Planetfall
(1957)
Hal Clement

Contents

	Chapter I
	Chapter II
	Chapter III
	Chapter IV
	Chapter V
	Chapter VI
	Chapter VII
	Chapter VIII

Chapter I

	A conservation service vessel is quite fast and maneuverable as craft of that general type go. But there was little likelihood that this one would catch up with its present target. Its pilot knew that. He had known it since the first flicker of current in his detectors had warned him of the poacher's presence. But with the calm determination so characteristic of his race, he made the small course-correction which he hoped would bring him through the target area at action speed. 

	The correction had to be small. Had the disturbance been far from his present line of flight, he would never have detected it, for his instruments covered only a narrow cone of space ahead of him. Too many pilots in the old days, with full-sphere coverage, had been unable to resist the temptation of trying to loop back to investigate disturbances whose source-areas they had already passed. 

	At one-third the speed of light, such a reversal of course would have wasted both energy and time. No one could make a reversal in any reasonable period, and, certainly, no poacher or other law-breaker was going to wait for the maneuver to be completed. 

	Even as it was, this pilot's principal hope lay in the possibility that the other vessel would be too preoccupied with its task of looting to detect and react to his approach in time. Detection was only possible if, like his own ship, the poacher carried but a single operator. Unfortunately, a freighter was quite likely to have at least two, even on a perfectly legal flight, and the Conservation pilot had known of cases where poaching machines had had crews as large as four. 

	Even the presence of two would render his approach almost certainly useless, since the loading and separating machinery would require only one manipulator, and the full attention of any others could be freed for lookout duty. Nevertheless, he bored on in, analyzing and planning as he traveled. 

	The poacher was big—as big as any he had ever viewed. It must have had a net load capacity of something like a half billion tons—enough to clean the concentrates off a fair-sized planet, particularly if it also boasted adequate stripping and refining apparatus. There was no way of making certain about this last factor, for no such equipment was drawing power as yet. And that, in a way, was peculiar, for the poacher must have been in his present position for some time. 

	Had the driving energies of the poacher been in use, the Conservation ship would have detected them long before, and would have experienced less difficulty in making the necessary course-change. With a scant five light-years in which to make the turn, the acceleration needed for the task was rather annoying. Not that it caused the pilot any actual physical discomfort. It was purely an emotional matter. His economy-conditioned mind was appalled by the waste of energy involved. 

	Four light-years lay behind him when the poacher reacted outrageously. For the barest instant the attacker dared to hope that he might still get within range. Then it became evident that the giant freighter had seen him long before, and had planned its maneuver with perfect knowledge of his limitations. 

	It began to accelerate almost toward him, at any angle which would bring it safely past. It would sweep past just out of extreme range if he kept on his present course —and probably well beyond trustworthy shooting distance, if he tried to intercept it. For an instant, the, agent was tempted. But before a single relay had clicked in his own small craft he remembered what the poacher must already have known—that the planet, which had perhaps already been robbed, came first. 

	It must be checked for damage, even though it was uninhabited as far as anyone knew. The mere fact that the poacher had stopped there meant that it must have something worth taking. It must, therefore, be tied as soon as possible into the production network whose completeness and perfection was the only barrier between the agent's race and galaxy-wide starvation. 

	He held his course, therefore, and broadcast a general warning as he went. He gave the thief's specifications, its course, as of the last possible observation, plus the fact that it seemed to be traveling empty. The absence of cargo was an encouraging sign. Perhaps no damage had been done to the world ahead. Unfortunately, it might also mean that the raider had a higher power-to-mass ratio than any freighter the agent had ever seen or heard of. He assumed that the ship was without cargo, and worded his warning accordingly. 

	His temper was not improved by an incident which occurred just before the giant vessel passed beyond detection range. A beam, quite evidently transmitted from the fleeing mass of metal, struck his antenna, and the phrase—"Now, don't you just hope they'll get us!"—came clearly along the instrument. 

	Again, relays almost closed on the Conservation flier, but the agent contented himself with repeating his warning broadcast and adding to it the data which had inevitably come along with the poacher's taunt—data concerning the personal voice of the speaker. Then he turned his attention to the problem of the planet ahead. 

	He would need more energy, of course. The interstellar speed of his craft had to be reduced to the general velocity of the stars in this part of the galaxy, for he could not make the survey that would be needed, merely by viewing the planet as he flashed by. He could, of course, get a pretty good idea of the metals that were present through such flash-technique, but he needed information as to their distribution. If he were lucky—if the poacher had actually failed to load up—there would almost certainly be concentrates worth recording and reporting to Conservation. 

	The sun involved was obvious enough, since it was the only one within several light years. The agent thought fleetingly of the loneliness, even terror, which would descend upon the average ground-gripper in close proximity to the nearly empty space at the galaxy's rim, and timed and directed his deceleration to bring him to rest some twenty-four diameters from the sun's photosphere. 

	The poacher had begun to travel long before he drew close enough to detect individual planets, and he was faced with the problem of discovering just which planet or planetoid had been visited. There were certainly enough to choose among and he was reasonably sure he had detected them all as he approached. 

	The possibility that he had been moving directly toward one for the whole time, and had, as a result, failed to observe any apparent motion for it, was too remote to cause him concern, particularly since it turned out that he had been well away from the general orbital plane of the system. He had the planets, then. But which ones were important? 

	Since he would have to check them all anyway, he didn't worry too much about selection. After using up the energy and time needed to stop in this forlorn speck of a planetary system, it would be senseless to leave anything unexamined. Why, he reasoned, should anyone else have to come back later to do what he had left undone? Still, he thought, it would be pleasant to determine quickly what the poacher had accomplished, if anything. 

	The innermost planet was definitely not the plundered victim. It had plenty of free iron, of course, and the agent noted with satisfaction that the metal was not concentrated at its core. If it ever became necessary to seek iron so far out in the galaxy, stripping it from so small a world would be relatively easy. 

	However, the important metals seemed to be dissolved and distributed with annoying uniformity through the tiny globe—a fact which was hardly surprising. The planet was too small, and its temperature was too high to permit either water or ammonia to exist in liquid form. The ordinary geological processes which produced ore deposits simply could not function here. 

	The second world was more hopeful—in fact, it seemed ideal on first survey. There was water, though not in abundance. Nevertheless, in the billions of years since the planet had formed a certain amount of hydrothermal activity had gone on in its crust, and a number of very good copper, silver, and lead concentrations appeared to exist. The agent decided to land and map these, after he had completed his preliminary survey of the system. If this were the world the poachers had been sweeping, they had evidently failed to get much. Venus might be the plundered planet. 

	It proved not to be, however. Earth's water is not confined to its lithosphere—it covers three-quarters of the planetary surface. It washes mountains into the seas, freezes at the poles and, at high elevations, even at the equator. It finds its way down into the rocks and joins other water molecules which have been there since the crust solidified. It picks up ions, carries them a little way, and trades them for others. 

	In short, Earth contains enough water to produce geological phenomena. The agent saw this almost in his first glance. He wasted a brief look at the encircling dry satellite, then he turned all of his attention on the primary planet itself. He even began to ease his ship outward from the orbit it had taken up, twenty million miles from Sol. 

	This, he decided, must be the world of the poacher's selection. Even without analysis, anyone with the rudiments of a geological education would know that there must be metal concentrations here—and a civilization that uses half a trillion tons of copper a year can be expected to have at least a few trained geologists. 

	The agent pointed the nose of his little cruiser at the tiny disc, shining brightly eighty million miles away. He drove straight toward it, combing its surface as he went with the highest-resolution equipment he could bring to bear. All over the surface, and for a mile below, those radiations probed and returned with their information. The agent swore luridly as the indicators told their tragic story. 

	There had been concentrations, all right. There were still a few. But someone had been scraping busily at the best of them, and had left little that was economically worth recovering. It was the old story. If good deposits and poor ones were worked at the same time, the profit was of course smaller. But at least the deposits lasted longer. 

	An eternity had passed since any legal operator of the agent's race had worked the other way, stripping the cream for a quick profit and letting the others go. Such a practice would have crippled the industry of the agent's home planet millions of years before, had it not been checked sternly by the formation of the Conservation Board. 

	Crippled industry, to a race at the stage of development his had attained, was the equivalent of a death sentence. Not one in a thousand of his people could hope to escape death by starvation, if the tremendously complex system of commerce were to break down. 

	The agent knew that—like most of his profession, he had seen border worlds where momentary imperfections in the system had taken their toll. 

	His fury at the sight of this planet mingled with—and was fed by—the memory of the horrors he had seen. Apparently, he had been wrong. The poachers had gotten away with their load—in fact, scores of them must have been at work. 

	No one ship, not even the monster he had seen so recently, could have done such a job without assistance on a planet of this size. The Conservation Department had suspected, before now, that it faced a certain degree of organization among the poachers. Here was infuriating evidence that the suspicion was all too well-founded. 

	Thought followed reaction through the agent's reception apparatus and through his mind, before his ship was within a million miles of the planet. 

	At that range no precise mapping was possible. In a sense, surface-mapping was no longer necessary, since the surviving deposits were hardly worth the gathering—but the tectonic charts would have to be obtained as usual. 

	A world like this was in constant change. A million, or ten, or a hundred million years from now the natural processes within its crust would have brought new concentrations into being. These forces must be charted, so that proper predictions could be obtained. Only through such research and prediction could Conservation beat the poachers to the next crop of metal, when it appeared. 

	The agent began to decelerate again, now matching his velocity with that of the planet itself. At the same time, he began a more detailed analysis of the surface, refining it constantly as the distance diminished. The water he already knew about. He had supposed the gaseous envelope to consist of methane and water vapor, with perhaps some ammonia, formed at the same time as the rest of the planet. But his instruments told a different story. 

	Earth had lost its primary atmosphere. The tragedy had occurred before the first member of the agent's race had ventured away from his own planetary system. The agent found the free oxygen, and swore again. He knew what that meant—photosynthesis. The planet was infected by those carbon compounds that behaved almost like life, except for their ferociously rapid rate of reaction. 

	They were not very dangerous, of course, but due care had to be maintained. A good many planets in the liquid-ammonia-liquid-water temperature range had them, and techniques had long since been worked out for conducting analysis, and even for mining in their presence, destructive as they often were to machinery. 

	The Conservation vessel, naturally, was constructed of alloys reasonably proof against any attack by free oxygen or the usual run of the carbon compounds. In fact, if this world had any unique developments of the latter the agent could always lift his ship out of the atmosphere. Such a retreat seemed to put a stop to the growth of photosynthetic life. 

	It never occurred to the agent that concealment might be in order. In the first place, he was on a perfectly legal mission. In the second place, he didn't think that there might be anyone on the planet to observe his arrival. 

	Oxygen being what it is, he had automatically classified the world as uninhabited and uninhabitable. As a result, the events of the half-second following his machine's penetration of Earth's ozone layer demanded a rather drastic revision of his outlook. 

	The radar beams, for an instant, made him suppose that another ship was on this world, and was trying to communicate with him. He had almost begun to answer before he realized that the radiation was not modulated, and could hardly be speech—or, more accurately, that its modulation was too simple and regular to represent words. Even though such radiation did not mean intelligence, however, it obviously did imply the presence of life. 

	Somehow, an organism must have evolved in an oxygen atmosphere with the ability to reduce metal oxides or sulfides, and keep them reduced to free metal. At the moment, it seemed to be a low order of life. But if it continued to develop as the agent's own species had done, his corner of the galaxy might become rather an interesting place in time. A man might have drawn a somewhat similar conclusion from hearing the chirp of a cricket under analogous circumstances. 

	At first, the agent supposed the radiation to have meaning similar to that of the cricket's chirp, too—it came and it went, regularly and monotonously, from a seemingly fixed source, and had an apparent willingness to go on until the sun cooled. But, a few milliseconds after the first pulses struck his receptors, others began to come in. They shared the simplicity of pattern shown by the first, but there were more of them. 

	As the ship moved, and its distance from some of the sources changed, it became evident that the waves were being directed in beams, rather than broadcast in all directions—and that the beams were following the ship. Intelligent or not, something was at least aware of his presence. 

	A score of hypotheses ran through the agent's mind during the next few milliseconds, for thought can move rapidly, when the neurons involved are of metal, and the impulses they carry are electronic currents, rather than potential differences between the surfaces of a colloid membrane. But none of these theories managed to satisfy him. 

	Even he could not continue to theorize at the moment either—for the hull of his vessel was glowing bright red and the surface of the planet was coming up rather rapidly to meet him. He had to land within the next few seconds, assuming that he did not want to do his theorizing hanging motionless in the atmosphere. 

	The outer surface of his hull was a trifle hard to manage at its present temperature. But none of the myriad of relays further in had been affected and the sliver of metal obeyed his thoughts as it always had, slowing to a dead halt a few yards above the surface and then settling down for a landing while the agent analyzed the material directly underneath. 

	It was pure luck that there was no vegetation below him—luck, at least, for any local fire-fighters. The hot walls did respond to control, albeit a trifle sluggishly. Particles of sand and clay, coming in contact with the hull, began to dance, like bits of sawdust on a vibrating plate. And like sawdust, the dance carried them into a particular pattern. 

	The pattern took the form of a hollow under the hull, while the excess soil heaped up around it on all sides. The ship eased gently downward into the crater thus formed, which deepened as it continued to sink. The settling of the vessel, and the deepening of the hole, continued for perhaps twenty feet, before the hull touched solid rock. 

	When it did, more relays moved, and the rock itself flowed away in fine dust. This continued for only another foot, and then the ship was resting in a perfectly fitting cradle of stone, and the displaced soil was drifting back around it, covering its still red-hot circumference. The sand smoothed itself into a low mound which almost, but not quite, covered the vessel. 

	Had the agent cared about concealment, of course, he could have dug a little deeper—but all he wanted was good contact with bedrock. There was much mapping to do, and the matter of local life would have to wait until it was done. 

-

Chapter II 

	The radar beams had stopped—or had, at least, ceased to reach the Conservation Agent—before he had gone underground. The point where he had landed was not in line-of-sight range of any of their stations. Needless to say, however, their operators had not forgotten him. 

	The agent was not considering possible radar operators. In fact, he would not have considered them even if radar had been, to him, something produced by a machine. He was far too busy listening. 

	If a human being puts his ear close against a wall, or a door-jamb in a fairly large building he will pick up a remarkable variety of sounds. He will hear doors closing, windows rattling, and assorted creaks and thuds whose origin is frequently difficult or impossible to determine. The one thing he will not hear is silence. 

	The crust of a planet is much the same on a vastly greater scale. It is always full of vibrations, ranging from gigantic temblors—as square miles of solid rock slip against similar areas on the two sides of a fault plane—to ghostly echoes of sound and the faint act of thermal oscillations as the sun's heat shifts from one side of a mountain to the other, and the rocks expand and contract to adjust to the new temperature. 

	These waves travel, radiating from their point of origin, being refracted and reflected as they enter regions of differing density or elasticity, losing energy as they go by heating infinitesimally the rock through which they pass. They may die out entirely in random motion—heat —while still inside the body of the planet. Or a good, healthy wave-train may get all the way to the other side. 

	If it does so on Earth, it takes about twenty minutes. Then a fair proportion of it bounces from the low-density zone that is the bottom of the atmosphere, or the top of the lithosphere, whichever you prefer, and starts back again. 

	And every variation in density, or crystal structure, or elasticity, or chemical composition, has some effect on the way such waves travel. They may speed up or slow down. Transverse waves, or the transverse components of complex waves, may damp out—have you ever tried to skip rope with a stream of water?—and the compressional waves alone go through. Transverse waves, polarized in one direction, may be refracted through an interface, where the same sort of wave striking the same interface—at the same angle but polarized differently—may be reflected from it. 

	The important thing is that constantly varying conditions affect the waves. And that means that the waves carry information. 

	It is confused, of course. Temblors come from all directions, from all distances, due to many different causes, and through all sorts of rock. Interpreting them is not just a matter of sitting down to listen. One might as well tune in a dozen different radios to as many different musical programs, while sitting in the middle of a battlefield with a thunderstorm going on, and try to decide how many flutes were being used in one of the orchestras. The information is there, but selectivity and analysis are needed. 

	The agent was equipped for such selectivity, such analysis. His sensitive gear could detect any motion of the rock, down to thermal oscillation of the ions, at frequencies ranging from the highest a silicate group could maintain, to the lowest harmonic of a planet the size of Jupiter. 

	If his instruments proved inadequate, he could listen himself. But since just listening would involve the projection of a portion of his own body through the hull and bringing it into contact with the rock, the act would put a crippling strain on his stonelike flesh, and would consume several millennia of time. He did not plan to take this alternative. Machines were built to be used. Why not use them? 

	His own senses reacted at electronic speed—were, in fact, electronic in nature, as were his thought patterns. The process of receiving a group of impulses, and of solving the multiple-parameter equations necessary to deduce all the facts as to their origin and transmission, called for just such a fast-acting computer as his mind, though even he took some time about it. 

	This, primarily, was because he was careful. A temblor originating nearby would naturally have fewer unknowns worked into its waveform by the time it reached him. Therefore, it represented a simpler problem. Also, when solved, that problem provided quantities which could be fitted directly into the equations since their wave-trains must have come through the same rocks as they approached him. 

	His picture of the lithosphere around him grew gradually, therefore, and by concentric shells. He saw the layers of different sorts of rock and, far more important, the stresses playing on each layer—stresses that sometimes damped out to zero in the endless, tiny twitchings of the planet's crust and that sometimes built up until the strength of the rocks, and the vastly greater weight of overlying materials could no longer resist them, and something gave. 

	He sensed the change, as trapped energy built up the temperature in a confined volume, until the rock could no longer be called solid, even though the pressure kept it from being anything that could be called liquid. He saw the magma pockets formed in this way migrate, up, down and across in the crust, like monstrous jellyfish in an incredibly viscous sea. 

	He saw certain points on the planet where they had reached so nearly to the surface that the weight above could no longer restrain the pressure of their dissolved gases. An explosive volcanic eruption is quite a sight, even from underneath. 

	His senses, through the vessel's instruments, probed down toward the core of the world, where magma pockets were more frequent. In such pockets, held in solutions which might some day carry them to the upper crust, they would be accessible—the copper and silver and molybdenum, and other metals his people needed. They would lay diffused through the material of the planet. 

	Those were the things that interested him. He needed to know the forces at work down there—not in general, as a climatologist knows why Arizona is dry, but in sufficient detail to be able to predict when and where these metals would reach the upper crust and form ore bodies. The fastest electronic computers man has yet built would be a long time working out such problems, given the data. The agent was certainly no faster, and was less infallible. 

	He knew this to be so, and, therefore, spent much of his time checking and rechecking each step of the work. The task took all his attention, and, for the time being, he was totally indifferent to impulses originating near the surface—much less to a number of feeble ones which originated above the surface. 

	There was something a good deal more interesting than human reactions to claim the attention of the Conservation operative. He had, of course, confirmed long since his original impression that the ore beds of the planet had been looted. His principal job now was to decide how long the normal diastrophic and other geological processes would require to replace them. 

	On a purely general basis, replacement should take tens of millions of years for a planet of Earth's size and constitution. Magma pockets would have to work their way up from the metal-rich depths to the outer crust. Then they would have to come into contact with materials which would dissolve or precipitate, as might be the case, the particular metals he sought. 

	The geological processes which depended so heavily on water or ammonia, in the liquid state, and concentrated the metallic compounds into ore deposits, could occur only near the surface. Of course, a magma pocket, commencing five hundred miles down, may not go upward. It may travel in any direction whatever, or not at all. 

	The density, the chemical composition, the melting point of the surrounding material, its ability to retain, in solution, the radioactives which may have been responsible for the pocket in the first place, were all vital factors. Equally vital was the question of whether its crystalline makeup is such as to absorb or release energy as increasing temperature reorganizes it—the proximity of one or more of the vast iron-pockets, whose coreward settling contributes its share of energy. All of these things influence the path as well as the very existence of the pocket. 

	It would be relatively easy to predict, on a purely statistical basis, the number of ore-bodies to be formed in a given ten-million-year period. But the agent needed much more than that. When a freighter is dispatched to pick up metal at one specific point and deliver it to another, the schedule is apt to suffer if the ship has to wait a million years for its load. Interrupted schedules are not merely nuisances. In a civilization spread throughout the core of the galaxy, none of whose member worlds are self-sufficient, they can be catastrophic. 

	So the agent measured carefully, and, as he did so something a trifle queer began to appear. Impulses that did not quite fit into the orderly pattern he had deduced kept arriving—impulses of a nature he found, at first hard to believe. 

	Then he remembered that the poachers had been her for quite a while before his own arrival, and an explanation lay before him. The impulses were of the sort that his own hull must have broadcast, while he was digging his present refuge. There could be only one thing which the poachers would logically have left behind them. They could have left evidence of their digging. 

	They had shown, he decided, a rather unusual amount of foresight for their kind, coupled with a ruthlessness which made the agent wonder whether they had even felt the radar beams that had greeted his own arrival. What the poachers had done was not a thing to do to an inhabited planet. 

	The out-of-place impulses were from mole robots, slowly burrowing their way into the world's heart. Each one, as the agent patiently computed its position, course, and speed, was headed for a point where the release of a relatively minute amount of energy would swing delicately balanced forces in a particular direction. The direction was obvious enough. The poachers expected to be back for another load, and were stimulating Earth's diastrophic forces to provide it. 

	This was a technique often used by legitimate metal-producers, but only on worlds that were uninhabited. Orogeny, even when stimulated in this fashion, may take half a million years to raise a section of landscape a few thousand feet. That still would not provide time to escape for a being who, without mechanical assistance, would take something like the same length of time to travel a few hundred. 

	From the agent's point of view, the presence of such depth-charges meant that Earth was going to become, in a fairly short time, a writhing, buckling, seething surface of broken rock, molten lava and folding, crumpling, tilting rafts of silicate material on a fearfully disturbed sea of stress-fluid. 

	Such heartless behavior might prove unavoidable—since he wouldn't be there at the time. But—what had produced those radar beams? 

	It revolted him that any planet with life should be treated in such a manner. Whether or not the life was currently intelligent was beside the point. Few mutations were needed to transform a life species, from something as unresponsive as the planet that had pawned them, into a species capable of understanding the internal mechanism of a star in detail, for any distinctions of that nature to carry weight. If those beams had originated from living bodies, something would have to be done about the moles. 

	The agent simply did not have the equipment to do a thing. He could fight his little ship. He could investigate and analyze. He could communicate all the way across the galaxy, if something like the ionized layers of a planet's atmosphere did not interfere. 

	But he had no mole robots on his vessel, no weapons that would penetrate rock, or even atmosphere, for any great distance. He could not himself stand the temperatures at depths to which some of the poacher's moles had already penetrated. Consequently, he could not follow them in his own ship, even if it were able to dig as rapidly as the robots. It was indeed a problem! 

	Sending for help was possible, but almost certainly useless. His patrol area was so far out near the galactic rim that any message would take several millennia to reach a point where it would do any good—and the ships which answered it would be at least three times as long in covering the distance as the radiation that summoned them. 

	By then most, if not all, of the robots would have reached their designated target points. They would have shut off the fields which held their shape against the pressure of the surrounding rock. Once that protection was gone, no material substance in the universe could keep the half-ton of fissionable isotopes forming their cargoes at subcritical separation. All that energy would come out, and the little that wasn't heat to start with soon would be. 

	Of course, even such amounts of energy are small in comparison with the usual supplies of a planet's crust. But once released in carefully calculated spots and at even more carefully calculated times they would do exactly what the poachers wanted. The Conservation agent, checking the placement of the moles, could find no fault with the computations of the poachers' geophysicist. He was in his own way an operator of genius! 

	He could, of course, arrange for official freighters to be on hand when the action bore fruit, and would certainly do so, as a last resort. But he must first attack the question of whether or not life was being endangered. For the first time since the beginning of his analysis, the agent directed his attention to the surface layers of the world. 

	Then he almost stopped again, as a new theory struck him. This planet had free oxygen in its atmosphere. Would its life, if any, be near the surface? But his hesitation was only momentary. He recalled the radar beams which were his only reason for suspecting life. They could not possibly have passed through any significant amount of rock. While his senses swept the surrounding crust, in ever-widening circles, he pondered the question of just how a living creature could endure such an environment. Think hard now, concentrate! 

	There was one obvious possibility. It might be riding a machine designed to protect it, as he was himself—which would imply that life was not native to this world. If that were the case, locating the creature or creatures should be easy. However, in such circumstances it would have to be assumed that the population was very small, since furnishing machines for all of a large population was a manifest impossibility. It would be unwise too—even if such a thing were possible. 

	A more fantastic idea was that, while the life of this world might have a carbon composition like his own, its metallic parts were of more inert substances—perhaps of the platinum-group metals. The agent knew no reason why these should not serve as well as calcium, in a nervous system. He might have thought of aluminum, had he been familiar with its behavior in an oxygen-water environment. 

	Then, there was the notion that a ship of his own race might be down and crippled—the most fantastic of all. 

	No such ship would be this far out in the galaxy, and it is hard to imagine a mishap which would leave the operator alive and safe from the environment, while crippling his communication facilities to the point where nothing but crude whistles came through. 

	Furthermore, there had been too many points of origin for the beams that had touched him. It might prove a tough nut to crack. 

	In fact, it was simply impossible to decide whether one of these hypotheses, or something which had not yet occurred to him would prove closest to the truth. For the time being, there was nothing to do but search. Naturally, it did not take long for the rhythmic impulses originating only a few miles away to catch his attention. 

	They were seismic, of course, since he was doing all his listening through the rock—but it quickly became evident that they were originating at the very boundary between lithosphere and atmosphere. Almost as quickly, he realized that the sources were moving. 

	This latter fact complicated the analysis rather seriously. It took the agent some time to conclude that sets of more or less solid objects, apparently always in pairs, were striking the lithosphere from outside. Sometimes there were relatively long periods of regular, repeated thuds, as one or more of the pairs did its hammering and such periods were always accompanied by motion of the point at which the blows were occurring. 

	At other times, the hammering was irregular, both in frequency and energy, and usually, though not always, these sequences radiated from a relatively fixed broadcasting point. There seemed to be six basic units producing the impulses. Well, he was making progress, at any rate. Systematic thought could be a joy in itself! 

	Quite evidently, if this disturbance were caused by local life, that life must be civilized to the point where it could design and build machines. Furthermore, six machines, machines so close together, really did call for thought. It suggested something about the population density of the planet. 

	On the worlds the agent knew, scarcely one individual in a thousand manned a machine capable of moving him about. To equip the rest similarly would not only be the height of folly, it would be impossible, because enough material could never be obtained; still more because very few of them were temperamentally suited to physical activity. Even if this race had equipped, say, one in a hundred of its members, the finding of such a number congregated in one spot implied either a tremendous population density or—could it be that they were looking for him? 

	He had never stopped to think what a two-dimensional search would be like. But these machines, he was beginning to think, must be confined to surface travel—perhaps sub-surface as well—and their operators were assuming that he was on or near the surface of the lithosphere. 

	The agent cast his memory back over the paths these things had been following, and decided that they might indeed be explained on the assumption they were seeking something and had a very restricted range of sensory perception. He dwelt for an instant on the last assumption, finding it unpleasant. 

	The radar beams, then, must have been used to track him. He had felt no such impulses, since digging in, although a portion of his hull remained exposed. But his attention had been so completely taken up with his work that he might not have noticed. He began to listen more carefully for electromagnetic radiation, and heard it immediately. On the instant, any doubts that might have remained concerning the intelligence of this race were disposed of. 

	There was a single source, which seemed to accompany only one of the machines, though the agent found it a little harder to locate precisely than the seismic sources. Apparently radio waves were being reflected from surfaces not in his mental picture of this part of the planet, thus confusing slightly his attempts at orientation. He was disturbed by the seeming fact that only one of these operators talked—and wondered why there had been no answer. 

	That problem was quickly solved, however. More careful listening disclosed a response coming from a fixed point some distance away. The agent did not attempt to make a seismic check on the environs of this source of radiation, since there was already enough to occupy his attention. 

	Still, why should only one of these machines, or its driver, be engaged in a long-range conversation? Surely the others, if they could be trusted to drive such devices, must occasionally have ideas of their own. It did not occur to him that the impulses might not represent speech—their pattern complexity was too great for anything else, though their tone was rather monotonous—quite literally. The frequency was constant and only the amplitude was modulated. 

	One possibility, of course, was that there was only one operator present, who was reporting to or discussing matters with his more distant fellow while he controlled all six of the nearby machines. In that case, however, the impulses he was using to control the subsidiary vehicles should be detectable, and nothing of the sort had reached the agent's senses. 

	Could it be that the orders were transmitted by metallic connections instead of radiation? They would have to be flexible, of course, since the relative positions of the machines were constantly changing! Yes, that could be it! 

-

Chapter III

	It became increasingly evident to the Conservationist that he could lie there, until he was trapped in an earthquake, making up five hundred theories per second, without getting one whit closer to knowledge of what was happening around him. He was going to have to examine the machines more closely. The only question was one of tactics. Should he go to them, or have them come to him? 

	He decided first to try the second gambit, since it offered more promise of drawing out information as to their nature and abilities. He would thus be able to determine precisely what stimuli affected their senses of equipment, and the extent of their capability in analyzing what they did detect. 

	Naturally, not a wave of their radiation had, thus far, conveyed any meaning to the Conservationist. More accurately, the few patterns that even remotely matched patterns of his own language did not deceive him for an instant by such chance similarities. Nor did he suppose the natives would have any better luck with his language. 

	His first attempt at attracting their attention consisted merely of broadcasting sustained notes on a variety of frequencies, other than the one they were using. As he had rather expected, these produced no noticeable reaction. Travel and conversation went on unaffected. When he repeated the attempts, using the same wave-lengths as the natives, however, the results were just as unsatisfactory. It was extremely frustrating. 

	Travel stopped, and after he had repeated the signal a few times, all six of the vehicles seemed to come together at one spot. In the pauses between his own transmissions, the native speech sounded almost continuously. Yet he felt doubt that he had even been heard. 

	He had rather expected that there might be an attempt to respond to him in kind, but this did not occur, even though he tried sending out his wave in various long and short pulses which should have been easy to copy. At least, he used lengths corresponding to those of the radar pulses which he had felt at his arrival, and which had, presumably, been emitted by members of this race. 

	They failed to respond to the patterns, however, even when in desperation he increased the lengths of the bursts of radiation to three or four thousand microseconds. The very speech patterns of the natives changed carrier amplitude in shorter periods than that—they must, he felt, be able to distinguish such intervals! 

	The agent began to speculate upon the general intelligence-level of this alien new race. He had to remind himself forcibly that, since they could move around so rapidly, they must be able to design and build complex machines. It was startling, to say the least. 

	Then it occurred to him that all the vehicles he was watching might be remote controlled, that the electro-magnetic waves he was receiving were the control impulses. Yes, yes, that must be it! He spent some time, trying to correlate the radio signals with the motions of the machines. The attempt, of course, failed completely, since men are at least as likely to talk while standing still, as while walking around. 

	This proving a poor check on his hypothesis—it did not disprove it, since the machines might be able to do many things besides move around—he tried duplicating some of their complete signal groups, watching carefully to see whether any motion of the vehicles resulted. He realized that the controlling entity might not like what he was doing, but he was sure that satisfactory explanations could be made, once contact was established. 

	The result of the experiment was a complete stoppage of motion, as nearly as he could tell. It was not quite what he had expected. But there was some gratification in getting any result at all. For several whole seconds there was silence, both seismic and electromagnetic. 

	Then the native speech—it had to be speech—began again, in groups which still seemed long to the agent, but which were certainly much shorter than most of those used before. He duplicated each group as it came. 

	"Who's that? Hello?" 

	"Who's that? Hello?" 

	"What in hell is going on? This isn't funny, Mack!" 

	"What's going on? This isn't funny, Mack." 

	"Cut it out, you joker! If you've got a message for us, unload it and take off!" 

	"Who's that? Hello?" The agent decided the last signal group was too long to be worth imitation, so he went back to one of the earlier groups. This action resulted in brief silence, followed by a pattern, brief, but with a fresh modulation, which he mimicked accurately. For several whole minutes, the conversation, if it could be called that, went on. He felt real pride now, a self-congratulatory kind of exaltation in being able to carry off his cleverly assumed masquerade with perfect confidence, vigor and, certainly, no small measure of success. 

	The Conservation agent had decided long since what the native machines would almost certainly do, and was pleased to detect them getting into motion once more. But when they had gone far enough for him to determine their direction of travel, he discovered, with some disappointment, that they were not moving toward him. 

	He would have had little trouble determining their motives, had they been moving straight away from him. But the angle they took carried them more or less in his direction, albeit considerably to one side. He found this a complete mystery, at first. Finally he noticed that the group was traveling along a depressed portion of the lithosphere's surface, and seized upon, as a working hypothesis, the idea that their machines found it difficult, or impossible, to climb slopes of more than a few degrees. 

	In that case, of course, they might not be able to reach him, directly or otherwise, since he had buried himself some distance up the side of a valley. He considered again leaving his position and coming to meet them, but reached the same decision as before—that he could learn more by seeing what they did on their own. 

	They spoke rarely as they traveled—but the agent found that he could always make them broadcast, by ceasing to radiate his own signal. Had they not been pursuing such an odd course, he would have supposed, from that fact, that they were using his radiation to lead them to him. His radiation! However, they kept on their course until they were somewhat past its nearest point to his position before they paused. Then there was a brief interchange of signals with some distant native, apparently in an atmosphere machine, and travel was resumed, at right angles to the original direction. 

	Now, however, the vehicles were heading away from the buried ship, had, in fact, turned left. The Conservationist gave up theorizing for the moment and contented himself with observing. He repressed his mounting excitement and became as still as a figure of stone. 

	They did not travel very far in the new direction. In less than half an hour they stopped again, held another brief conversation, and then began to retrace their steps to, and finally across, their original route. Apparently, they were still interested in the agent's broadcasts. At any rate, they continued repeating the early "Hello" and "Who's that" signals to which he had originally responded, whenever he stopped radiating. They were not following the radiation, but certainly—almost certainly —they had some interest in it. 

	Then, quite abruptly, they stopped traveling and appeared to lose interest in the whole matter. The group broke up, and its members wandered erratically about for some time. Then they drew together once more and gradually quieted down completely, or at least to the point where the agent could not be sure that the occasional impulses coming from that area were due to their motion. 

	He had just developed another theory, and this new trick bothered him seriously. He would have preferred to ignore it, but he could not. It had occurred to him that these creatures might be able to detect electromagnetic radiation of the sort he had been broadcasting, but not be able to identify the direction from which it came. He had heard of eases of physical injury among his own people which had produced such a result. 

	The idea that such a disability might be universal in this race called for a severe stretch of the agent's imagination, but he toyed with it all the same. As a result, he had just come to realize that the peculiar motions of the things he had been observing could indeed be accounted for by the assumption that they were searching for him under some such handicap—when they stopped moving. This was hard to reconcile with any sort of search procedure. What possible reason could stop them? He wished sometimes there could be fewer complexities in his existence. What possible reason? 

	Lack of fuel? Inconceivable, assuming even minimum intelligence on the part of the operator or operators. 

	Surface impossible for the machines to travel over? Unlikely, since several of them had come some distance toward him during their erratic wandering after the halt of the main body. And there had been others in the atmosphere. 

	Sun-powered mechanisms, halted by the fact that night had fallen? It was possible, though it seemed a trifle odd for such a device to be used on a rotating planet, where it must be sunless half the time. Also, it seems doubtful that the machines were large enough to intercept the requisite amount of solar radiation. The agent had a fair idea of their size and mass, from the minimum observed separation, plus the energy with which they struck the ground. 

	Not interested in him at all; and stopped simply because they had reached their intended destination? This seemed all too painfully probable, if the course of their travels were considered by itself—yet nearly impossible, if their reaction to his broadcasting were taken into account. 

	It was at this point that the agent began to consider seriously the possibility that he might never be able to get the information of their danger across to the inhabitants of this planet. Their behavior, so far, seemed to lack any element he could recognize as common sense. He was open-minded enough to realize that this might work both ways, yet such a possibility did not augur well for the chances of successful communication between the two intelligences involved. There were cynics even among his own people who claimed that folly and ignorance always went arm in arm, and were biological constants throughout space. 

	Once more, he was facing the question of whether he should go to meet these gadgets, or wait where he was and, in the latter case, how long he should wait. Certainly, if he were to check the possibility that they were sunpowered, he should not stir until after night was over. 

	But none of the other hypotheses could very well be tested without actually examining, at close hand, the natives and their machines. He decided, then, to wait until sunrise, and for a reasonable period thereafter. Then, if these things did not resume their journey in his general direction, he would seek them out. 

	As it turned out, he did not have to move. The appearance of the sun saw the vehicles already in motion, which was informative in a negative way. After a brief period of random traveling, they congregated once more, seemed to confer silently for a time, and then resumed travel along their former route. Also, they broadcast once more the signal the agent had come to interpret as a request for him to start transmitting. 

	The events of the preceding afternoon were repeated in some detail. The group continued past the agent's station on their straight-line course for a short distance, then stopped, and once more made a right-angle turn. This time, it was to the right, toward the hidden alien—and the agent realized that this theory about their sensory limitations must be at least partly correct. 

	They had to go through elaborate maneuvers to locate the source of a radio broadcast—maneuvers which suggested that even their ability to judge the intensity of the radiation was rather crude. It took them about a tenth of the planet's rotation period, this time, to narrow the field down as far as their radio senses appeared to permit. 

	Before mid-morning they had made two more right-angle turns, and then spread out to cover, individually, the remaining area of uncertainty. The agent settled comfortably in his hole and awaited discovery. This should tell him much. 

	Just how close would these things have to come to detect him directly? Would he be able to pick up their nerve-currents first? What would they do when they found him? How long would it take them to realize that he was not a native of their world? And, most important, would they have some constructive ideas about means of communication? Who did he think he was fooling? At the moment the agent would have admitted to anyone that he himself had none. And if he was up against a blank wall in that respect, how could he reasonably expect them to come up with something really new and brilliant? 

	He kept his own senses keyed up, striving to detect the first clue, other than radio and seismic waves, of the nearness of the Earthly machines. Presumably, they were more or less electrical in nature, and he knew that electric and magnetic fields must, sooner or later, draw close enough to give him a picture of their structure. A little closer than that, and the electric fields of the operators' nervous systems should permit him to deduce their shapes and structures—assuming, of course, that at least one operator was with the present group of machines, which could hardly yet be considered certain. 

	Although it was the machine with the radio that actually stumbled on the buried vessel, the radio was not in use at the time. As a result, the agent decided, rather quickly, that no operator was in fact present. The radio was, of course, put to use the moment the ship was sighted—but its structure and nature was obvious to the alien, and it was quite evidently not an intelligent being. 

	It was, however, the only object in the vicinity with functioning, electrical circuits. Moreover, there was no direct sign of life in any of the machines which gathered quickly around the ship. Finding it a little hard to believe even his own theories, the agent once more examined the radio—only to reach the same conclusion. 

	Its organization was not sufficiently complex to compare with a single living crystal, much less an entire nervous system. The conclusion seemed inescapable. Not only was the machine carrying it being controlled from a distance, but even the vehicle itself operated without detectable electrical forces. 

	The machine, of course, could not be invisible. His failure to see it meant merely that he was employing the wrong means—anything material can be seen, in some way or other. There remained the question of just what were the proper means in this particular case. 

	Free metals affected electric or magnetic fields, or both, in ways which permitted their recognition. Only a few fragments of such material were present—fragments quite evidently shaped by intelligence, but not themselves part of either an intelligent body, or even a complex mechanism. 

	Non-conducting crystal reflected and refracted many kinds of radiation. Perhaps these things, then, could be seen. The only trouble with this idea was that eyes were not a normal part of the agent's physical makeup. While his ship possessed several which were used in navigation —stars were most easily detected and recognized by light waves—they all happened to be underground at the moment. He had never anticipated a use for them on the surface of the planet, not being himself a chemist. 

	The machines were now all moving about on the ground in his immediate vicinity. One of them even moved onto the exposed section of his hull for a few moments and it gave him his first chance to approximate their mass really accurately. Unfortunately he could not determine precisely how much of the energy radiating from their footsteps was due to weight. 

	The machine on his hull carried a tiny ionization tube, whose behavior at the moment was being affected by the mild radioactivity of the ship—activity only natural after a million years in interstellar space. The purpose of the tube was no more obvious than that of the electromagnetic radiator. Neither could move or think. The only possibility seemed to lie in a connection with the remote control of these machines. Perhaps, they were sensing devices of some sort. 

	There seemed no logical reason for not raising the ship far enough to get a look at these alien machines. He had discovered all he could expect to learn, from where he was. They did receive him. They were interested, and they, therefore, had at least glimmerings of intelligence. They could not—or, at least, their machines could not—determine the direction from which radio-waves were coming. 

	It was still not clear to him whether these machines were under the control of one individual, or several. There seemed no way of investigating this important question for some time to come. What the agent wanted to know, as soon as possible, was just what sort of mechanism could operate without perceptible electrical fields —and that seemed to demand that he see them. Yes, he must see them. 

	His hull had long since cooled, and could be controlled without difficulty. He started it vibrating again, and, simultaneously, applied enough drive to counteract the weight of the ship and its contents. For a fleeting instant, he wondered whether the distant operators could detect the flickering of the myriads of relays that responded to his thoughts, or even the electrical fields of the thoughts themselves. 

	If the latter were true, they could certainly not interpret them properly. In that case, the machines would have found him much earlier, and the agent would, by now, have been holding a conference with them about the best means of intercepting the mole robots. That possibility, he decided, could be ignored. 

	The patrol flier lifted easily, until over half its bulk was above the ground. Its pilot held it there, briefly, while the rhythm of the hull packed and firmed the powdered soil that had drifted beneath it. Then he cut his power once more, and began to look about him with his newly uncovered eyes. 

-

Chapter IV

	The star-traveler already knew, of course, that he was in a valley, partway up one of the sides. The hills bounding it were not particularly high, especially by the standards of this planet. In fact, the Conservationist had a pretty accurate idea of the dimensions of the Himalayas, distant as they were—though he had been more interested in determining the rate at which they were rising. He gave the local elevations only a passing thought, then sought to examine what lay closer to his vision-outlets: outlets which the Parsons group had quite correctly labeled "eyes." 

	He failed. The details five miles away were clear and clouds of what must be water or ammonia droplets hanging at still greater distances in the atmosphere were still clearer. But, as he brought his attention to objects nearer and nearer to his ship, they grew, shapeless, and increasingly harder to examine. 

	Cursing himself for forgetting, he recognized the reason. His eyes were perfectly good instruments—for the purpose toward which they had been designed. They were carefully shaped lenses of calcium fluoride, designed with almost a full hemisphere of field and their curved focal surface was followed faithfully by the photosensitive material of his own flesh. The tiny metallic crystals in his stony tissues would, of course, be affected electrically by light, and, like many of his race, he had learned to interpret the light-images formed by lenses. 

	There was just one catch. There was no provision for changing either the shape or the position of the lenses. But actually, why should there be? They were designed to enable him to determine the directions of the stars, whose distances were for all practical purposes always infinite. He had never needed focusing arrangements until now. 

	The eyes were a foot across and almost as great in focal length. Objects a hundred yards away were blurs. At six feet they were scarcely interruptions to the background. He could just tell, by sight, that there were moving objects in his vicinity, and get a vague idea of their size. Beyond that, details were indistinguishable. 

	The nearest repair-shop where his machinery could be modified was about six thousand light-years toward the galactic center. He could, of course, pull his flesh back from one or more of the lenses until the eye involved focused at a distance of a few feet—if the situation would wait for the necessary years or centuries. However, even if the situation did wait, the natives and their machines probably wouldn't. 

	He could wait until they departed, and examine them when they were far away. Better than this, he could fly to a distance at which they were reasonably distinct in his sight. The question raised in that connection was, of course, how the natives would react to such a move on his part. However, if he did not move, he would probably learn nothing. Therefore, he resumed his rise from the soil, cleared its surface, and hurled his vessel half a mile upward. 

	To observe, and, in effect, to photograph the details of what lay below took only a few microseconds. Then he moved a few hundred yards to one side and repeated the procedure. Three seconds after takeoff, he was settling back into his original location with a fairly clear picture of the strange equipment surrounding it firmly painted in his mind. 

	He understood now why the seismic impulses had come in pairs. Each of the machines was supported by two struts, which were so hinged as to permit several degrees of freedom of motion. During his brief period of observation, they had traveled enough—away from the point where his ship had been resting—to permit him to analyze their startling method of travel. This seemed to consist in balancing on one strut, falling in the desired direction, and catching one's mass with the other before collapsing completely. The process was repeated cyclically. 

	It appeared, mathematically, that the value of the planet's gravitational acceleration would put an upper limit on the rate of travel possible by this means. The agent found himself a little dubious about the engineering advantages of it. If one had to travel on the surface, wheels seemed easier—although an irregular surface might present further difficulties. Few Conservationists, surely, had confronted problems so difficult to resolve. 

	At least, he had eliminated the last possible doubt that the things were not-metallic, non-electric machines, since he had actually seen them move in a manner which verified and complemented his seismic observation. This implied that the natives were not merely cultured, but had developed a physical science equal to, perhaps greater, than that of the agent's own race. The latter was certainly possible, since he had not the faintest idea of what was the operative principle of the devices. It was a disturbing speculation, but he refused to enlarge upon it emotionally. Obviously they had some electrical equipment. The signal detector and broadcasting device, as well as the ionization cylinder, were quite evidently as artificial as his own ship. Their science, regardless of its development, could not be entirely alien. It might be possible for him to learn something about it. If so, it was important that he begin—for the equipment needed to stop the moles would have to be obtained from these people in rather short order. 

	The agent examined once more, as precisely as his sensory equipment permitted, every detail of the things around him, which were now returning slowly, after their hasty withdrawal. He broadcast his "Hello" again, and carefully noted the way it affected the receiver. 

	When the answer came, he checked with equal care the source of the modulating energy. 

	The result was interesting. The receiver apparently did not consider the carrier waves important. It damped them out and used, through most of its circuitry, a secondary signal consisting of the original modulations. This was caused to vary the strength of a magnetic field which, as nearly as the agent could tell, was used to impart mechanical motion to an object principally nonmetallic. 

	He could get only a rough idea of its size and shape from the space left for it in the mechanism. The evidence seemed to indicate that the whole device simply rebroadcast the modulation of the original signal mechanically into the atmosphere. 

	He knew, of course, that a gas could carry compression waves, though it had never occurred to him that they might be of any particular use. He had simply never stopped to wonder why his method of digging was more effective on a planet with atmosphere. It did no good to blame oneself for such oversights when the fat was in the fire. Anyway, he was sure of one thing. The waves were being used to carry the signals controlling the machines. Certainly no others were. 

	They also served for communication, since similar waves appeared to be received by the same disc in the signal device, and were used to modulate its broadcast electromagnetic impulses. This process seemed pointless, except as a means of long-distance communication. Probably pressure waves did not transmit energy so effectively through a gas as electromagnetic radiation carried it through space. So far, so good. 

	It all tied in, more or less, with the evident fact that these machines were not electrical, even if it did not begin to explain how they actually worked. Some sort of more precise analysis would, of course, be needed. The metal he could detect about the things seemed quite purposeless, and he did not see that it was likely to help. 

	It was present in small, disconnected bits and was devoid of electrical energy, if you brushed aside the minute currents generated by its motion in the planet's magnetic field. 

	The machines, then, were made virtually entirely of non-conductors, and should be about as easy for the agent to examine as a device consisting exclusively of gas jets and magnetic fields would be for a human being. 

	This meant that the analysis would have to be by highly indirect methods. A chemist, with his laboratory machine, might be able to do the job in microseconds. But a traveling device, like the scoutship, had no equipment designed with any such purpose in mind. 

	He suspected that this was one of the situations where the sensile members of his race—the great majority—would leap at the chance to show their superiority over one who was bound to a machine. It had always been that way. It was a common enough feeling among those whose lives were primarily intellectual. The doers, like the agent, countered it with a clear recognition of the necessity for their work. At the moment, however, the agent rather wished that a normal person had been present, to show his intellectual superiority. 

	Then he realized that his own possession of machinery did not disqualify him as an intelligent being. If a member of his race could solve this problem, it was as likely to be himself as anyone else. He would have to use all his knowledge, of course, not just the specialized information which was all the millennia of flight demanded. 

	Enough knowledge should be there. He had, of course, been young when he elected this life, but he had had much thinking time before his career was actually begun. Also, there had been a good deal of time to think as he drifted among the stars, and opportunities to gather data that planetbound thinkers had never possessed. 

	He would have to go back to the most elemental principles of thought—if he could. First, he had decided, on the basis of what seemed adequate evidence, that the planet was inhabited—that its inhabitants used machines and, therefore, had freedom of motion—and that these machines were based on a technology almost, but not quite wholly, alien to his own. 

	Nevertheless, the devices must operate under the same physical laws that applied elsewhere in the universe. This meant that they must take in some form of energy, must perform a desired action, and must eventually account for the energy as heat. 

	The energy was not electric or magnetic, since he could have detected the presence of that kind of energy directly. It was not gravitational, since the gravitational -potential of these machines—when measured as a function of their distance from the planet's center—had actually increased since he had first detected them. It was barely possible, of course, that some primary source beyond his detection-range might work on such a basis. But for the moment that hardly bothered him. It could be filed away for future reference. 

	There was almost certainly no direct mechanical link with a distant energy source. He felt sure that he would have seen any such, during his brief trip aloft. 

	Chemical energy, however, remained a distinct possibility. Normally—which usually meant, he reflected wryly, circumstances in which intelligence had not taken a hand—chemical reactions were too slow to provide useful energy, even though they were responsible for life. However, on a planet infested with such weirdly active carbon compounds, it would not do to be dogmatic on the matter. 

	It was known that reactions, in such circumstances, did go with enormous speed, though little actual quantitative work had been done on the matter of energy involved. It was quite conceivable, in any case, that there might be some method of turning chemical directly into mechanical energy, without involving electricity as an intermediate stage. 

	Looked at from this viewpoint, several more possibilities as to the planet became evident. Its natives could survive, either by nature or intelligent adaptation, in an oxygen-rich atmosphere. Oxygen was one of the most virulently active elements in existence. Hence, it might not be too surprising to find such a people developing a chemical technology and bypassing the electricity a living creature should logically use—but wait. They had not bypassed electricity. 

	There were auxiliary machines, among the vehicles facing him, which did use it. Perhaps, these people had originally developed a normal technology, but, for some unaccountable reason, had never mastered space-flight! That was more than likely, if one assumed they did not merely tolerate oxygen, but needed it. 

	In that case, they would inevitably exhaust, in a relatively short time, the metal resources of a single planet. 

	They would be faced with the choice of developing machines that did not make demands on the metal supply, or of sinking to barbarism during the millions of years it would take new metal deposits to concentrate to usability. 

	This race might have succeeded in accomplishing the former—in which case, the exhaustion of the local ore veins could not be blamed on the poachers after all. The marauder might have planted the torpedoes in momentary pique, believing that a regular freighter had been there first and hoping to throw the production schedule of this planet out of step with that which had been recorded for it. 

	It was a very attractive idea, but the agent decided he should not go quite so far in pure speculation. There should be other possible sources of energy besides chemical activity, promising as such energy appeared to be. He could, for example, detect a pressure against his hull which seemed to be due to currents in the atmosphere. These must necessarily carry energy, though it seemed, at first estimate, that it could hardly be quantitatively adequate to run these machines. 

	There was nuclear energy. Obviously, these aliens did not use it directly, yet the possibility remained that it was their primary source and was stored in some non-self-destructive form within them. Strength was lent to this possibility by the presence of the ionization tube, which might well be used to locate radioactive materials. If, of course, the normal senses of the creatures were inadequate for the task. Atomic energy not under rigid control was always a rather frightening thing to contemplate, and he did not dwell on certain other unlikely possibilities concerning it. 

	He had already thought of solar energy, but had seen nothing to offset any of his earlier objections to this theory. On the whole, the chemical idea seemed the most worth following up. 

	He searched his memory for the little he knew about the high-speed chemical reactions of free-oxygen environments, and found a few helpful items. For one, they did involve solar energy—they employed it usually in breaking down water. The oxygen was freed to the surroundings, and the hydrogen combined with oxides of carbon to produce carbohydrates. 

	These, in turn, could react upon each other, with simple compounds and with some of the free oxygen, to produce incredibly complex substances whose detailed structure had never been worked out by any chemist of his people. This situation should, of course, result in a continual increase of free oxygen in the planet's atmosphere at the expense of the water. 

	Observation indicated that, actually, an equilibrium was usually attained in this respect. Whether the oxygen re-combined spontaneously with the hydrogen in the compounds, or whether still other high-speed reactions, of the same general type as the photosynthetic ones, did the trick, was still a matter of debate. Even the agent could understand, however, that the combination of oxygen with almost any of the complex carbon-hydrogen compounds would return the energy originally supplied by the sun. 

	If the compounds had any reasonable density, it should be possible to store quite a fuel supply in a very small space that way, using atmospheric oxygen to combine with it whenever desired. Even without precise figures, he felt sure that this would constitute an adequate energy-source for the machines he had been watching. 

	Was there anything he had overlooked? No—he was nothing if not thorough when he undertook a task of objective scientific analysis. A doer had his own pride to safeguard, and if he was not an intellectual in a strict sense, he did possess a first-rate mind. 

	How could this theory be checked experimentally? If it proved correct, there should be, somewhere on or within these machines, a store of hydrogen-carbon compounds. They should be absorbing atmospheric oxygen at a fairly high rate. And they should be exhausting water and, possibly, oxides of carbon. 

	He had no means for recognizing the hydrogen-carbon compounds, even if he found them, so there seemed little point in trying to take one of the mechanisms apart. No point even if its operator proved willing to allow it. However, there seemed to be a possible way of attacking the problem through the other facts. If an oxidizing reaction of the sort he had envisioned went on in a confined space, what would happen to the pressure? He pondered the problem. 

	Producing solid oxides would reduce pressure by removing oxygen. The formation of carbon dioxide would leave it unchanged, for there would be the same number of molecules after the reaction as before. Making water or carbon monoxide would give a pressure increase, since each molecule of oxygen would go into two molecules of the product. 

	All this, of course, assumed that water and the oxides of carbon were gases at this temperature. The method offered him two out of three chances of learning something—better, really, since it was likely that two, or all three, of the reactions occurred together. Only if CO, alone were produced, would there be a negative result? The catch seemed to be how one was to seal one of these devices in a gas-tight container, with a limited amount of atmosphere. 

	The container, of course, was available. His own ship had a good deal of waste space, left deliberately to allow for later modifications, if and when they were developed. He could open his hull for maintenance at virtually any point, and the openings were naturally designed to seal gas-tight, since his occupation was more than likely to lead him into corrosive atmospheres such as this. 

	He would have to be sure that he let the planet's air only into chambers where it could not reach either his own tissues or the ship's circuitry. No, wait. The test should take only minutes or hours, not years. Both his flesh and the silver wires could stand oxygen that long, and he could get rid of it later by opening the hull to the vacuum of space. That made matters easier—much easier. 

	But how could he detect the change in pressure, if it did occur? He did have manometers, of course. But they were vented to the outside of his hull. No one had foreseen a need for measuring internal pressure. He would have to do some more hard thinking. 

	What effects would pressure produce, besides merely mechanical ones? There would not be enough change, in the electrical properties of the exposed wires, for even the agent to detect. The change would probably not be fast enough to alter the temperature noticeably. And even if it did alter it, he would not be able to tell whether the change were due to gas laws, or simply the operation of the machine. 

	In the temperature range of this world, it was not really certain that all the products were gaseous, anyway. The mere fact that he had detected them in that form, during his approach, meant nothing. The infra-red spectrographic equipment he had used would have picked up trace quantities. It was unfortunate that its receivers were also aimed outward. 

	The agent could not, for the life of him, recall the vapor-pressure curves of any of the expected products—though, come to think of it, something was liquid here. The clouds he could see proved that, as did their precipitation on his half. He could not assume that it was one of the products he sought, however, and his best bet was still to maintain pressure change. If he could do it ... 

	As usual, the solution was ridiculously simple, once the traveler had thought of it. Most of the access-doors in the hull opened outward and all were operated electrically. He had perfect control over the current supplied to their operating motors. He knew that if he refrained from latching one or more of the doors, and simply held it shut with the motor, he could sense directly the amount of effort needed to keep it sealed against the internal pressure. 

	As far as he was concerned, it was a quantitative solution—if the pressure increased. If it decreased—well, he would know it, from the extra effort needed to open the door. He was concentrating on immediate small details now—and very wisely. 

	With his machine, action could follow thought without delay. The moment he had his answer, a door swung open in the side of the great metal egg he was driving, and Earth's air poured in. Good as his seals were, the ship had not, of course, retained any significant amount of gas in the millennia it had been in space. 

	He did not bother to develop a plan for enticing one of the machines through the opening. He assumed, quite justly, that any intelligent mind must have a fair proportion of curiosity in its make-up. The fact that sell preservation might oppose this influence did not, as far as the agent knew or suspected, apply to the present situation. The risk of sacrificing even an expensive remote-controlled machine should be well worth taking in such circumstances. He simply waited for one of the devices to be driven into his ship. 

	Before this happened, however, there was a good deal of conversation among the machines present and, he presumed, the distant broadcaster—if, of course, it could be called conversation. The agent was still unable to reconcile this supposition with the absence of intelligent life in the present group. 

	At last, however, the expected event occurred. One of the machines swung about and moved toward the opening in the hull. Just outside, it halted, and the agent guessed at a brief burst of atmospheric pressure waves, though his manometers did not react fast enough to catch them. Then it entered. 

	It traveled on four struts instead of two. It became completely horizontal and advanced on the supporting struts. Evidently the upper ones, which the agent had seen, could be used for locomotion when desirable. Its entrance was slower than by its usual rate of motion, though the agent could not imagine why. The suggestion that slower motion made detail observation easier would never have occurred to a being whose perception and recording operations occupied fractions of a microsecond. Whatever the reason for the delay, it finally managed to get inside. 

	The agent wasted no time. Ready to observe anything and everything that resulted, he shut the access hatch. Results, by his reaction-time standards, were slow—additional evidence that remote control was involved. The electromagnetic unit burst into activity the instant things finally began to happen. Some of the machines outside began to tap on the hull with dimly perceptible solid fragments, apparently pieces of silicate rock. The agent tried to find regularities in the blows that might be interpreted as communication code of some sort. He failed. 

	One of the devices, standing a little distance away, moved one of its attached fragments of metal until a hollow cylinder—which formed part of it—was in line with the hull. After a long moment the more distant end of the cylinder filled with gas, sufficiently ionized to be clearly perceptible to the alien. 

	The gas must have been under considerable pressure, for almost instantly it began to expand, driving before it a smaller fragment of metal which had plugged the tube. This fragment became progressively easier to perceive as its speed through the planet's magnetic field increased. 

	It emerged from the near end of the cylinder with sufficient momentum to continue in a nearly linear course, until it made contact with the hull. The agent watched with mounting excitement as it flattened, spread out and finally broke into many pieces. Incredible! He analyzed it, both electrically and mechanically, from the way it broke up. But he could make no sense of the operation. 

	After a time, the pounding ceased, and the two machines remaining outside drew together. No obvious activity came from them for some time. 

	Inside the hull, more interesting, possibly more understandable, events were taking place. The moment the door had closed, the machine trapped within had attempted to withdraw. Its action was a trifle faster than that of the ones still outside. The agent could not decide whether this meant that the escape reaction was automatic or that a distant controller had turned his attention to the captive machine first. 

	It had pounded aggressively on the inside of the door in the same seemingly planless fashion as its fellows. Then it had slowed down, and began to move another of the strangely fashioned pieces of metal distributed about its frame. This abruptly became clearly perceptible, as an electric current began to flow through portions of its structure. 

	The source of the current was a seemingly endless supply of metallic ions—quite evidently chemical energy could be used for something. The current's function was less obvious, since it was led through a conductor whose greatest resistance was concentrated in a tight metal spiral. 

	This must in some way have been shielded from atmospheric oxygen, at fairly high temperature if the ion cloud around it meant anything; it nevertheless remained uncorroded. Heating the wire seemed all that the device accomplished—the agent refused to believe that the ion cloud was intense enough to help either in action or perception. The light and heat radiated were inconsiderable, but—wait! Perhaps that was it—perhaps this machine had eyes! 

	The agent examined the electrical device more closely, and discovered that part of its uncharged structure consisted of a roughly paraboloidal piece of metal, which must certainly have been able to focus light into a beam, of sorts. 

	A few moments later, it became evident that it did just that. The agent's body was exposed in several places in this part of the ship, and, time after time, one part would be struck by radiance, while the rest were in more or less complete darkness. Furthermore, a few minutes' observation showed that when the machine moved at all it followed the direction in which the light beam happened to be pointing at the time. 

	Sometimes it did not move, though the beam kept roving around the chamber. The agent deduced from that one of two things. Either the device had several eyes, or the one it had was movable over virtually the entire sphere of possible directions. The thing was making an orderly survey of the interior of the space in which it was trapped. But it was carefully refraining from touching anything except the floor on which it stood. 

	That portions of this floor consisted of the agent's tissue made no difference to either party—as far as either knew. But the agent began to wonder how much of the exposed machinery of the ship would be comprehensible to the presumed distant observer. 

	Still more, he wondered how this presumed observer maintained contact with his machine. There was no energy whatever—in any form that the agent could detect —getting through his hull, either to or from the trapped machine. A minor exception to this might be the pressure waves generated by the stones striking his hull. But he had already failed to find in these blows any pattern at all, much less one which could be correlated with the actions of the machine inside. 

	Naturally, the thought that this might be an automatic device, similar to the mole robots, could hardly help occurring to the Conservationist. If this were the case, its present behavior was far more complicated than that of any such machine he had ever encountered. But hold on —he had already faced the implications inherent in that idea. So the technology of this world was more advanced, in some ways, than his own. There were still things the natives didn't know—things which would most certainly hurt them. Any concern he might have felt about himself was drowned in this larger solicitude. 

	He wondered whether he could so operate any of his own machinery to or through his prisoner, so as to convey a message of any sort. Certainly, if it used light as a vehicle of perception, it could detect motion on the part of the relays. For example—they were larger by quite a margin than the wave length of the radiation the hot wire was emitting in greatest strength. 

	There were several hundred thousand of them in the dozen square yards exposed to the direct-line vision of the captive, which should be enough to form some sort of pattern. Some sort of pattern, that is, if their owner could figure out how to operate them without making the ship misbehave. 

	He was still pondering this problem, along with the question of just what would be a meaningful pattern to the operators of the machine, when his attention was once more drawn to the outside. 

	The machines there seemed to have taken up a definite course of action. They had once more approached the hull, and were doing something to it which he could not at first quite understand. It quickly enough became evident, however. The brightness of the images he was receiving through the eyes, to which he had naturally been paying very little attention, began rapidly to decrease. Within a minute or so, the lenses ceased to transmit at all. 

	His tactile "sense" consisted in part of the ability to analyze the response of his hull to the vibrating impulses he applied to it. If such impulses were followed faithfully he could be sure that there was no mass in contact with the surface. On the other hand, if they were damped to any extent, he could form a fairly accurate idea of the amount and even some of the physical properties of such a mass. 

	In the present case, he discovered almost instantly that his eye lenses had been covered with a most peculiar substance. It not only adhered tenaciously to them, but seemed to absorb without noticeable reaction the same vibrations which had sent the soil dancing out of his way like summer chaff in a breeze. This did not particularly bother him, since the eyes were nearly useless for watching the machines anyway. But he kept trying to shake the material off, while he considered the implications of the move. 

	One was that the machines depended, far more heavily than he had suspected, on the sense of sight, and must suppose that he did likewise. Another was that they were about to take measures which they did not want observed by him. He did not worry seriously about anything they could do to his ship, but he began to listen very carefully for their footsteps all the same. 

	Another possibility was that they simply did not want him to fly away with the captive machine. To a race dependent upon sight, no doubt the idea of flying without it was unthinkable. He wondered, fleetingly, whether he should move a few hundred yards, just to see what effect the act had on them. Then the actions they were already performing caught his attention, and he shelved the notion. He became alarmed at what appeared to be an abrupt change of plan. 

	Two of the things were leaving the neighborhood, in a direction more or less toward the other electromagnetic radiator. Making allowances for the difficulty these machines apparently suffered in traveling over uneven terrain, the agent felt reasonably sure that this was their goal. The other two remained near him and settled down to relative motionlessness, as nearly as he could tell. He comforted himself with the thought that whatever plan they were attempting might demand some time to mature. 

	Perhaps the departing machines were going after additional equipment, though it appeared their goal might be attained more rapidly by sending other machines from the control point. However, it was quite possible that no others were available—such was likely enough to be the case on any of his own worlds, where only one individual in five hundred was machine-equipped, and over half of these were incapable of locomotion. Pride swelled in him at the thought, but he dismissed it as unworthy. 

	His soliloquy was interrupted by something that had not happened to him since his ship had first lifted from the world on which it had been built. The incident itself was minor, but its implications were not. The hull vibration, which he was still applying near all of his aboveground eyes, stopped near one of them. 

	He had not stopped it. The command for the carefully planned motion pattern was still flowing along his nerves. It should have been inducing the appropriate response in a fairly large group of relays. Something had gone wrong, and it produced a sudden crisis in his thinking. 

	The ship, of course, was equipped with a fantastic number of test-circuits, and he began to use them for all they were worth. It took him about three milliseconds to learn a significant fact. All the inoperative relays were close to, or actually within, the compartment where the captive machine was located. Closer checking showed that the trouble was mechanical—the tiny switches were being held in whatever position they had been in when the trouble struck. 

	Worse, the paralysis was spreading. It was spreading with a terrifying rapidity. The basic cause was not hard to guess, even with the details far from obvious. The agent instantly unsealed the door barring his captive from the outside world, and felt thankful that the controls involved still functioned. 

	The thing lost no time in getting out, and the pilot lost even less in getting the door securely sealed after it. For the time being, he completely ignored what went on outside, while he strove to remedy the weird disability. He was far from consoled by the thought, when it struck him, that he had proved what he wanted to know. 

	Something solid had blocked the relays—had, more accurately, formed around their microscopic moving parts. Whatever it was must have come in gas form for he would have felt the localized weight of a liquid, even inside. Most of the interior of his ship, as well as his own flesh, was still far colder than the planet on which he was lying. 

	Quite evidently one of the exhaust products of the captive machine, released as a gas, had frozen wherever it touched a cold surface. It might have been either water or one of the oxides of carbon. The agent neither knew nor cared. He proceeded to run as much current as possible through all his test-circuits, with the object of creating enough resistance-heat to evaporate the material. 

	The process took long enough to make him doubt seriously that his conclusion could be correct. But eventually the frozen relays began to come back into service. He could have speeded up the process, by going up a few miles and exposing his interior to the lowered pressure, and he knew enough physics to be aware of the fact. 

	It spoke strongly for the shock he had received that he never thought of this until evaporation was nearly complete. It was lucky for his peace of mind that he never realized what the liquid water formed in the process might have done to his circuits. Fortunately, formed as it had been, it contained virtually no dissolved electrolytes and caused no shorts. 

	He realized, suddenly, that he had permitted his attention to stray from the doings of the nearby machines for what might be an unwise length of time, and at once resumed his listening. Apparently, they were still doing nothing. No seismic impulses were originating in the area where he had last perceived them. That eased his mind a trifle, and he returned to the problem of the material covering his eyes. 

	This stuff seemed to be changing slightly in its properties. Its elasticity was increasing, for one thing, and the change seemed to be taking place more rapidly on the side from which the air currents were coming. The agent could think of no explanation for this. He tried differing vibration patterns on the stuff, manipulating them with the skill of an artist—but a long time passed before he had anything approaching success. 

	At last, however, a minute flake of the material cracked free and fell away—and he could really see! He could actually make out what was going on! 

-

Chapter VI

	The reason was obvious, of course. With an aperture of thirty centimeters and a focal length of about twenty-seven, the focus of the Conservationist's eye-lenses was highly critical; with the aperture about half a millimeter, as it had been left by the fragment of clay he had broken off, it became a minor matter. 

	He recognized the machines easily, near the edge of his new field of view, and began to work on the covering of a better-located eye. He did not succeed quite so well here, as the fragment he finally detached was larger, and the image correspondingly less clear, but it was still a good-enough job to enable him to follow the actions of the devices visually. 

	They were not traveling, as he has deduced already. Furthermore, a fourth machine, hitherto unnoticed, had joined them. All four had settled to the ground, so that their main frames took the weight normally carried by the traveling struts, which appeared merely to be propping the roughly cylindrical shapes in a more or less vertical attitude. The different ways in which this was accomplished, in different cases, did not surprise the agent. It would not have occurred to him to expect any two machines to be precisely alike, except perhaps in such standard subcomponents as relays. And it was, of course, fortunate that every new development happened in sequence, enabling him to analyze carefully as he went along. 

	The upper struts were moving rather aimlessly in general, but it did not take long for him to judge that their primary function was manipulation. The objects being handled at the moment were for the most part meaningless—apparently stones, bits of metal without obvious function, utterly unrecognizable objects which might be aggregates of the unfamiliar carbon compounds, though the agent knew no way to prove it. There were one or two exceptions. The device that had projected the slug of metal at his hull was easy to recognize, even though he had not perceived all of it at the time it was being used. 

	He tried to decide what parts of the machines functioned as their eyes, and was able to find them. It was not difficult, for no other portion was reasonably transparent. He discovered that all these vision organs were now turned toward him, but saw nothing surprising in the fact. The operators must have been familiar with the rest of the landscape, and did not expect anything of interest to show up on it. 

	Then the traveler noticed that all four of the machines were rising to their struts. As he watched, they began to move toward him. 

	At the same time, one of them extended a handling member toward a smaller fabrication, which almost immediately turned out to be another electromagnetic radiator. It was put to use at once, being swiftly raised to the upper part of the largest machine in the vicinity of the eyes, while a minor appendage of the handling limb which held it closed a switch. 

	This started the carrier frequency, after a delay which the agent was able to identify as due to the slow growth of the ion-clouds in portions of the apparatus—apparently they were produced by heating metal—and to the inherent lag of mechanical operations. The relays in the device were fantastically huge. They took whole milliseconds to operate and since they rather obviously had components consisting of multicrystalline pieces of metal, they must have had a sharply limited service life. 

	Evidently the natives had not gone far enough with metal technology even to get the most out of one world's supplies. This was a side-issue, however. A far more interesting development involved the modulation of the carrier. The agent found it possible actually to see the way this was being carried out. 

	An opening in the machine, not far below the eyes, rimmed with a remarkably flexible substance at whose nature he could only guess, began to open, shut and go through a series of changes of shape. He found it possible to correlate many of these contortions with the modulation of the electromagnetic signal. Apparently the opening was part of a device for generating pressure-wave patterns in the atmosphere. 

	The agent supposed that whatever plan the distant observers had been maturing must be moving into action, and he wondered what the machines were about to do. 

	He was naturally a little surprised, since he had not expected any developments of this sort so soon. 

	Then he wondered still more, for the advance toward him which had been commenced halted, as suddenly as it had begun. Whatever had motivated them had either ceased—or the whole affair was part of an operation whose nature was still obscure. It would be the better part of valor to assume the latter, he decided. 

	He watched all four of the machines with minute care. They were now balanced on their support struts. They were neither advancing nor retreating, and the upper members were moving in their usual random fashion. All eyes were still fixed on his ship. 

	Then he noticed that the pressure-wave assemblies of all four were functioning, although three did not possess any broadcaster whose signal could be modulated. He watched them in fascination. Sometimes—usually, in fact —only one would be generating waves. At others, two, three or all four would be doing so. Even the one with the broadcaster did not always have its main switch closed at such times. Something a little peculiar was definitely occurring. 

	It had already occurred to the agent that the atmospheric waves carried the control impulses for these machines. Why should the machines themselves be emitting them, however? Receivers should be enough for such machines. Then he recalled another of his passing thoughts, which might serve as an explanation. Perhaps there was only one operator for all of them. And after all, why not? It might be better to think of the whole group as a single machine. 

	In that case, the pressure waves, traveling among its components, might be coordination signals. They just might be. At any rate, some testing could be done along this line. Whatever limitations he and his ship might have on this world, he could at least set up pressure waves in its atmosphere. Perhaps he could take over actual control of one or more of these assemblies. He had had the idea earlier, in connection with radio waves, and nothing much had come of it. But there seemed no reason not to try it again with sound. Nothing could surpass the experimental method when it was pursued with one strongly likely probability in mind. 

	A logical pattern to use would be the one that had been broadcast back to the distant observer a few moments before. It had been connected with a fairly simple, definite series of actions, and he had both heard and seen its production. He tried it, causing his hull to move in the, complex pattern his memory had recorded a few seconds before. He tried it a second time. 

	"The thing's howling like a fire-siren!" 

	Just as when he had tried the same test with radio waves, there was no doubt that an effect had been produced, though it was not quite the effect the agent had hoped for. The handling appendages on all four of the things dropped whatever they were holding and snapped toward the upper part of their bodies. Once there, their flattened tips pressed firmly against the sides of the turrets on which their eyes were mounted. 

	For a moment, none of them produced any waves of its own. Then, the one with the broadcaster began to use it at great length. The agent wondered whether or not to attempt reproduction of the entire pattern it used this time, and decided against it. It was far more likely to be a report than involved in control. He decided to wait and see whether any other action ensued. 

	What did result might have been foreseen even by one as unfamiliar with mankind as the Conservationist. The machine with the broadcaster began producing more pressure waves, watching the ship as it did so. The agent realized, almost at once, that the controller was also experimenting. He regretted that he could not receive the waves directly, and wondered how he could make the other—or others—understand that their signals should be transmitted electromagnetically. 

	As a matter of fact, the agent could have detected the sound waves perfectly well, had it occurred to him to extend one of his seismic receptor-rods into the air. A sound wave carries little energy and only a minute percentage of that will pass into a solid from a gas. But an instrument capable of detecting the seismic disturbance set up by a walking man a dozen miles away is not going to be bothered by quantitative problems of that magnitude. However, this fact never dawned on the agent. Yet few would deny that he had done very well. 

	As it happened, no explanation was necessary for the hidden observer. He must have remembered, fairly quickly, that all the signals the agent had imitated had been radioed, and drawn the obvious conclusion. At any rate, the broadcaster was very shortly pressed into service again. A signal would be transmitted by radio, and the agent would promptly repeat it in sound waves. 

	Since the Conservationist had not the faintest idea of the significance of any of the signals this was not too helpful—but the native had a way around that. A machine advanced to the hull of the ship and scraped the clay from one of its eyes. The particular eye was the most conveniently located one, to the agent's annoyance. But fortunately it was not the only one through which he could see the things. 

	Then, an ordered attempt was begun, to provide him with data which would permit him to attach meanings to the various signal groups. Once he had grasped the significance of pointing, matters went merrily on for some time. 

	They pointed at rocks, mountains, the sun, each other —each had a different signal group, confirming the agent's earlier assumption that they were not identical devices. But there also seemed to be a general term which took them all in. 

	He was not quite sure whether this term stood for machines in general, or could be taken as implying that the devices present were part of a single assembly, as he had suspected earlier. While the lessons went on, two of them wandered about the valley seeking new objects to show him. One of these objects proved the spark for a very productive line of thought. 

	Its shape, when it was brought back and shown to him, was as indescribable as that of many other things he had been shown by them. Its color was bright green and the agent, perceiving a rather wider frequency band than was usable by human eyes, did not see it or think of it as a green object. He narrowed its classification down to a much finer degree. 

	He did not know the chemical nature of chlorophyll, but he had long since come to associate that particular reflection spectrum with photosynthesis. The thing did not seem to possess much rigidity. Its bulbous extensions sagged away from either side of the point where it was being supported. The handling extension that gripped it seemed to sink slightly into its substance. 

	He had never seen such a phenomenon elsewhere, and had no thought or symbol from the term pulpy. However, the concept itself rang a bell in his mind, for the machines facing him seemed fabricated from material of a rather similar texture. It was a peculiarity of their aspect that had been bothering him subconsciously ever since he had seen them moving. Now a nagging puzzlement—subconscious frustration was always unpleasant—was lifted from his mind. 

	The connection was not truly a logical one. Few new ideas have strictly logical connection with pre-existing knowledge. Imagination follows its own paths. Nevertheless, there was a connection, and, from the instant the thought occurred to him, the agent never doubted seriously that he was essentially correct. The natives of this planet did not merely use active carbon compounds as fuel for their machines. They constructed the machines themselves of the same sort of materials. 

	Under the circumstances it was a reasonable thing to do—if one could succeed at it. The reactions of such chemicals were undoubtedly rapid enough to permit as speedy action as anyone could desire—at least as fast as careful thought could control. The agent's race had long since learned the dangers inherent in machines capable of responding to casual, fleeting thoughts and his ship's pickup-circuits were less sensitive, by far, than they might have been. 

	It was obvious why these devices were controlled from a distance, instead of being ridden by their operators, too. There must be some dangerous reactions, indeed, going on inside them. The agent decided it was just as well that his temporary prisoner had merely looked at the inside of his ship, without touching anything, and resolved to take no more such chances. 

	At any rate, there should be no more need for that sort of experiment. Language lessons were well under way. He had recorded a good collection of nouns, some verbs the machines had acted out, even an adjective or two. He was puzzled by the tremendous length of some of the signal groups, and suspected them of being descriptions, rather than individual basic words. 

	But even that theory had difficulties. The signal which, apparently, stood for the machines themselves, one which should logically have called for a rather long and detailed description, was actually one of the shortest—though even this took several hundred milliseconds to complete. The agent decided that there was no point in trying to deduce grammar rules. He could communicate with memorized symbols, and they would have to suffice. 

	Of course, the symbols that could be demonstrated on the spot were hardly adequate to explain the nature of Earth's danger. The Conservationist had long since decided just what he wished to say in that matter, and was waiting impatiently for enough words to let him say it. 

	It gradually became evident, however, that if he depended on chance alone to bring them into the lessons he was going to wait a long time. This meant little to him, personally. But the mole robots were not waiting for any instruction to be completed. They were burrowing on. The agent tried to think of some means for leading the lessons in the desired direction. 

	This took a good deal of imagination on his part, obvious as his final solution would seem to a human being. The idea of having to learn a language had been utterly strange to him, and he was still amazed at the ingenuity the natives showed in devising a means for teaching one. It was some time before it occurred to him that he might very well perform some actions, just as they were doing. If he did not follow his own acts with signal groups of his own, these natives might not understand that he wanted theirs. The time had come for a more direct and audacious approach to the entire problem, and at the thought of what he was about to do his spirits soared. 

	He did it. He lifted the ship a few feet into the air, settled back to show that he was not actually leaving, and then rose again. He waited, expectantly. 

	"Up." 

	"Rise." 

	"Go." 

	Each of the watching machines emitted a different signal, virtually simultaneously. Three of them came through very faintly, since the speakers were some distance from the radio. But he was able to correlate each with the lip-motions of its maker. He was not too much troubled by the fact that different signals were used. He was more interested in the evidence that a different individual was controlling each machine. This was a little confusing, in view of his earlier theories. But he stuck grimly to the problem at hand. 

-

Chapter VII

	The agent dropped back to the ground and went through his actions again. This time only the individual with the radio spoke. The word it used was rise. This was not the one it had used the other time. To make sure, the agent went through the act still again, and got the same word. Evidently, once their minds were made up, they intended to stick to their decisions. What could he think? 

	Then he tried burrowing into the ground, which seemed a useful action to be able to mention. The word given on the radio was dig, though two of the other machines apparently had different ideas once more. 

	It did not occur to him that these things might be detecting the by-products of his digging as well as his deliberate attempts to produce sound waves, or that his efforts to focus his third eye lens, a little while before, had actually been the cause of their sudden interest in his ship at that moment. He was much too pleased with himself at this point to entertain such extraneous ideas. 

	Having taken over the initiative in the matter of language lessons, he concentrated on the words he wanted, and, within a fairly short time, felt sure that he could get the basic facts of Earth's danger across to his listeners. After all, only four signal groups were involved in the concept. Satisfied that he had these correctly, he proceeded to use them together. In his progress now he felt the surge of a very personal kind of pride. 

	"Man dig—mountain rise." 

	For some unexplained reason the listening machines did not burst into frantic activity at the news. For a moment, he hoped that the controllers had turned to more suitable equipment to cope with the danger, leaving inactive that which they had been using. But he was quickly disabused of that bit of wishful thinking. The machine with the radio began to speak again. 

	"Man dig." It bent over and began to push the loose dirt aside with the flattened ends of its upper struts. The agent realized, with some dismay, that its operator must suppose he was merely continuing the language lesson. He spoke again, more loudly, the two signal groups which the other seemed to be ignoring. 

	"Mountain rise." 

	All the machines looked at the hill across the valley, but nothing constructive seemed likely to come from that. If they waited for that one to rise noticeably, it would be too late to do anything about enlightening them as to the robots. He tried, frantically, to think of other words he had learned, or combinations which would serve his purpose. One seemed promising to him. 

	"Mountain break—Earth break—man break." The verb did not quite fit what was to happen, according to its earlier demonstration, but it did carry an implication of destruction, at least. His audience turned back to the ship, but gave no obvious sign of understanding. 

	He thought of another concept which might apply, but no word for it had yet appeared in the lessons. So, to illustrate it, he turned his ship's weapon on a patch of soil, a hundred yards from the bow. Twenty seconds' exposure to that needle of intolerable flame reduced the ground which it struck to smoking lava. 

	Even before he had finished the word fire came from one of the watchers. The observer made no comment on the fact that the tube which threw slugs of metal had been leveled at his hull, during most of the performance. He simply made use of the new word. 

	"Man dig—Earth fire—mountain fire." 

	One of the machines produced its ionization tube and cautiously approached the patch of cooling slag. This had a slight amount of radioactivity from the beam, and its effect on the tube gave rise to much mutual signaling on the part of the machines. This culminated in a lengthy radio broadcast, not addressed to the agent. Then the language lessons were resumed, with the natives once more taking the initiative. 

	"Iron—copper—lead." Samples were shown individually. 

	"Metal." All the samples were shown together. 

	"Melt" This was demonstrated, when they finally made him understand that the weapon should be used again. 

	"Big—little." Pairs of stones, of cacti, coins and figures, scratched in the dirt, illustrated this contrast. 

	Numbers—no difficulty. 

	"Ship." This proved confusing, since the agent had supposed the word man covered any sort of machine. Finally, slightly fuller sentences became possible. "Fire-metal under ground," the men tried. 

	The agent repeated the statement, leaving them in doubt. More time passed, while yes and no were explained. Then the same phrase brought a response of "Yes." 

	"Men dig." 

	"Yes—men dig—mountain melt—mountain rise." 

	"Where?" This word took still more time, and was solved, at last, only by a pantomime involving all the men. Here and there were covered in the same act. However, knowing what the question meant did not make it much easier for the agent to answer it. 

	He had no maps of the planet, and would have recognized no man-made charts, with the possible exception of a globe, which is not standard equipment on a small field expedition. 

	After still more time, the men managed to get a unit of distance across to him, however, and he could use the ion beam for pointing. In this way, he did his best to indicate the locations of the moles. 

	"There! Eighty-one miles. Two miles down." And, in another direction. "There! Fifteen hundred-twelve miles. Eighteen miles down." He kept this up through the entire list of the forty-five moles he had detected and located. 

	The furious note-taking that accompanied his exposition did not mean anything to him, of course, though he deduced correctly the purpose of the magnetic compass one of the listening machines was using. He realized that giving positions to an accuracy of one mile was woefully inadequate for the problem of actually locating the moles. 

	But he could do the final close-guiding later, when the native machines approached their targets. He could come to their aid if they did not have detection equipment of their own which would work at that range. Just what possibilities in that direction might be inherent in organic engineering the agent could not guess. At any rate, the natives did not seem to feel greater precision was needed. They made no request for it. 

	In fact, they did not seem to want anything more. He had expected to spend a long time explaining the apparatus needed to intercept and derange the moles. But that aspect of the matter did not appear to bother the natives at all. Why, why? It should have bothered them. 

	In spite of appearances, the agent was not stupid. The problem of communication with an intelligence not of his own race had never, as far as he knew, been faced by any of his people. He had tried to treat it as a scientific problem. It was hardly his fault that each phenomenon he encountered had infinitely more possible explanations than ordinary scientific observation, and he could hardly be expected to guess the reason why. 

	Even so, he realized it could not be considered a proven fact that the natives had read the proper meaning from his signaling. He actually doubted that they had, in the way and to the extent that some mid-nineteenth century human physicists doubted the laws of gravity and conservation of energy. He determined to continue checking as long as possible, to make sure that they were right. 

	The human beings, partly as a result of greater experience, partly for certain purely human reasons, also felt that a check was desirable. With their far better local background, they were the first to take action. To them, fire metal, when mentioned in conjunction with a positive test for radioactivity, implied only one kind of fire. 

	Man dig was not quite so certain. They apparently could not decide whether the alien being was giving information or advice—whether someone was already digging at the indicated points, or that they should go there themselves to dig. The majority inclined to the latter view. 

	To settle the question, one of them took the trench-shovel, which was part of their equipment, and arranged a skit that eventually made clear the difference between the continuative—digging—and the imperative dig! 

	While this was going on, another thought occurred to the agent. Since these things had used different words for the machines he was watching and the one he was riding, perhaps man was not quite the right term for the mole-robots he was trying to tell about. He wondered how he could generalize. By the end of the second run-through of the skit he had what he hoped was a solution. 

	"Man digging—ship digging," he said. 

	"Digging fire metal?" 

	"Man digging fire metal—ship digging fire metal." 

	"Where?" 

	He ran through the list of locations again, though somewhat at a loss for the reason it was needed, and was allowed to finish, because, though he did not know it, no one could think of a way to tell him to stop. He felt satisfied when he had finished—there could hardly be any doubt in the minds of his listeners now. 

	They were talking to each other again—the reason was now obvious enough. The operators must be in different locations, must be communicating with each other through their machines. He had little doubt of what they were saying, in a general way. 

	Which was too bad—in a general way. 

	"It's vague—infernally vague." 

	"I know—but what else can he mean?" 

	"Perhaps he's lust telling about some of our own mines, asking what we get out of them or trying to tell us he wants some of it." 

	"But what can 'flame metal' mean but fissionables? And what mine of ours did he point out?" 

	"I don't know about all of his locations, but the first one he mentioned--the closest one—certainly fits." 

	"What?" 

	"Eighty-one miles, bearing thirty degrees magnetic. 

	That's as close as you could ask to Anaconda, unless this map is haywire. There are certainly men digging there!" 

	"Not two miles down!" 

	"They will be, unless we find a substitute for copper." 

	"I still think this thing is telling us about beings of its own kind, who are lifting our fissionables. They could do it easily enough, if they dig the way this one does. I'm for at least calling up there, and finding out whether anyone has thought of drilling test cores under the mine level—and how deep they went. There's no point walking around here, looking for anything else. We've found our fireball, right here." 

	The agent was interested but not anxious when the machines turned back to him, and direct communication was brought once more into operation. He was beginning to feel less tense, and confident that everything was going to come out all right if he stuck with it. 

	"Eighty-one miles that way. Men digging. Go now." 

	They illustrated the last words, turning away from his ship and starting in the proper direction. The agent could not exactly relax, fitting as he did into the spaces designed for him in his ship, but he felt the appropriate emotion. 

	They were getting started on one of the necessary steps, at least. Presumably, the other and more distant ones would be tackled as soon as the news could be spread. These machines moved slowly, but their control impulses apparently did not. 

	It occurred to him that, since none of the devices had been left on hand to communicate with him, the natives might be expecting him to appear at the nearest digging site—the one they had mentioned. The more he thought of it, the more likely such an interpretation of their last message seemed. So, with the men barely started on their walk back to the waiting jeep, the Conservationist sent his ship whistling upward on a long slant toward the northeast. 

-

Chapter VIII

	The moment he rose above the valley, the Conservationist picked up the radar beams again—the beams that had startled him when he first approached the strange planet. As had happened on the earlier occasion, a few milliseconds served to bring many more of them to bear upon him. 

	He was quite evidently being watched on this journey. But he no longer expected these beams to carry intelligent speech. More or less casually, he noted their points of origin. He wondered, for brief moments, whether it might not be worthwhile to investigate them later, but felt fairly certain that it wouldn't. He turned his full attention on his goal. 

	The crusts of clay had fallen from his eyes as he flew, and he was once again limited to long-distance vision. He could make out the vast, terraced pits of the great copper mine as he approached, but could not distinguish the precise nature of the moving objects within. He did not consider sight a particularly useful or convenient sense anyway, so he settled to the ground, half a mile from the pit's edge, bored in as he had before, and began probing with seismic detectors and electrical senses. 

	He had, of course, already known of the presence of the hole. A fair amount of seismic activity had reached his original landing-spot from this place, enabling him to deduce its shape fairly accurately. Now, however, he realized—and for the first time—the amount of actual work going on. There were many machines of the sort he had already seen, which was hardly surprising. But there were many others as well, and the fact that most of them were metallic in construction startled him considerably. 

	There was a good deal of electrical activity, and at first he had hopes of finding an actual native. But these hopes quickly faded when he discovered there was nothing at all suggestive of thought-patterns. Some of the machines were magnetically driven. Others used regular electrical impulses for, apparently, starting the chemical reactions which furnished their main supply of energy. 

	The really surprising fact was the depth of the pit. If this work had begun since the receipt of his information, the wretched, guilty robots would be caught without difficulty. It took some time, by his perception standards, for a truer picture of the situation to be forced on his mind. 

	The pit had not been started recently. The progress of the diggers was fantastically slow. Clumsy metal scoops raised a few tons of material at a time and deposited it in mobile containers that bore it swiftly away. Fragments of the pit-wall were periodically knocked loose by expanding clouds of ionized gas, apparently formed chemically. The shocks initiated by these clouds were apparently the origin of most of the temblors he had felt from this source, while he was still eighty miles away. 

	His electrical analysis finally gave him the startling, incredible facts. This was a copper mine—extracting ore far poorer in quality than his own people could afford to process. This race was certainly confined, for some reason, to its home planet, and had been driven to picking leaner and ever leaner ores to maintain its civilization. 

	The development of organic machines had given them a reprieve from barbarism and final extinction, but surely could not save them forever. Why in the galaxy, did they not use the organic robots for digging directly, as he had seen them do, during the language lessons? One would think that metal would be far too precious to such planet-bound people, for them to waste even iron on bulky, clumsy devices such as those at work here! 

	Even granting that the machines he had originally seen, and which seemed the most numerous, were not ideally designed for excavation work, surely, surely, better ones could be made. A race that could do what this race had done with carbon compounds could have no lack of ingenuity—or, more properly, of creative genius. 

	Very slowly, he realized why they had not—and why his mission was futile. He realized why these people would be doomed, even if the moles had never been planted. He noticed something relevant during the conversation, but had missed its full staggering implication. The organic compounds were soft. They bent and sagged and yielded to every sort of external mechanical influence—it was a wonder, thinking about it, that the machines he had seen held their shapes so well. No doubt, there was a frame-work of some sort, perhaps partly metallic even though he had not perceived it. 

	But such things could never force their way through rock. The only way they could dig was with the aid of metallic auxiliaries—simple ones, such as those used to illustrate the verb to him, or more capacious and complex ones like those in use here. 

	This race was doomed, had been doomed long before the poachers ever approached their planet. They needed metal, as any civilization did. They were bound to their world, but kept from moving about even upon it, for not one in a thousand of these people could conceivably travel by machine, as the agent's race did. The organic engines could not possibly be used as vehicles. They could not be so used because their very essential nature of chemical violence made them untouchable. 

	These people were trapped in a vicious circle, using their metal to dig more metal, sparing what little they could for electrical machinery and other equipment essential to a civilization, always having less and less to spare, always using more and more to get it. The idea that they could survive, until the planet's natural processes renewed the supply, was ridiculous. 

	It was, in short, precisely the same tragic circle that the agent's own race was precariously avoiding, millennium after millennium, by its complex schedule of freighters that distributed the metal from each planet in turn among thousands of others; then either waited for nature to renew the supply, or "tickled up" uninhabitable worlds as the poachers had done to this one. 

	Metal kept the machines operating. The machines kept food flowing to that vast majority of individuals who could not travel in search of it. A single break in the transport schedule could starve a dozen worlds. It was a fragile system, at best, and no member of the race liked to think about—much less actually face—examples of its failure. 

	The agent's mounting discomfort as he considered the matter of Earth was natural and inevitable. This race was what his own might have been, hundreds of millions of years before, had means of space-travel not been developed. They would probably be extinct before the poachers' torpedoes began to take effect, which was, no doubt, a mercy. 

	The agent could not help them. Even if the communication problem were cracked, they could not be brought into the transport network of civilization for untold millennia. No, they were truly lost—a race under sentence of extinction. The reorganization necessary was frightening in its complexity, even to him. Teaching them to build and use the equipment of his ship would be utterly useless, since it was entirely metallic, and they would be even worse off than with their organic devices; 

	They were already, probably by chemical means, stripping ores more efficiently than his own people, so he could hardly help them there. No, it was a virtual certainty that, when the planet's crust began to heave as giant bathyliths built up beneath it, when rivers of lava poured from vents scattered over the planet, no one would be there to face it. 

	This was a relief, in a way. The agent could picture, all too vividly, the plight of seeing a close friend engulfed only a few miles away, and having to spend hours or years of uncertainty, wondering when his own area would be taken—and then knowing. 

	That was the worst. There was plenty of warning, as far as awareness was concerned. Anywhere from minutes to years and millennia, if one was a really good computer. You knew, and if you had a mobile machine, you could move out of the way. Even these organic machines traveled fast enough for that. But only machines would let a being get out of the way—and there would be no machines here by then. 

	He wished with every atom of his being that he had never detected the poachers, had never seen this unfortunate planet or heard of its race. No good had come of it—or very little, anyway. There would, admittedly, be metal here before long, brought up with the magma flows, borne by subcrustal convection-currents in the stress-fluid that formed most of the worlds bulk. 

	The poachers would be coming back for it, and he could at least deprive them of that. He would beam a report in toward the heart of the galaxy, making sure it did not radiate in the direction they had taken. Then there would be freighters to forestall them. 

	It was ironic, in a way. If any of this race should have survived the disturbance that would bring back the metal, that disturbance would be the salvation both of their species and their civilization. Most probably, however, the only witnesses would be a few half-starved, dull-minded barbarians, who would wonder, dimly, what was happening for a little while before temblors shattered their bodies forever. 

	There was nothing to keep him here, and the place was distasteful. More of the organic robots were approaching his position, but he did not want to talk any more. He wanted to forget this planet, to blot the memory of it forever from his mind. 

	With abrupt determination, he sent the dirt boiling away from his hull in a rising cloud of dust, pointed his vessel's blunt nose into the zenith and applied the drive. He held back just enough to keep his hull temperature within safe limits, while he was still in the atmosphere. 

	Then, with detectors fanning out ahead, he swung back to the line of his patrol orbit, and began accelerating away from the Solar system. Ignorant of events behind him, he never sensed the flight of swept-winged metal machines that hurtled close below while he was still in the air, split seconds after he had left the ground. 

	He did not notice the extra radar beam that fastened itself on his hull, while the machine projecting it flung itself through the sky, computing an interception course. This was too bad, for the relays in that machine would have made him feel quite at home, and its propulsion mechanism would have given him more food for thought. 

	He might have sensed its detonation, for his pursuer had a nuclear warhead. But its built-in brain realized, as quickly as the agent himself could have, that no interception was possible within its performance limits. It gave up, shutting off its fuel and curving back toward its launching station. Even the aluminum alloys in its hull would have interested the agent greatly—but he was trying to think of anything except Earth, its inhabitants and their appalling technology. 

	His patrol orbit would carry him back to this vicinity in half a million years or so. The freighters would have been there by that time. 

	He wondered if he could bring himself to look at the dead world. 

The End