of a vast rock which he
found upon the ice close to the side of the glacier. The mean of five
years gave him a motion for this block of 40 feet a year.

401. Another block, the transport of which he subsequently measured
more accurately, gave him a velocity of 400 feet a year. Note his
explanation of this discrepancy:--"The enormous difference of these
two observations arises from the fact that one block stood near the
centre of the glacier, which moves most rapidly, while the other
stood near the side, where the ice is held back by friction." So
clear and definite were Rendu's ideas of the plastic motion of
glaciers, that had the question of curvature occurred to him, I
entertain no doubt that he would have enunciated beforehand the
shifting of the point of maximum motion from side to side across the
axis of the glacier (§ 25).

402. It is right that you should know that scientific men do not
always agree in their estimates of the comparative value of facts
and ideas; and it is especially right that you should know that your
present tutor attaches a very high value to ideas when they spring
from the profound and persistent pondering of superior minds, and
are not, as is too often the case, thrown out without the warrant
of either deep thought or natural capacity. It is because I believe
Rendu's labours fulfil this condition, that I ascribe to them so
high a value. But when you become older and better informed, you may
differ from me; and I write these words lest you should too readily
accept my opinion of Rendu. Judge me, if you care to do so, when your
knowledge is matured. I certainly shall not fear your verdict.

403. But, much as I prize the prompting idea, and thoroughly as
I believe that often in it the force of genius mainly lies, it
would, in my opinion, be an error of omission of the gravest kind,
and which, if habitual, would ensure the ultimate decay of natural
knowledge, to neglect verifying our ideas, and giving them outward
reality and substance when the means of doing so are at hand. In
science thought, as far as possible, ought to be wedded to fact.
This was attempted by Rendu, and in part accomplished by Agassiz and
Forbes.


§ 60. _Viscous Theory._

404. Here indeed the merits of the distinguished glacialist last
named rise conspicuously to view. From the able and earnest advocacy
of Professor Forbes, the public knowledge of this doctrine of glacial
plasticity is almost wholly derived. He gave the doctrine a more
distinctive form; he first applied the term viscous to glacier ice,
and sought to found upon precise measurements a "Viscous Theory" of
glacier motion.

405. I am here obliged to state facts in their historic sequence.
Professor Forbes when he began his investigations was acquainted with
the labours of Rendu. In his earliest work upon the Alps he refers
to those labours in terms of flattering recognition. But though as
a matter of fact Rendu's ideas were there to prompt him, it would
be too much to say that he needed their inspiration. Had Rendu
not preceded him, he might none the less have grasped the idea of
viscosity, executing his measurements and applying his knowledge to
maintain it. Be that as it may, the appearance of Professor Forbes on
the Unteraar glacier in 1841, and on the Mer de Glace in 1842, and
his labours then and subsequently, have given him a name not to be
forgotten in the scientific history of glaciers.

406. The theory advocated by Professor Forbes was enunciated by
himself in these words:--"A glacier is an imperfect fluid, or viscous
body, which is urged down slopes of certain inclination by the
natural pressure of its parts." In 1773 Bordier wrote thus:--"As the
glaciers always advance upon the plain, and never disappear, it is
absolutely essential that new ice shall perpetually take the place
of that which is melted: it must therefore be pressed forward from
above. One can hardly refuse then to accept the astonishing truth,
that this vast extent of hard and solid ice moves as a single piece
downwards." In the passage already quoted he speaks of the ice being
pressed as a fluid from above. These constitute, I believe, Bordier's
contributions to this subject. The quotations show his sagacity at an
early date; but, in point of completeness, his views are not to be
compared with those of Rendu and Forbes.

407. I must not omit to state here that though the idea of viscosity
has not been espoused by M. Agassiz, his measurements, and maps of
measurements, on the Unteraar glacier have been recently cited as the
most clear and conclusive illustrations of a quality which, at all
events, closely resembles viscosity.

408. But why, with proofs before him more copious and characteristic
than those of any other observer, does M. Agassiz hesitate to accept
the idea of viscosity as applied to ice? Doubtless because he
believes the notion to be contradicted by our every-day experience of
the substance.

409. Take a mass of ice ten or even fifteen cubic feet in volume;
draw a saw across it to a depth of half an inch or an inch; and
strike a pointed pricker, not thicker than a very small round file,
into the groove; the substance will split from top to bottom with a
clean crystalline fracture. How is this brittleness to be reconciled
with the notion of viscosity?

410. We have, moreover, been upon the glacier and have witnessed the
birth of crevasses. We have seen them beginning as narrow cracks
suddenly formed, days being required to open them a single inch. In
many glaciers fissures may be traced narrow and profound for hundreds
of yards through the ice. What does this prove? Did the ice possess
even a very small modicum of that power of stretching, which is
characteristic of a viscous substance, such crevasses could not be
formed.

411. Still it is undoubted that the glacier moves like a viscous
body. The centre flows past the sides, the top flows over the
bottom, and the motion through a curved valley corresponds to fluid
motion. Mr. Mathews, Mr. Froude, and above all Signor Bianconi,
have, moreover, recently made experiments on ice which strikingly
illustrate the flexibility of the substance. These experiments merit,
and will doubtless receive, full attention at a future time.


§ 61. _Regelation Theory._

412. I will now describe to you an attempt that has been made of
late years to reconcile the brittleness of ice with, its motion in
glaciers. It is founded on the observation, made by Mr. Faraday in
1850, that when two pieces of thawing ice are placed together they
freeze together at the place of contact.

413. This fact may not surprise you; still it surprised Mr. Faraday
and others, and men of very great distinction in science have
differed in their interpretation of the fact. The difficulty is to
explain where, or how, in ice already thawing the cold is to be
found requisite to freeze the film of water between the two touching
surfaces.

414. The word _Regelation_ was proposed by Dr. Hooker to express the
freezing together of two pieces of thawing ice observed by Faraday;
and the memoir in which the term was first used was published by Mr.
Huxley and Mr. Tyndall in the Philosophical Transactions for 1857.

415. The _fact_ of regelation, and its application irrespective of
the _cause_ of regelation, may be thus illustrated:--Saw two slabs
from a block of ice, and bring their flat surfaces into contact; they
immediately freeze together. Two plates of ice, laid one upon the
other, with flannel round them overnight, are sometimes so firmly
frozen in the morning that they will rather break elsewhere than
along their surface of junction. If you enter one of the dripping
ice-caves of Switzerland, you have only to press for a moment a slab
of ice against the roof of the cave to cause it to freeze there and
stick to the roof.

416. Place a number of fragments of ice in a basin of water, and
cause them to touch each other; they freeze together where they
touch. You can form a chain of such fragments; and then, by taking
hold of one end of the chain, you can draw the whole series after it.
Chains of icebergs are sometimes formed in this way in the Arctic
seas.

417. Consider what follows from these observations. Snow consists of
small particles of ice. Now if by pressure we squeeze out the air
entangled in thawing snow, and bring the little ice-granules into
close contact, they may be expected to freeze together; and if the
expulsion of the air be complete, the squeezed snow may be expected
to assume the appearance of compact ice.

418. We arrive at this conclusion by reasoning; let us now test it by
experiment, employing a suitable hydraulic press, and a mould to hold
the snow. In exact accordance with our expectation, we convert by
pressure the snow into ice.[H]

[H] A similar experiment was made by the Messrs. Schlagintweit prior
to the discovery which explains it, and which therefore remained
unsolved.

419. Place a compact mass of ice in a proper mould, and subject
it to pressure. It breaks in pieces: squeeze the pieces forcibly
together; they re-unite by regelation, and a compact piece of ice,
totally different in shape from the first one, is taken from the
press. To produce this effect the ice must be in a thawing condition.
When its temperature is much below the melting point it is crushed
by pressure, not into a pellucid mass of another shape, but into a
white powder.

420. By means of suitable moulds you may in this way change the shape
of ice to any extent, turning out spheres, and cups, and rings, and
twisted ropes of the substance; the change of form in these cases
being effected through rude fracture and regelation.

421. By applying the pressure carefully, rude fracture may be
avoided, and the ice compelled slowly to change its form as if it
were a plastic body.

422. Now our first experiment illustrates the consolidation of the
snows of the higher Alpine regions. The deeper layers of the névé
have to bear the weight of all above them, and are thereby converted
into more or less perfect ice. And our last experiment illustrates
the changes of form observed upon the glacier, where, by the slow and
constant application of pressure, the ice gradually moulds itself to
the valley, which it fills.

423. In glaciers, however, we have also ample illustrations of
rude fracture and regelation. The opening and closing of crevasses
illustrate this. The glacier is broken on the cascades and mended
at their bases. When two branch glaciers lay their sides together,
the regelation is so firm that they begin immediately to flow in
the trunk glacier as a single stream. The medial moraine gives no
indication by its slowness of motion that it is derived from the
sluggish ice of the sides of the branch glaciers.

424. The gist of the Regelation Theory is that the ice of glaciers
changes its form and preserves its continuity under _pressure_ which
keeps its particles together. But when subjected to _tension_, sooner
than stretch it _breaks_, and behaves no longer as a viscous body.


§ 62. _Cause of Regelation._

425. Here the fact of regelation is applied to explain the plasticity
of glacier ice, no attempt being made to assign the cause of
regelation itself. They are two entirely distinct questions. But a
little time will be well spent in looking more closely into the cause
of regelation. You may feel some surprise that eminent men should
devote their attention to so small a point, but we must not forget
that in nature nothing is small. Laws and principles interest the
scientific student most, and these may be as well illustrated by
small things as by large ones.

426. The question of regelation immediately connects itself with that
of "latent heat," already referred to (383), but which we must now
subject to further examination. To melt ice, as already stated, a
large amount of heat is necessary, and in the case of the glaciers
this heat is furnished by the sun. Neither the ice so melted nor
the water which results from its liquefaction can fall below 32°
Fahrenheit. The freezing point of water and the melting point of ice
touch each other, as it were, at this temperature. A hair's-breadth
lower water freezes; a hair's-breadth higher ice melts.

427. But if the ice could be caused to melt without this supply of
solar heat, a temperature lower than that of ordinary thawing ice
would result. When snow and salt, or pounded ice and salt, are mixed
together, the salt causes the ice to melt, and in this way a cold of
20 or 30 degrees below the freezing point may be produced. Here, in
fact, the ice consumes _its own warmth_ in the work of liquefaction.
Such a mixture of ice and salt is called "a freezing mixture."

428. And if by any other means ice at the temperature of 32°
Fahrenheit could be liquefied without access of heat from without,
the water produced would be colder than the ice. Now Professor James
Thomson has proved that ice may be liquefied by mere _pressure_, and
his brother, Sir William Thomson, has also shown that water under
pressure requires a lower temperature to freeze it than when the
pressure is removed. Professor Mousson subsequently liquefied large
masses of ice by a hydraulic press; and by a beautiful experiment
Professor Helmholtz has proved that water in a vessel from which
the air has been removed, and which is therefore relieved from the
pressure of the atmosphere, freezes and forms ice-crystals when
surrounded by melting ice. All these facts are summed up in the
brief statement _that the freezing point of water is lowered by
pressure_.[I]

[I] Professor James Thomson and Professor Clausius proved this
independently and almost contemporaneously.

429. For our own instruction we may produce the liquefaction of ice
by pressure in the following way:--You remember the beautiful flowers
obtained when a sunbeam is sent through lake ice (§ 11), and you have
not forgotten that the flowers always form parallel to the surface of
freezing. Let us cut a prism, or small column of ice with the planes
of freezing running across it at right angles; we place that prism
between two slabs of wood, and bring carefully to bear upon it the
squeezing force of a small hydraulic press.

430. It is well to converge by means of a concave mirror a good light
upon the ice, and to view it through a magnifying lens. You already
see the result. Hazy surfaces are formed in the very body of the
ice, which gradually expand as the pressure is slowly augmented.
Here and there you notice something resembling crystallisation;
fern-shaped figures run with considerable rapidity through the ice,
and when you look carefully at their points and edges you find them
in visible motion. These hazy surfaces are spaces of liquefaction,
and the motion you see is that of the ice falling to water under the
pressure. That water is colder than the ice was before the pressure
was applied, and if the pressure be relieved, not only does the
liquefaction cease, but the water re-freezes. The cold produced by
its liquefaction under pressure is sufficient to re-congeal it when
the pressure is removed.

431. If instead of diffusing the pressure over surfaces of
considerable extent, we concentrate it on a small surface, the
liquefaction will of course be more rapid, and this is what Mr.
Bottomley has recently done in an experiment of singular beauty and
interest. Let us support on blocks of wood the two ends of a bar of
ice 10 inches long, 4 inches deep, and 3 wide, and let us loop over
its middle a copper wire one-twentieth, or even one-tenth, of an
inch in thickness. Connecting the two ends of the wire together, and
suspending from it a weight of 12 or 14 pounds, the whole pressure
of this weight is concentrated on the ice which supports the wire.
What is the consequence? The ice underneath the wire liquefies; the
water of liquefaction escapes round the wire, but the moment it is
relieved from the pressure it freezes, and round about the wire, even
before it has entered the ice, you have a frozen casing. The wire
continues to sink in the ice; the water incessantly escapes, freezing
as it does so behind the wire. In half an hour the weight falls; the
wire has gone clean through the ice. You can plainly see where it
has passed, but the two severed pieces of ice are so firmly frozen
together that they will break elsewhere as soon as along the surface
of regelation.

432. Another beautiful experiment bearing upon this point has
recently been made by M. Boussingault. He filled a hollow steel
cylinder with water and chilled it. In passing to ice, water, as you
know, expands (§ 45); in fact, room for expansion is a necessary
condition of solidification. But in the present case the strong
steel resisted the expansion, the water in consequence remaining
liquid at a temperature of more than 30° Fahr. below the ordinary
freezing point. A bullet within the cylinder rattled about at this
temperature, showing that the water was still liquid. On opening the
tap the liquid, relieved of the pressure, was instantly converted
into ice.

433. It is only substances which _expand_ on solidifying that behave
in this manner. The metal bismuth, as we know, is an example similar
to water; while lead, wax, or sulphur, all of which contract on
solidifying, have their point of fusion _heightened_ by pressure.

434. And now you are prepared to understand Professor James Thomson's
theory of regelation. When two pieces of ice are pressed together
liquefaction, he contends, results. The water spreads out around the
points of pressure, and when released re-freezes, thus forming a kind
of cement between the pieces of ice.


§ 63. _Faraday's View of Regelation._

435. Faraday's view of regelation is not so easily expressed, still
I will try to give you some notion of it, dealing in the first place
with admitted facts. Water, even in open vessels, may be lowered many
degrees below its freezing temperature, and still remain liquid;
it may also be raised to a temperature far higher than its boiling
point, and still resist boiling. This is due to the mutual cohesion
of the water particles, which resists the change of the liquid
either into the solid or the vaporous condition.

436. But if into the over-chilled water you throw a particle of ice,
the cohesion is ruptured, and congelation immediately sets in. And if
into the superheated water you introduce a bubble of air or of steam,
cohesion is likewise ruptured, and ebullition immediately commences.

437. Faraday concluded that _in the interior_ of any body, whether
solid or liquid, where every particle is grasped so to speak by the
surrounding particles, and grasps them in turn, the bond of cohesion
is so strong as to require a higher temperature to change the state
of aggregation than is necessary _at the surface_. At the surface
of a piece of ice, for example, the molecules are free on one side
from the control of other molecules; and they therefore yield to
heat more readily than in the interior. The bubble of air or steam
in overheated water also frees the molecules on one side; hence the
ebullition consequent upon its introduction. Practically speaking,
then, the point of liquefaction of the interior ice is higher than
that of the superficial ice. Faraday also refers to the special
solidifying power which bodies exert upon their own molecules.
Camphor in a glass bottle fills the bottle with an atmosphere of
camphor. In such an atmosphere large crystals of the substance may
grow by the incessant deposition of camphor molecules upon camphor,
at a temperature too high to permit of the slightest deposit _upon
the adjacent glass_. A similar remark applies to sulphur, phosphorus,
and the metals in a state of fusion. They are deposited upon solid
portions of their own substance at temperatures not low enough to
cause them to solidify against other substances.

438. Water furnishes an eminent example of this special solidifying
power. It may be cooled ten degrees and more below its freezing
point without freezing. But this is not possible if the smallest
fragment of ice be floating in the water. It then freezes accurately
at 32° Fahr., depositing itself, however, not upon the sides of the
containing vessel, but _upon the ice_. Faraday observed in a freezing
apparatus thin crystals of ice growing in ice-cold water to a length
of six, eight, or ten inches, at a temperature incompetent to produce
their deposition upon the sides of the containing vessel.

439. And now we are prepared for Faraday's view of regelation.
When the surfaces of two pieces of ice, covered with a film of
the water of liquefaction, are brought together, the covering
film is transferred from the surface to the centre of the ice,
where the point of liquefaction, as before shown, is higher than
at the surface. The special solidifying power of ice upon water
is now brought into play _on both sides of the film_. Under these
circumstances, Faraday held that the film would congeal, and freeze
the two surfaces together.

440. The lowering of the freezing point by pressure amounts to
no more than one-seventieth of a degree Fahrenheit for a whole
atmosphere. Considering the infinitesimal fraction of this pressure
which is brought into play in some cases of regelation, Faraday
thought its effect insensible. He suspended pieces of ice, and
brought them into contact without sensible pressure, still they
froze together. Professor James Thomson, however, considered that
even the capillary attraction exerted between two such masses would
be sufficient to produce regelation. You may make the following
experiments, in further illustration of this subject:--

441. Place a small piece of ice in water, and press it underneath the
surface by a second piece. The submerged piece may be so small as to
render the pressure infinitesimal; still it will freeze to the under
surface of the superior piece.

442. Place two pieces of ice in a basin of warm water, and allow them
to come together; they freeze together when they touch. The parts
surrounding the place of contact melt away, but the pieces continue
for a time united by a narrow bridge of ice. The bridge finally
melts, and the pieces for a moment are separated. But capillary
attraction immediately draws them together, and regelation sets in
once more. A new bridge is formed, which in its turn is dissolved,
the separated pieces again closing up. A kind of pulsation is thus
established between the two pieces of ice. They touch, they freeze,
a bridge is formed and melted; and thus the rhythmic action continues
until the ice disappears.

443. According to Professor James Thomson's theory, pressure is
necessary to liquefy the ice. The heat necessary for liquefaction
must be drawn from the ice itself, and the cold water must escape
from the pressure to be re-frozen. Now in the foregoing experiments
the cold water, instead of being allowed to freeze, _issues into
the warm water_, still the floating fragments regelate in a moment.
The touching surfaces may, moreover, be convex; they may be reduced
practically to _points_, clasped all round by the warm water, which
indeed rapidly dissolve them as they approach each other; still they
freeze immediately when they touch.

444. You may learn from this discussion that in scientific matters,
as in all others, there is room for differences of opinion. The frame
of mind to be cultivated here is a suspension of judgment as long
as the meaning remains in doubt. It may be that Faraday's action
and Thomson's action come both into play. I cannot do better than
finish these remarks by quoting Faraday's own concluding words,
which show how in his mind scientific conviction dwelt apart from
dogmatism:--"No doubt," he says, "nice experiments will enable us
hereafter to criticise such results as these, and separating the true
from the untrue will establish the correct theory of regelation."


§ 64. _The Blue Veins of Glaciers._

445. We now approach the end, one important question only remaining
to be discussed. Hitherto we have kept it back, for a wide
acquaintance with the glaciers was necessary to its solution. We had
also to make ourselves familiar by actual experiment with the power
of ice, softened by thaw, to yield to pressure, and to liquefy under
such pressure.

446. Snow is white. But if you examine its individual particles you
would call them _transparent_, not white. The whiteness arises from
the mixture of the ice particles with small spaces of air. In the
case of all transparent bodies whiteness results from such a mixture.
The clearest glass or crystal when crushed becomes a white powder.
The foam of champagne is white through the intimate admixture of a
transparent liquid with transparent carbonic acid gas. The whitest
paper, moreover, is composed of fibres which are individually
transparent.

447. It is not, however, the air or the gas, but the _optical
severance_ of the particles, giving rise to a multitude of reflexions
of the white solar light at their surfaces, that produces the
whiteness.

448. The whiteness of the surface of a clean glacier (112), and of
the icebergs of the Märgelin See (357), has been already referred to
a similar cause. The surface is broken into innumerable fissures by
the solar heat, the reflexion of solar light from the sides of the
little fissures producing the observed appearance.

449. In like manner if you freeze water in a test-tube by plunging it
into a freezing mixture, the ice produced is white. For the most part
also the ice formed in freezing machines is white. Examine such, ice,
and you will find it filled with small air-bubbles. When the freezing
is extremely slow the crystallising force pushes the air effectually
aside, and the resulting ice is transparent; when the freezing is
rapid, the air is entangled before it can escape, and the ice is
_translucent_. But even in the case of quick freezing Mr. Faraday
obtained transparent ice by skilfully removing the air-bubbles as
fast as they appeared with a feather.

450. In the case of lake ice the freezing is not uniform, but
intermittent. It is sometimes slow, sometimes rapid. When slow the
air dissolved in the water is effectually squeezed out and forms a
layer of bubbles on the under-surface of the ice. An act of sudden
freezing entangles the air, and hence we find lake ice usually
composed of layers alternately clear, and filled with bubbles. Such
layers render it easy to detect the planes of freezing in lake ice.

451. And now for the bearing of these facts. Under the fall of the
Géant, at the base of the Talèfre cascade, and lower down the Mer
de Glace; in the higher regions of the Grindelwald, the Aar, the
Aletsch and the Görner glaciers, the ice does not possess the
transparency which it exhibits near the ends of the glaciers. It
is white, or whitish. Why? Examination shows it to be filled with
small air-bubbles; and these, as we now learn, are the cause of its
whiteness.

452. They are the residue of the air originally entangled in the
snow, and connected, as before stated, with the whiteness of the
snow. During the descent of the glacier, the bubbles are gradually
expelled by the enormous pressures brought to bear upon the ice.
Not only is the expulsion caused by the mechanical yielding of the
soft thawing ice, but the liquefaction of the substance at places of
violent pressure, opening, as it does, fissures for the escape of the
air, must play an important part in the consolidation of the glacier.

453. The expulsion of the bubbles is, however, not uniform; for
neither ice nor any other substance offers an absolutely uniform
resistance to pressure. At the base of every cascade that we have
visited, and on the walls of the crevasses there formed, we have
noticed innumerable blue streaks drawn through the white translucent
ice, and giving the whole mass the appearance of lamination. These
blue veins turned out upon examination to be spaces from which the
air-bubbles had been almost wholly expelled, translucency being thus
converted into transparency.

454. This is the _veined_ or _ribboned structure_ of glaciers,
regarding the origin of which diverse opinions are now entertained.

455. It is now our duty to take up the problem, and to solve it if we
can. On the névés of the Col du Géant, and other glaciers, we have
found great cracks, and faults, and _Bergschrunds_, exposing deep
sections of the névé; and on these sections we have found marked the
edges of half-consolidated strata evidently produced by successive
falls of snow. The névé is stratified because its supply of material
from the atmosphere is intermittent, and when we first observed
the blue veins we were disposed to regard them as due to this
stratification.

456. But observation and reflexion soon dispelled this notion. Indeed
it could hardly stand in the presence of the single fact that at the
bases of the ice-falls the veins are always _vertical_, or nearly so.
We saw no way of explaining how the horizontal strata of the névé
could be so tilted up at the base of the fall as to be set on edge.
Nor is the aspect of the veins that of stratification.

457. On the central portions of the cascades, moreover, there are
no signs of the veins. At the bases they first appear, reaching in
each case their maximum development a little below the base. As you
and I stood upon the heights above the Zäsenberg and scrutinised the
cascade of the Strahleck branch of the Grindelwald glacier, we could
not doubt that the base of the fall was the birthplace of the veins.
We called this portion of the glacier a "Structure Mill," intimating
that here, and not on the névé, the veined structure was manufactured.

[Illustration: SECTION OF ICEFALL, AND GLACIER BELOW IT, SHOWING
ORIGIN OF VEINED STRUCTURE.]

458. This, however, is, at bottom, the language of strong _opinion_
merely, not that of _demonstration_; and in science opinion ought to
content us only so long as positive proof is unattainable. The love
of repose must not prevent us from seeking this proof. There is no
sterner conscience than the scientific conscience, and it demands,
in every possible case, the substitution for private conviction of
demonstration which shall be conclusive to all.

459. Let us, for example, be shown a case in which the stratification
of the névé is prolonged into the glacier; let us see the planes of
bedding and the planes of lamination existing side by side, and still
indubitably distinct. Such an observation would effectually exclude
stratification from the problem of the veined structure, and through
the removal of this tempting source of error, we should be rendered
more free to pursue the truth.

460. We sought for this conclusive test upon the Mer de Glace,
but did not find it. We sought it on the Grindelwald, and the Aar
glaciers,[J] with an equal want of success. On the Aletsch glacier,
for the first time, we observed the apparent coexistence of bedding
and structure, the one _cutting_ the other upon the walls of the same
crevasse. Still the case was not sufficiently pronounced to produce
entire conviction, and we visited the Görner glacier with the view of
following up our quest.

[J] M. Agassiz, however, reports a case of the kind upon the glacier
of the Aar.

[Illustration: STRUCTURE AND BEDDING ON ALETSCH GLACIER.]

461. Here day after day added to the conviction that the bedding and
the structure were two different things. Still day after day passed
without revealing to us the final proof. Surely we have not let our
own ease stand in the way of its attainment, and if we retire baffled
we shall do so with the consciousness of having done our best.
Yonder, however, at the base of the Matterhorn, is the Furgge glacier
that we have not yet explored. Upon it our final attempt must be made.

462. We get upon the glacier near its end, and ascend it. We are soon
fronted by a barrier composed of three successive walls of névé, the
one rising above the other, and each retreating behind the other. The
bottom of each wall is separated from the top of the succeeding one
by a ledge, on which threatening masses of broken névé now rest. We
stand amid blocks and rubbish which have been evidently discharged
from these ledges, on which other masses, ready apparently to tumble,
are now poised.

463. On the vertical walls of this barrier we see, marked with the
utmost plainness, the horizontal lines of stratification, while
something exceedingly like the veined structure appears to cross the
lines of bedding at nearly a right angle. The vertical surface is,
however weathered, and the lines of structure, if they be such, are
indistinct. The problem now is to remove the surface, and expose the
ice underneath. It is one of the many cases that have come before
us, where the value of an observation is to be balanced against the
danger which it involves.

464. We do nothing rashly; but scanning the ledges and selecting a
point of attack, we conclude that the danger is not too great to
be incurred. We advance to the wall, remove the surface, and are
rewarded by the discovery underneath it of the true blue veins. They,
moreover, are vertical, while the bedding is horizontal. Bruce, as
you know, was defeated in many a battle, but he persisted and won at
last. Here, upon the Furgge glacier, you also have fought and won
your little Bannockburn.

[Illustration: STRUCTURE AND BEDDING ON FURGGE GLACIER.]

465. But let us not use the language of victory too soon. The
stratification theory has been removed out of the field of
explanation, but nothing has as yet been offered in its place.


§ 65. _Relation of Structure to Pressure._

466. This veined structure was first described by the distinguished
Swiss naturalist, Guyot, now a resident in the United States. From
the Grimsel Pass I have already pointed out to you the Gries glacier
overspreading the mountains at the opposite side of the valley of the
Rhone. It was on this glacier that M. Guyot made his observation.

467. "I saw," he said, "under my feet the surface of the entire
glacier covered with regular furrows, from one to two inches wide,
hollowed out in a half-snowy mass, and separated by protruding plates
of harder and more transparent ice. It was evident that the glacier
here was composed of two kinds of ice, one that of the furrows,
snowy and more easily melted; the other of the plates, more perfect,
crystalline, glassy, and resistant; and that the unequal resistance
which the two kinds of ice presented to the atmosphere was the cause
of the ridges.

468. "After having followed them for several hundred yards, I reached
a crevasse twenty or thirty feet wide, which, as it cut the plates
and furrows at right angles, exposed the interior of the glacier to
a depth of thirty or forty feet, and gave a beautiful transverse
section of the structure. As far as my eyes could reach, I saw
the mass of the glacier composed of layers of snowy ice, each two
of which were separated by one of the hard plates of which I have
spoken, the whole forming a regularly laminated mass, which resembled
certain calcareous slates."

469. I have not failed to point out to you upon all the glaciers
that we have visited the little superficial furrows here described;
and you have, moreover, noticed that in the furrows mainly is lodged
the finer dirt which is scattered over the glacier. They suggest
the passage of a rake over the ice. And whenever these furrows were
interrupted by a crevasse, the veined structure invariably revealed
itself upon the walls of the fissure. The surface grooving is indeed
an infallible indication of the interior lamination of the ice.

470. We have tracked the structure through the various parts of the
glaciers at which its appearance was most distinct; and we have
paid particular attention to the condition of the ice at these
places. The very fact of its cutting the crevasses at right angles
is significant. We know the mechanical origin of the crevasses; that
they are cracks formed at right angles to lines of tension. But since
the crevasses are also perpendicular to the planes of structure,
these planes must be parallel to the lines of tension.

471. On the glaciers, however, tension rarely occurs alone. At the
sides of the glacier, for example, where marginal crevasses are
formed, the tension is always accompanied by pressure; the one
force acting at right angles to the other. Here, therefore, the
veined structure, which is parallel to the lines of tension, _is
perpendicular to the lines of pressure_.

472. That this is so will be evident to you in a moment. Let the
adjacent figure represent the channel of the glacier moving in the
direction of the arrow. Suppose three circles to be marked upon the
ice, one at the centre and the two others at the sides. In a glacier
of uniform inclination all these circles would move downward, the
central one only remaining a circle. By the retardation of the sides
the marginal circles would be drawn out to ovals. The two circles
would be _elongated_ in one direction, and _compressed_ in another.
Across the long diameter, which is the direction of strain, we have
the marginal crevasses; across the short diameter _m n_, which is the
direction of pressure, we have the _marginal veined structure_.

[Illustration]

473. This association of pressure and structure is invariable. At
the bases of the cascades, where the inclination of the bed of the
glacier suddenly changes, the pressure in many cases suffices not
only to close the crevasses but to violently squeeze the ice. At
such places the structure always appears, sweeping quite across
the glacier. When two branch glaciers unite, their mutual thrust
intensifies the pre-existing marginal structure of the branches,
and developes new planes of lamination. Under the medial moraines,
therefore, we have usually a good development of the structure. It
is finely displayed, for example, under the great medial moraine of
the glacier of the Aar.

474. Upon this glacier, indeed, the blue veins were observed
independently three years after M. Guyot had first described them. I
say independently, because M. Guyot's description, though written in
1838, remained imprinted, and was unknown in 1841 to the observers on
the Aar. These were M. Agassiz and Professor Forbes. To the question
of structure Professor Forbes subsequently devoted much attention,
and it was mainly his observations and reasonings that gave it the
important position now assigned to it in the phenomena of glaciers.

475. Thus without quitting the glaciers themselves, we establish
the connexion between pressure and structure. Is there anything in
our previous scientific experience with which these facts may be
connected? The new knowledge of nature must always strike its roots
into the old, and spring from it as an organic growth.


§ 66. _Slate Cleavage and Glacier Lamination_.

476. M. Guyot threw out an exceedingly sagacious hint, when he
compared the veined structure to the cleavage of slate rocks. We must
learn something of this cleavage, for it really furnishes the key to
the problem which now occupies us. Let us go then to the quarries
of Bangor or Cumberland, and observe the quarrymen in their sheds
splitting the rocks. With a sharp point struck skilfully into the
edge of the slate, they cause it to divide into thin plates, fit for
roofing or ciphering, as the case may be. The surfaces along which
the rock cleaves are called its _planes of cleavage_.

477. All through the quarry you notice the direction of these planes
to be perfectly constant. How is this laminated structure to be
accounted for?

478. You might be disposed to consider that cleavage is a case of
stratification or bedding; for it is true that in various parts of
England there are rocks which can be cloven into thin flags along the
planes of bedding. But when we examine these slate rocks we verify
the observation, first I believe made by the eminent and venerable
Professor Sedgwick, that the planes of bedding usually run across the
planes of cleavage.

479. We have here, as you observe, a case exactly similar to that of
glacier lamination, which we were at first disposed to regard as due
to stratification. We afterwards, however, found planes of lamination
crossing the layers of the névé, exactly as the planes of cleavage
cross the beds of slate rocks.

480. But the analogy extends further. Slate cleavage continued to
be a puzzle to geologists till the late Mr. Daniel Sharpe made the
discovery that shells and other fossils and bodies found in slate
rocks are invariably flattened out in the planes of cleavage.

481. Turn into any well-arranged museum--for example, into the School
of Mines in Jermyn Street, and observe the evidence there collected.
Look particularly to the fossil trilobites taken from the slate rock.
They are in some cases squeezed to one third of their primitive
thickness. Numerous other specimens show in the most striking manner
the flattening out of shells.

482. To the evidence adduced by Mr. Sharpe, Mr. Sorby added other
powerful evidence, founded upon the microscopic examination of slate
rock. Taking both into account, the conclusion is irresistible that
such rocks have suffered enormous pressure at right angles to the
planes of cleavage, exactly as the glacier has demonstrably suffered
great pressure at right angles to its planes of lamination.

483. The association of pressure and cleavage is thus demonstrated;
but the question arises, do they stand to each other in the relation
of cause and effect? The only way of replying to this question is to
combine artificially the conditions of nature, and see whether we
cannot produce her results.

484. The substance of slate rocks was once a plastic mud, in which
fossils were embedded. Let us imitate the action of pressure upon
such mud by employing, instead of it, softened white wax. Placing a
ball of the wax between two glass plates, wetted to prevent it from
sticking, we apply pressure and flatten out the wax.

485. The flattened mass is at first too soft to cleave sharply; but
you can see, by tearing, that it is laminated. Let us chill it with
ice. We find afterwards that no slate rock ever exhibited so fine a
cleavage. The laminæ, it need hardly be said, are perpendicular to
the pressure.

486. One cause of this lamination is that the wax is an aggregate
of granules the surfaces of which are places of weak cohesion; and
that by the pressure these granules are squeezed flat, thus producing
planes of weakness at right angles to the pressure.

487. But the main cause of the cleavage I take to be the lateral
sliding of the particles of wax over each other. Old attachments
are thereby severed, which the new ones fail to make good. Thus the
tangential sliding produces lamination, as the rails near a station
are caused to exfoliate by the gliding of the wheel.

488. Instead of wax we may take the slate itself, grind it to fine
powder, add water, and thus reproduce the pristine mud. By the proper
compression of such mud, in one direction, the cleavage is restored.

489. Call now to mind the evidences we have had of the power of
thawing ice to yield to pressure. Recollect the shortening of the
Glacier du Géant, and the squeezing of the Glacier de Léchaud, at
Trélaporte. Such a substance, slowly acted upon by pressure, will
yield laterally. Its particles will slide over each other, the
severed attachments being immediately made good by regelation. It
will not yield uniformly, but along special planes. It will also
liquefy, not uniformly, but along special surfaces. Both the sliding
and the liquefaction will take place principally at right angles to
the pressure, and glacier lamination is the result.

490. As long as it is sound the laminated glacier ice resists
cleavage. Regelation, as I have said, makes the severed attachments
good. But when such ice is exposed to the weather the structure is
revealed, and the ice can then be cloven into tablets a square foot,
or even a square yard in area.


§ 67. _Conclusion_.

491. Here, my friend, our labours close. It has been a true pleasure
to me to have you at my side so long. In the sweat of our brows we
have often reached the heights where our work lay, but you have been
steadfast and industrious throughout, using in all possible cases
your own muscles instead of relying upon mine. Here and there I have
stretched an arm and helped you to a ledge, but the work of climbing
has been almost exclusively your own. It is thus that I should like
to teach you all things; showing you the way to profitable exertion,
but leaving the exertion to you more anxious to bring out your
manliness in the presence of difficulty than to make your way smooth
by toning difficulties down.

492. Steadfast, prudent, without terror, though not at all times
without awe, I have found you on rock and ice, and you have shown
the still rarer quality of steadfastness in intellectual effort. As
here set forth, our task seems plain enough, but you and I know how
often we have had to wrangle resolutely with the facts to bring out
their meaning. The work, however, is now done, and you are master of
a fragment of that sure and certain knowledge which is founded on the
faithful study of nature. Is it not worth the price paid for it? Or
rather, was not the paying of the price the healthful, if sometimes
hard, exercise of mind and body, upon alp and glacier--a portion of
our delight?

493. Here then we part. And should we not meet again, the memory of
these days will still unite us. Give me your hand. Good bye.




INDEX.


  A

  Accurate measurements of the motions of glaciers, by Agassiz and
    Forbes, 60-62.
  Æggischhorn, view from the, 137.
  Agassiz's measurements, 60;
    conclusions, 107;
    discovery by, 150;
    observations made by, 187.
  Aiguille du Dru, pyramid of, 43;
    cloud-banner of, 90.
  ---- des Charmoz, 43;
    clouds about, 90.
  ---- Noire, 51.
  ---- Verte, height of, 53
  ---- du Midi, stone avalanches of, 56.
  Air, its expansion, 24;
    a chilling process, 25;
    experiments illustrating, 25, 26.
  Aletsch glacier, 136;
    length of, 136;
    arm of, 137.
  Alpine ice, origin in the sun's heat, 7.
  Ancient glaciers of England, Ireland, Scotland, and Wales, 150, 151.
  Architecture of snow, 29-34, 91;
    of lake-ice, 35, 169.
  Arveiron, vault of, 66;
    description and cause of, 92.

  B

  Bel Alp, description of the, 139, 140.
  Bergschrund, formation of the, 102, 103, 179.
  Blue veins of glaciers, 176;
    whiteness of snow, 176;
    whiteness of ice, 177;
    freezing of lake-ice, 177;
    explanation of cause of whiteness of glaciers, 178;
    translucency converted into transparency, 178;
    vertical veins, 179;
    structure and bedding on glaciers, 181, 182;
    stratification theory, 183;
    observations, 187.
  Bodies of guides found on Glacier des Bossons, 57, 144.
  Boulders, size of, 148, 149.

  C

  Changes of volume resulting from heat and cold, 118-122;
    illustrations of, 119, 120;
    consequences from, 122;
    opinions of Count Rumford, 12, 124.
  Chapeau, refreshment at, 41.
  Cleavage and glacier lamination, 187;
    analogy between, 188, 189;
    planes of, 188;
    observations of Prof. Sedgwick, 188;
    discovery by Daniel Sharp, 188;
    additional evidence of Mr. Sorby, 189;
    association of pressure and cleavage established, 189:
    relation of cause and effect, 189;
    artificial conditions of Nature combined, 189, 190.
  Clouds, their formation, 3-6;
    in tropical regions, 25;
    illustration of the formation of, 26.
  Col du Géant, snows and ice-cascade of the, 46, 47;
    snow-fall on the plateau of, 48, 49;
    cracks on, 179.
  Conclusion, 191, 192.
  Condensers needed, 154.
  Conditions necessary for the production of natural phenomena, 99.
  Conscience, scientific, 180.
  Crevasses, 41;
    work among the, 52;
    widening of, 54;
    drifting of bodies buried in, 57;
    birth of, 98;
    features of, 100;
    characteristic, 102;
    transverse, 103;
    stalactites of Alpine, 100;
    marginal, 105-107;
    longitudinal, 109;
    curvature of glacier related to number of, 110-112.
  Crystallization of metals, 29;
    of sugar, 30;
    of saltpetre, 30;
    of alum, 30;
    of chalk, 30;
    of carbon, 30;
    reversal of the process of, 36.

  D

  De Saussure's theory of glacier-motion, 156.
  Dilatation theory, 155, 156.
  Dirt-bands of Mer de Glace, observed by Prof. Forbes, 130;
    description and explanation of, 131, 132.
  Distillation, oceanic, 18; ordinary, 21.
  Dôme du Goûté, broken crags of, 50.
  Drifting of huts on ice, 59, 60.
  Dr. William Hopkins's conclusions regarding the obliquity of the
    lateral crevasses, 107.

  E

  Égralets, passage through the, 53.
  Electric light, dark waves of, 14.
  Equivalent points, comparison of velocities of, 75, 76, 107.
  Erratic blocks, 147-150.
  Evaporation, caused by the heat-rays of the sun, 13.
  Expansion of water, 121, 155.
  Experiments to show the heat-power of the dark rays, 14-19;
    illustrative, 22, 23, 25, 20;
    Dr. Franklin's experiment, 112.
  Extract from Bordier's book, 157, 162.

  F

  Faraday's theory regarding regelation, 171;
    special solidifying power exerted by substances upon their own
      molecules, 172, 173;
    opinion of Prof. James Thomson, 174, 175;
    experiments illustrating the subject, 174;
    quotation from Faraday, 175.
  Fog, its formation in ball-rooms, 5.
  Forces, of crystallization, 30;
    of gravitation, 30;
    of Nature, 31;
    attractive and repulsive, 127.
  Freezing mixture, 120, 168.

  G

  Glacier, the source of the Rhone, 7;
    fed by mountain-snow, 7, 21;
    melted by the sun's dark rays, 13;
    terminal moraines of, 38;
    questions regarding motions of, 54-58;
    action of the ends of, 58;
    motion at top and bottom of, 80, 81;
    lateral compression of, 81, 82;
    longitudinal compression of, 84, 85;
    slow movement in winter of, 87;
    motion of Grindelwald, 94;
    motion of Great Aletsch, 94;
    motion of Morteratsch, 95;
    crevasses at the side of, 106;
    action, 146, 147;
    ice, 155;
    veined or ribboned structure of, 178;
    blue veins of, 170;
    tables, 113, 114;
    mills, 116-118;
    theory of Scheuchzer regarding, 155;
    ancient, 145-147;
    impurities thrust out by, 144;
    whiteness of a clean, 43, 170;
    measurements by Hugi and Agassiz of, 59, 60;
    epoch, 152-167.
  Glacier des Bois, description of, 38-43.
  ---- des Bossons, 56;
    mass of ice upon, 134.
  ---- of Aletsch, sand-cones of, 116.
  ---- des Périades, 51.
  ---- du Talèfre, boundary of, 53;
    width of, 92;
    chasms of the, 98.
  ---- de Léchaud, motion of, 80;
    width of, 82;
    compression of, 190.
  ---- du Géant, _névé_ of the, 49;
    motion of, 79;
    width of, 82;
    cracks above the ice-falls of, 119;
    honey-combing of, 115;
    shortening of, 190.
  ---- Unteraar, movement of, 61;
    appearance of Prof. Forbes on, 161;
    measurements by Agassiz on, 162.
  ---- Görner, sand-cones of, 116;
    description of, 140-144;
    moraines of, 143;
    advance and retreat of, 144;
    objects of interest on, 144;
    structure and bedding on, 181.
  Grand Plateau, crevasse on, 57, 118.
  Greenland's icy mountains, 21.
  Growth of knowledge, 59.
  Guesses in science, 74.

  H

  Harmony of life and its conditions, 125.
  Heat, waves of, 12;
    invisible, 14;
    office of the invisible waves of, 36;
    absorption of solar, 100;
    demanded for the liquefaction of ice, 131;
    latent, 153.
  Hoar-frost, 5, 6;
    not melted by light-waves, 13, 18.
  Hôtel des Neuchâtelois, movements of, at Riffelberg, 141.

  I

  Ice, structure of, 35, 36, 119;
    towers of, 104;
    sea, 132-134;
    retreat of, 145;
    development in the Alps of, 150;
    freezing of pieces of, 164;
    liquefaction by pressure of, 108, 160;
    translucent, 177;
    difference between hard and soft, 87.
  Icebergs, of arctic seas, 133;
    described by Sir Leopold McClintock, 133, 134;
    drifting of, 134;
    origin of, 134;
    of Switzerland, 136-138;
    colors of, 138;
    formation of chains of, 165.
  Ice-lens, concentration of sun's rays by means of, 37.
  Ice-river through the vale of Hasli, 146.
  Icicles, 99;
    how produced, 100;
    a theory of, 100-102
  Imagination, scientific use of, 34.
  Impurities thrust out by glaciers, 144.
  Infinite Wisdom, designs of, 124.

  J

  Jardin, description of, 53.
  Jungfrau, 136, 137.

  K

  Killarney, luxuriant vegetation of, 27, 151.

  L

  La Grande Jorasse, crests of, 43;
    roses of cloud about, 90.
  Lake of Geneva, an expansion of the river Rhone, 6.
  Latent heat, 153, 167.
  Lateral moraines, origin of, 54.
  Light, wave-theory of, 10, 11;
    inference from the phenomena of, 11;
    length of wave of, 12.
  Likeness of glacier-motion to river-motion, 72-76.
  Liquefaction of ice, 168, 169;
    experiment by Mr. Bottomley, 170;
    experiment by Mr. Boussingault, 170, 171.
  Locus of the point of swiftest motion, 76.
  Longitudinal crevasses, how formed, 109;
    examples of, 110.

  M

  Magillicuddy's Reeks, 27, 150, 151.
  Magnet, poles of, 32;
    repelling corners and ends of, 126.
  Märgelin See, 138; icebergs in the, 138.
  Marginal crevasses, explanation of, 106-109.
  Mauvais Pas, 41.
  Measurements of glaciers by Hugi and Agassiz, 59, 60.
  Medial moraines, how accounted for, 55.
  Mer de Glace, 41;
    its sources, 43-45;
    view of the, 44;
    branching of the, 45;
    medial moraines of the, 51, 52;
    triangulation of, 62;
    motion of, 66, 70;
    daily motion of, 67;
    unequal motion of the two sides of, 70-72;
    motion of axes of, 78;
    summer condition of, 87;
    winter on, 88-92;
    dirt-bands of, 127;
    dimensions of, 145;
    winter motion of, 93;
    greater number of crevasses on eastern side of, 112;
    glacier tables of, 113;
    grand moulin of, 117, 118;
    approximate weight of, 154.
  Molecules, of water, 31, 34;
    expansion of, 125;
    forces acting upon, 127;
    exclusiveness of water, 132, 133.
  Montanvert, auberge of the, 41, 43;
    appearance in winter, 89.
  Moraine, lateral, 41;
    medial moraines of the Mer de Glace, 51, 112;
    cedars of Lebanon growing on ancient, 150;
    explanation of the cause of ridges on, 112, 113.
  Morteratsch glacier, cause of the widening of the medial
      moraine of, 97;
    motion of, 95, 96;
    sand-cones of, 116.
  Motion, of Mer de Glace, 93;
    of Grindelwald, 94;
    of Great Aletsch glacier, 94;
    of Morteratsch glacier, 95;
    sand-cones of, 116.
  Moulins, description of, 116;
    dangers from, 117;
    sounding of, 118.
  Mountain condensers, 27, 150.

  N

  _Névé_, explanation of term, 49;
    stratification of the, 179.

  O

  Obliquity of the lateral crevasses, 167;
    illustration, 107, 108.

  P

  Petit Plateau, 56.
  Piz Bernina, route to, 95.
  Place de la Concorde of Nature, 136.
  Plastic theory, 156;
    advocated by Bordier, 157;
    advocated by Rendu, 158.
  Poles, atomic, 32, 126;
    attractive and repellent, 30.
  Pontresina, village of, 95.
  Precious stones, examples of crystallizing power, 30.
  Precipitation, 23, 25;
    atmospheric, 27.
  Promontory of Trélaporte, 44;
    of Tacul, 51.
  Proofs of glacier-motions, 58.
  Pyramid of Aiguille du Dru, 43;
    of Aiguille des Charmoz, 43.

  Q

  Quotation from Rendu, 158.

  R

  Rain, its source, 3;
    tropical, 23, 25.
  Rainfall, observations on amount of, 28.
  Regulation, theory, 163-167;
    observations made by Mr. Faraday, 164;
    formation of chain of icebergs by, 165;
    cause of, 167;
    Faraday's view of, 171-176.
  Relation of structure to pressure, 183;
    veined structure described by Guyot, 183-185;
    illustration, 186;
    connection established, 187.
  Retina, how excited, 8;
    theory of Sir Isaac Newton, 9.
  Riffelberg Hotel, location of, 144.
  Rivers, their sources, 1, 7, 19.

  S

  Sand-cones, 116.
  Scientific tacts, connection of, 72.
  Siedelhorn, view from the summit of, 146.
  Snow, its conversion into ice, 156;
    consolidation in Alpine regions, 156;
    line, 49;
    its formation in Russian ball-rooms, 5;
    in subterranean stables, 5, 14;
    in polar regions, 21;
    its architecture, 29-32, 34, 91;
    absorbs solar heat, 100.
  Solidifying power of camphor and metals, 17.
  Source of the Severn, 2;
    the Thames, 2;
    the Danube, 2;
    the Rhine, 2;
    the Rhone, 2;
    the Gauges, 2;
    the Euphrates, 2;
    the Garonne, 2;
    the Elbe, 2;
    the Missouri, 2;
    the Amazon, 2;
    Albula, 2;
    the Arveiron, 38;
    the Aar, 50.
  Stalactites of Alpine crevasses, 100.
  Steam, its condensation, 3, 4.
  Sunbeams, office of, 10-21.
  Sun, its heat the source of Alpine ice, 7;
    vibratory motion of the atoms of the, 11;
    position of, 19;
    indirect heat, of the, 28.
  Switzerland, ancient glaciers of, 145-147.

  T

  Theodolite, description of, 63;
    use of, 64, 65.
  Theory, of Dilatation, 155;
    developed by De Charpentier, 156;
    sliding, 156;
    plastic, 156-160;
    viscous, 161-103;
    regulation, 163-167;
    of glacial epoch, 155-167.
  Trade-winds, 20.
  Transverse crevasses, formation of, 104, 105.
  Trélaporte, promontory of, 44;
    motion of the water through the narrows of, 79.

  U

  Universe, order of the, 32.

  V

  Valley, of Hasli, 146, 147;
    Black, 151.
  Vapor, in the atmosphere, 5.
  Viscous theory, advocated by Prof. Forbes, 161;
    rejected by M. Agassiz, 162.

  W

  Water, changes of volume of, 118-122;
    maximum density of, 120;
    effects of expansion of, 121;
    not a solitary exception to general law, 124;
    molecular expansion of, 125-127;
    temperature necessary to freeze the sea, 132;
    special solidifying power of, 173;
    freezing-point of, 168.
  Waves, of light, 8-11, 127;
    length of, 12;
    of heat, 12.
  Whiteness of a clean glacier, 43, 176;
    of Märgelin See, 138, 176;
    of snow, 176;
    of ice formed in freezing mixtures, 177.


THE END.


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Transcriber Note

On page 61 (153), Principal was changed to Professor. Minor typos were
corrected. Some images were moved to avoid splitting paragraphs.









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Rivers, Ice and Glaciers, by John Tyndall

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