Echo is naturally magnetic, probably more so than any other planetoid—and Neal Bormon cursed softly, just to relieve his feelings, as that magnetism gripped the small iron plates on the soles of the rough boots with which the Martians had provided him. Slavery—and in the twenty-ninth century! It was difficult to conceive of it, but it was all too painfully true. His hands, inside their air-tight gauntlets, wadded into fists; little knots of muscle bulged along his lean jaw, and he stared at the darkness around him as if realizing it for the first time. This gang had plenty of guts, to shanghai men from the Earth-Mars Transport Lines. They'd never get by with it.

And yet, they had—until now. First, Keith Calbur, and then himself. Of course, there had been others before Calbur, but not personal friends of Neal Bormon. Men just disappeared. And you could do that in the Martian spaceport of Quessel without arousing much comment—unless you were a high official. But when Calbur failed to show up in time for a return voyage to Earth, Bormon had taken up the search.

Vague clews had led him into that pleasure palace in Quessel—a joint frequented alike by human beings and Martians—a fantasmagoria of tinkling soul-lights; gossamer arms of frozen music that set your senses reeling when they floated near you; lyric forms that lived and danced and died like thoughts. Then someone had crushed a bead of reverie-gas, probably held in a Martian tentacle, under Bormon's nostrils, and now—here he was on Echo.

He gave an angry yank at the chain which was locked around his left wrist. The other end was fastened to a large metal basket partly filled with lumps of whitish-gray ore, and the basket bobbed and scraped along behind him as he advanced. Of the hundred or more Earthmen, prisoners here on Echo, only seven or eight were within sight of Bormon, visible as mere crawling spots of light; but he knew that each was provided with a basket and rock-pick similar to his own. As yet he had not identified anyone of them as Keith Calbur. Suddenly the metallic voice of a Martian guard sounded in Bormon's ears.

"Attention. One-seven-two. Your basket is not yet half filled, your oxygen tank is nearly empty. You will receive no more food or oxygen until you deliver your quota of ore. Get busy."

"To hell with you!" fumed Bormon—quite vainly, as he well knew, for the helmet of his space suit was not provided with voice-sending equipment. Nevertheless, after a swift glance at the oxygen gauge, he began to swing his rock-pick with renewed vigor, pausing now and then to toss the loosened lumps of ore into the latticed basket. On Earth, that huge container, filled with ore, would have weighed over a ton; here on Echo its weight was only a few pounds.

Neal Bormon had the average spaceman's dread of oxygen shortage. And so, working steadily, he at last had the huge basket filled with ore—almost pure rhodium—judging by the color and weight of the lumps. Nearby, a jagged gash of light on the almost black shoulder of Echo indicated the location of that tremendous chasm which cut two-thirds of the way through the small asteroid, and in which the Martians had installed their machine for consuming ore.

Locating this gash of light, Bormon set out toward it, dragging the basket of ore behind him over the rough, rocky surface.

The ultimate purpose of that gargantuan mechanism, and why this side of the planetoid apparently never turned toward the sun, were mysteries with which his mind struggled but could not fathom.

Presently, having reached the rim of the abyss, with only a narrow margin of oxygen left, he commenced the downward passage, his iron-shod boots clinging to the vertical wall of metallic rock, and as he advanced this magnetic attraction became ever more intense. The blaze of lights before him grew brighter and seemed to expand. Dimly, two hundred yards over his head, he could glimpse the opposite wall of the chasm like the opposing jaw of an enormous vise.

He joined the slow-moving stream of workers. They were filing past a guard and out on a narrow metal catwalk that seemed to be suspended—or rather poised—by thin rods in close proximity to a spacious disk which extended from wall to wall of the chasm. They moved in absolute silence. Even when tilted ore-baskets dumped a ton or more ore into the gaping orifice in the center of the disk, there was still no sound—for Echo, small and barren of native life, lacked even the suggestion of a sound-carrying atmosphere.

And that weird soundlessness of the action around him brought a giddy sense of unreality to Neal Bormon. Only the harsh, mechanical voice of the Martian guard, intoning orders with cold and impersonal precision, seemed actually real.

"Attention. One-seven-two. Dump your ore...."

These Earthmen were apparently known by numbers only. Bormon's own number—172—was on a thin metal stencil stretched across the outer surface of the glass vision plate of his helmet; he couldn't forget it.

He obeyed the Martian's order. Then he noticed that men with empty baskets were moving along a curved ramp, like a corkscrew, which led to a different level, whether above or below he could not possibly tell without a distinct mental effort.

He decided it was to a lower level as he moved onward, for the huge disk lost its circularity and became like the curving wall of a cylinder, or drum, down the outside of which the ramp twisted. Fresh ore was also being brought from this direction. And seeming to extend out indefinitely into blackness was a misty shaft, like the beam of a searchlight. Presently the ramp gave way to a tunnel-like passage.

Flexible metal-sheathed tubes dangled from the ceiling. These tubes were labeled: OXYGEN, WATER, NUTRIENT.

Bormon, patterning the actions of those he observed around him began to replenish his supply of these three essentials to life. His space suit was of conventional design, with flasks in front for water and nutrient fluid, and oxygen tank across the shoulders. By attaching the proper tubes and opening valves—except the oxygen inlet valve, which was automatic—he soon had his suit provisioned to capacity.

He had just finished this operation when someone touched his arm. He glanced up at the bulky, tall figure—an unmistakable form that even a month's sojourn on Echo had not been able to rob of a certain virility and youthful eclat.

For a moment they stared into each other's eyes through the vision plates of their helmets and Bormon was struck dumb by the change, the stark and utterly nerve-fagged hopelessness expressed on Keith Calbur's features.

Then Calbur tried to grin a welcome, and the effect was ghastly!

For a moment his helmet clicked into contact with Bormon's.

"Neal," he said, his voice sounding far away, "so they got you, too! We can't talk here.... I'm pretty well shot. Lived in this damn walking tent for ages. No sleep, not since they took me.... Some powder, drug, they put in the nutrient fluid—it's supposed to take the place of sleep—and you can't sleep! Only it doesn't.... You come along with me."

The darkness swallowed them up. Bormon had thrown his rock-pick into his empty basket. And now, by keeping one hand in contact with Calbur's basket, as it bobbed and jerked on ahead, he was able, even in the inky blackness, to keep from straying aside.

After seemingly interminable groping and stumbling, Calbur's light flashed on. They had entered a pocket in the rocks, Bormon realized, a small cavern whose walls would prevent the light from betraying their presence to the guard.

Calbur threw himself exhaustedly down, signifying that Bormon should do likewise, and with their helmets touching, a strange conversation ensued.

Bormon explained, as well as he was able, his presence there.

"When you didn't show up, Keith, in time to blast for Earth," he said, "all we could do was to report your absence to the space police. But they're swamped; too many disappearances lately. Moreover, they're trying to relocate that stream of meteoric matter which wrecked a freighter some time back. They know something is in the wind, but they'll never guess this! For weeks they've had the patrol ship, Alert, scouting around Mars. So, after making the run to Earth and back to Mars—I had to do that, you know—I got back in Quessel again and commenced to pry around, sort of inviting the same thing to happen to me that had happened to you—and here we are."

"We're here for keeps, looks like," answered Calbur grimly, his voice having lost part of that overtone of strained nerves. "A man doesn't last long, so the other prisoners say, two months at the most. These Marts use Earthmen because we're tougher, here at least, and last longer than Marts.... Hell, what wouldn't I give for a smoke!"

"But the purpose, Keith? What's the scheme?"

"I thought you knew. Just Marts with fighting ideas—a crowd backed by wealthy, middle-class Martians who call themselves Lords of Conquest. They're building ships, weapons. First, they're going to take over Mars from the present government, which is friendly to Earth, and then they're going to subdue Earth."

Calbur had switched off his light, as a matter of precaution, and his voice came to Bormon from a seemingly far distant point—a voice from out of the darkness, fraught with fantastic suggestion.

"Ships? You say they're building ships? Where?" Bormon asked, his own voice reverberating harshly within the confines of his helmet.

"In a cavern they've blasted out near the south magnetic pole of Mars. You know that's an immense, barren region—lifeless, cold—bordered on the north by impenetrable reed thickets. They need rhodium in large quantities for hull alloys and firing chambers. That's why they're mining it, here on Echo."

"They'll never get it to Mars," Bormon declared quickly. "Every freighter is checked and licensed by the joint governments of Earth and Mars."

"They won't?" Calbur laughed, distantly. "Listen, Neal—every crateful of ore that's dumped into their machine, here on Echo, gets to Mars within a few hours. And it isn't carried by ships, either!"

"You mean—?"

"I didn't get the answer, myself, until I'd been here for some time. You see, Echo is just a gob of metal—mostly magnetite, except for these granules of rhodium—forty miles in diameter, but far from round. Then there's that chasm, a mammoth crack that's gaped open, cutting the planetoid almost in half. The whole thing is magnetic—like a terrestrial lodestone—and there's a mighty potent field of force across that gap in the chasm. The walls are really poles of a bigger magnet than was ever built by Martians or human being. And of what does a big magnet remind you?"

After a moment of thought, Bormon replied, "Cyclotronic action."

There was a short silence, then Calbur resumed. "These Marts shoot the ore across space to the south magnetic pole of Mars. A ground crew gathers it up and transports it to their underground laboratories. As a prisoner explained it, it was simple; those old-time cyclotrons used to build up the velocity of particles, ions mostly, by whirling them in spiral orbits in a vacuum-enclosed magnetic field. Well, there's a vacuum all around Echo, and clear to Mars. By giving these lumps of ore a static charge, they act just like ions. When the stream of ore comes out of the machine, it passes through a magnetic lens which focuses it like a beam of light on Mars' south pole. And there you have it. Maybe you saw what looked like a streak of light shooting off through the chasm. That's the ore stream. It comes out on the day side of Echo, and so on to Mars. They aim it by turning the whole planetoid."

"Hm-m-m, I understand, now, why it's always dark here—they keep this side of Echo facing away from Mars and the sun."

"Right," said Calbur. "Now we'll have to move. These Marts are heartless. They'll let you die for lack of oxygen if you don't turn in baskets of ore regularly. But we'll meet here again."

"Just give me time to size things up," Bormon agreed. The effects of the reverie-gas was wearing off and he was beginning to feel thoroughly alive again and aware of the serious situation which confronted them. "Don't let it get you down, Keith," he added. "We'll find a way out."

But his words expressed a confidence that the passing of time did not justify.

Again and again he filled his ore-basket, dragged it to the hungry mouth of that prodigious mechanism in the abyss, and in return he received the essentials for continued life.

During this time he formed a better idea of conditions around him. Once he wandered far from the Martian's headquarters, so far that he nearly blinded himself in the raw sunlight that bombarded the day side of the tiny planetoid. Again, he was strangely comforted with the discovery of a small space ship anchored deep in the abyss although he was not permitted to go near it.

He soon found that nothing was to be expected of the horde of Earthmen who slaved like automatons over the few miles of Echo immediately adjacent to the chasm's rim. The accumulative effect of the drug seemed to render them almost insensible of existence.

But with Calbur, who had served for a shorter time, it was different.

"Keith, we've got to tackle one of the Mart guards," Bormon told him, during one of their conferences in the cave. "We'll take its ray-tubes, fight our way to that ship they've cached in the chasm below the cyclotron power plant, and blast away from here."

"How?" asked Calbur. "If you make a move toward one, it'll burn you down—I've seen it happen!"

"Listen, I've spent hours figuring this out. Suppose one of us were to stay here in this cave, helmet-light on, and near enough to the opening so that his light would show dimly on the outside. Wouldn't a Mart guard be sure to come along to investigate?"

"Yes, practically sure," agreed Calbur, but with no great interest. Hour by hour he was sinking closer to that animate coma which gripped the other Earthmen. "But what would that get you? If you lose too much time, you'll be cut off from rations."

"I know, but suppose also that one of us—I, for instance—was hiding in the rocks above the cave, with a big chunk of ore, ready to heave it down on the Mart?"

Calbur seemed to be thinking this over, and for a moment there was silence.

"When shall we try it?" he demanded suddenly, and there was a note of eagerness and hope in his voice. "It's simple enough. It might actually work."

"Right now! If we put it off, it'll soon be too late."

They discussed details, laying their plans carefully, Bormon prudently refraining any suggestion that this move was one born of sheer desperation on his part.

Everything settled, Calbur moved up near the opening, so that his helmet-light could be dimly seen from outside the cave. Bormon, dragging his ore-basket, climbed up in the rocks directly over the entrance, and presently found concealment that suited him. Near at hand he placed a loose chunk of rock which on Earth would have weighed perhaps eighty pounds. The trap was set.

He settled himself to wait. His own light was, of course, extinguished. Far off he could see crawling blobs of luminance as guards and human workers moved slowly over the surface of Echo. Otherwise stygian darkness surrounded him. But he had chosen a position which, he hoped, would not be revealed by the light of any Martian bent on investigating the cave.

There were, he had learned, actually less than a score of Martians here on Echo; about half of them stayed around that cyclotronic ore-hurler in the chasm. They depended on secrecy, and were in constant communication, by ether-wave, with spies not only on Earth and Mars but among the personnel of the space police itself. These spies were in a position to warn them to shut down operations in case the ore stream through space attracted notice and was in danger of being investigated. It was all being conducted with true Martian insidiousness.

Thus Bormon's thoughts were wandering when, at last, he became aware that a Martian guard was approaching. His cramped muscles suddenly grew tense. His heart began to pound; it was now or never—and he must not fail!

The Martian, reeling along rapidly on the mechanical legs attached to its space armor, appeared to suspect nothing. It approached amid a rosette of light which seemed to chase back the shadows into a surrounding black wall. It had evidently seen the gleam of Calbur's helmet-light, for it was heading directly toward the mouth of the cave above which Bormon crouched.

The moment for action arrived. Tense as a tirhco spring, Bormon leaped erect, hurled the jagged lump of rock down on the rounded dome of the Martian's armor. Then, without pausing to ascertain the result, he grasped the rim of his ore-basket and swinging it in a wide arc before him, leaped downward—

For a moment Martian, basket and Earthman were in a mad tangle. Bormon realized that the Martian had been toppled over, and that one of its ray-tubes was sending out a coruscating plume of fire as it ate into the rocks. The moment seemed propitious to Bormon!

Hands gripping and searching desperately, he found the oddly-shaped clamp that bound the two halves of the Martian's space armor together—and released it.

There was a hiss of escaping gas. Abruptly those metal handlers ceased to thrash about....

Bormon, thrilling with success, rose to his feet, turned off the Martian's ray-tube just as Calbur, delayed with having to drag his ore-basket, through the rather narrow opening, dashed into view.

There was no need for words. Bormon handed him a ray-tube.

Within a matter of seconds, each had burned through a link of the chain around his wrist. They were free from those accursed baskets! Calbur secreted the weapon in a pouch of his space suit, then swiftly they set to work, for their next move had been carefully planned.

Opening the armor fully, they began to remove the dead Martian, puffed up like a kernel of pop-corn by the sudden loss of its air pressure.

Having cleared the armor, Bormon climbed inside—space suit and all—folding up like a pocket knife so as to resemble somewhat the alien shape it was intended to hold, and tested the semi-automatic controls. Everything appeared to be in working order. Assuring himself of this as well as his knowledge of Martian mechanics would permit, he crawled out again to help Calbur.

Calbur was scrambling to collect ore. And under their combined efforts one of the baskets was presently filled—for the last time, Bormon fervently hoped!

Again he entered that strange conveyance, the Martian's armor, and after some experimental manipulation of the push-button controls, managed to get the thing upright on its jointed, metal legs and start it moving awkwardly in the direction of the chasm.

Behind him came Calbur, dragging the basket of ore—for lacking a disguise such as Bormon's, he must have some excuse for returning to the cabin, and he had wrapped the chain around his wrist to conceal the fact that it had been severed.

Bormon, in the narrow confines of his armor, disconnected the mechanical voder used by its deposed owner, for all Martians are voiceless.

His greatest fear was that one of the Martian guards would attempt to communicate with him. This would disclose the imposture immediately, since he would be unable to reply. For all Martian communication, even by ether-wave, is visual—the medium being a complicated series of symbols based on their ancient sign language, the waving of tentacles, which no human brain has ever fully understood. The means of producing these conventionalized symbols was a tiny keyboard, just below an oval, silvery screen, and as Bormon sent his odd conveyance stalking down the side of the chasm, toward that sweeping disk which he now knew to be formed by the ends of two cyclotronic D-chambers facing each other, he kept one eye on this silvery screen, but it remained blank.

He moved on down past the catwalk to the lower ramp. Here he must pass close to a Martian guard.

But this Martian seemed to give him no attention whatever.

Reaching a point opposite the ship, Bormon stepped from the ramp. Still that oval screen remained blank. No Martian was apparently paying enough attention to him to question his movements.

Again he caused the armor to advance slowly, picking his way along the rock surface. He reached the ship.

For a moment he was hidden behind the hull. One glance sent his hopes plunging utterly. Neither of the two fuel caps were clamped down, which could mean but one thing—the ship's tanks were empty!

It was a stunning blow. No wonder the Martians felt safe in leaving the ship practically unguarded. After a moment, anger began to mount above Bormon's disappointment. He would start to kill off Martians! If he and Calbur couldn't get away from Echo, then he'd see that at least some of these Marts didn't either. He might even wipe them all out. Calbur, too, had a ray-tube.

But what of Calbur? Quickly Bormon moved from behind the ship. Calbur was loitering on the ramp, ore-basket empty, evidently on the point of making a break to join him.

Frantically, Bormon focused the ether-wave on Calbur's helmet, hurling a warning.

"Stay where you are. It's a washout! No fuel...."

He began moving across the rocks toward the power-plant. That was the most likely spot to commence—more Marts close at hand. He'd take them by surprise.

Suddenly he was cold, calculating, purposeful. After all, there wasn't much chance of wiping them all out—and yet he might. He should strike at a vital point, cripple them, so as to give Calbur and the others a chance in case he only managed to kill a few before passing out of the picture.

A glittering neutrochrome helix on top of the power-plant gave him a suggestion. Why not destroy their communications, fix things so they couldn't call for help from Mars?

Abruptly he realized something was wrong. That silver oval six inches from his face was flashing a bewildering complexity of symbols. Simultaneously the Martian on the ramp began to move quickly and questioningly toward him.

The moment had arrived. Bormon swung the metal handler bearing the ray-tube into line and pressed the firing button....

Amid a splatter of coruscating sparks the Martian went down.

"Number one!" growled Bormon. Everything now depended on prompt action and luck—mostly luck! As quickly as possible he heeled around, aimed at the helix on the power-plant. It swayed slowly as that pale blue shaft ate into its supports, then drifted away.

He had lost sight of Calbur. Absolute silence still reigned, but on airless Echo that silence was portentous. Along the rim of the chasm he could see the glitter of Martian armor against the blackness of space. The alarm had been given. But for the moment he was more concerned with the imminent danger from those who tended the intricate controls in the power-plant, and the guard at the far end of the catwalk. This guard was protected by the catwalk itself and the stream of Earthmen slaves still moving uncomprehendingly along it. Bormon sent his space armor reeling forward, intent on seeking shelter behind the bulk of the power-plant.

He almost reached that protection. But suddenly sparks plumed around him, and his armor slumped forward—one leg missing. He fell, fortunately, just within the shelter of the power-plant.

Desperately he struggled to open the armor, so as to get the ray-tube in his own hand. But when he finally crawled forth it was to face three Martians grouped around him, their weapons—six in number—unwaveringly centered on him.

"Earthman," said the mechanical speaker coldly inside his helmet, "you have killed a Martian."

And then, with true Martian decisiveness and cruelty, they pronounced inhuman judgment on him.

"We in our kindness shall not immediately demand your life as forfeit. You shall wander unhindered over Echo, dying slowly, until your oxygen is gone. Do not ask for more; it is sealed from you. Do not again enter the chasm; it is death to you. Now go."

Hours later Bormon was indeed wandering, hopeless as a lost soul, over nighted Echo, awaiting the consummation of his sentence, which now seemed very near. Already his oxygen gauge indicated zero and he was face to face with the "dying slowly" process promised by the Martians—the terrible death of suffocation.

Now, as things began to seem vague and unreal around him, Bormon was drawing near that hidden cave where he and Calbur had often met for like a final flash of inspiration had come the thought that here, if anywhere, he would find Calbur.

It was strange, he reflected, how the life in a man forces him on and on, always hoping, to the very end. For now it seemed that the most important thing in the universe was to find Calbur.

He had husbanded the last of his oxygen to the utmost. But panting, now, for breath, he opened the valve a fraction of a turn and staggered on in the darkness. And suddenly, dimly as in a dream, he knew that at last he had found Calbur....

And Calbur was doing a queer thing. Gauntleted hands moving hastily in the chalky radiance cast by his helmet-light, he was tossing chunks of rhodium from his filled ore-basket—

Then their helmets clicked together, and he heard Calbur's voice, faint, urgent:

"Climb in the basket! I'll cover you with ore so they won't see you. I'll drag you in. Well get your tank filled—I swear it!"

The next instant, it seemed, Bormon felt himself being tumbled into the ore-basket. Chunks of ore began pressing down lightly on his body. Then the basket commenced to pitch and scrape over the rocks.

But his lungs were bursting! Could he last? He had to. He couldn't fool Calbur by passing out—not now. Something like destiny was working, and he'd have to see it through.

Something was tapping on his helmet. Bormon opened his eyes, and light was trickling down between the chunks of ore. No longer was there any scraping vibrations. Something, metallic, snakelike, was being pressed into his hand.

And then Bormon remembered. The oxygen tube! With a final rallying of forces only partly physical, he managed to stab the tube over the intake of his tank. The automatic valve clicked and a stream of pure delight swept into his lungs!

For a time he lay there, his body trembling with the exquisite torture of vitality reawakening, slowly closing the helmet-valve to balance the increase of pressure in the tank.

Suddenly that snakelike tube was jerked away from between the chunks of ore, and again the basket began a scraping advance.

Bormon's new lease on life brought its problems. What was about to happen? In a moment, now, Calbur would be ordered by the guard to dump his ore. They wouldn't have a chance, there on the catwalk. For Bormon's abrupt reappearance would bring swift extinction, probably to both.

The basket stopped. They had reached the ore-dump. Calbur's head and shoulders appeared. Behind the vision plate in his helmet there was a queer, set expression on his thin face. He thrust the ray-tube into Bormon's hands.

Bormon sprang erect, leaped from the basket. For a moment he stared around, locating the guard at the end of the catwalk. As yet the guard appeared not to have noticed anything unusual. But where was Calbur?

"Attention. One-six-nine. Dump your ore," ordered the guard, coldly, mechanically.

Something seemed to draw Bormon's eyes into focus on his own number stencil. One-six-nine, he read. Calbur's number! And then, suddenly, he realized the dreadful, admirable thing Keith Calbur had done....

For Calbur had leaped through the ore-chute, into the cyclotron's maelstromic heart! Despairing, he had chosen a way out. He had forfeited his life so that Bormon could take his place.

"Dump your ore," repeated the Martian guard, coldly.

"To hell with you!" snarled Bormon, and blasted with the tube.

He missed the Martian. Still weakened by the ordeal he had just passed through, and overwrought as an effect of Calbur's last despairing act, his aim was not true. Nevertheless, that coruscating shaft was fraught with far-reaching consequence. Passing three feet to the left of the Martian, it snapped two of the rods which braced the catwalk in position over the cyclotron drum. Thus released at the far end, the metal ribbon—for the catwalk was little more than that—curled and twisted like a tirhco spring, pitching Bormon, as from a catapult, straight along the path so recently chosen by Calbur.

Destiny had indeed provided them both with a strange exit from Echo, for in that split second Bormon realized that he was being hurled squarely into the gaping orifice of the cyclotron.

Far out in the vacuity between Echo and Mars, Captain Dunstan sat in his cabin aboard the Patrol Ship Alert—most powerful and, therefore, speediest craft possessed by the Earth-Mars Space Police.

On his desk lay two jagged pieces of ore, whitish-gray in color, which he had been examining.

His speculations were interrupted by the sudden bursting open of the cabin door. An officer, spruce in gray uniform and silver braid, entered hurriedly, his face flushed with excitement.

"Captain Dunstan, the most extraordinary thing has happened! We've just picked up two men—two men drifting with the meteoric stream, and in space suits—and they're alive!"

Captain Dunstan rose slowly. "Alive, and adrift in space? Then it's the first such occurrence in the history of space travel! Who are they?"

"I don't know, sir. So far we've got only one out of his suit. But I have reason to believe they're the men recently reported as missing by the E.M.T. Lines. He babbled something about Echo—that there's hell to pay on Echo. I imagine he means Asteroid No. 60. But—"

"Lead the way," said the captain, stepping quickly toward the doorway. "There's something mighty queer going on."

And so, by a lucky break, Neal Bormon found himself snatched from death and aboard the Alert, arriving there by a route as hazardous and strange as was ever experienced by spaceman.

And no less strange and unexpected came the knowledge of Keith Calbur's arrival there ahead of him.

Bormon, who was last to be drawn in by the grapple-ray and helped out of his space suit by the willing hands of the Alert's crew, was still capable of giving an understandable account of things; although Calbur, until the effects of the Martian drug wore off, would be likely to remain in his somewhat neurotic condition of bewilderment.

"These Marts," said Bormon, after a great deal of explaining on both sides, "don't know that you have discovered their stream of ore. They won't know it until their communications have been repaired."

Captain Dunstan nodded. "That explains why we were able, on this occasion, to approach the meteoric stream without its immediate disappearance. But I cannot understand," he confessed, "how two men could have passed through such an apparatus as you describe, and remain alive."

"Perhaps I can offer a possible explanation," said an officer whose insignia was that of Chief Electrobiologist. "If, as we suspect, this Martian invention is founded on the old and well-known cyclotronic principle, then we have nothing but reciprocal interaction of electric fields and magnetic fields. And these fields, as such, are entirely harmless to living organisms, just as harmless as gravitational fields. Moreover, any static charge carried by the bodies of these men would have been slowly dissipated through the grapple-ray with which they were drawn out of the ore stream."

This explanation appeared to satisfy the captain. "You say," he questioned, addressing Bormon, "that there are other men on Echo—Earthmen being used as slaves?"

"Yes, more than a hundred."

Captain Dunstan's mouth became a fighting, grim line. He gave several swift orders to his officers, who scattered immediately.

Somewhat later, Bormon found his way into the surgery where Calbur lay—not sleeping yet, but resting peacefully.

Assuring himself of this, Bormon, too, let his long frame slump down on a near-by cot—not to sleep, either, but to contemplate pleasantly the wiping-up process soon to take place on Echo, and elsewhere.