Lecture Demonstration
Mesklin 01.1
(1973)
Hal Clement

INTRODUCTION TO "LECTURE DEMONSTRATION"
From Astounding: John W Campbell Memorial Anthology
edited by Harry Harrison, Random House, 1973

	I have been a high-school teacher/or a quarter of a century, a student for nearly twice as long. "Lecture Demonstration" may show me as the former to people who did not know John Campbell, but not to those who did.

	He bought my first story over thirty years ago when I was a college sophomore. Since then we have exchanged thousands of words of correspondence 4 spent many, many hours in conversation. We sometimes agreed, some--its did not. I was trained in theory-—astronomy and chemistry-—and still to center my extrapolation on one/actor alone, like a politician. John’s vocation was in engineering, and he tended to remember better than I that the rules are working at once: when he focused on one, it was to start an argument.

	We were alike in one way. The phrase "of course" set either of us going. It’s such fun to take apart a remark where those words appear! To show why, or how, or under what circumstances it isn’t true at all! That attitude was the seed of Mission of Gravity, the story on which I am content (so far) to let my reputation rest, and how much of that story was John is for future Ph.D. candidates to work out. He provided none of the specific scientific points, for once, the general attitude underlying it was so obviously Campbellian that I had been flattered more than once to hear that "of course" Hal Clement was a Campbell pseudonym.

	He would certainly have had as much fun writing Mission of Gravity as I did. Low-gravity planets and high-gravity planets were old hat to science fiction fans, but 0/Course no one planet can vary greatly in its gravity So, naturally, Mesklin was born, thousands of times the mass of Earth, but whirling rapidly on its axis that its equatorial diameter is more than twice the polar value, less than eighteen minutes pass from noon to noon, and a man massing one hundred and eighty pounds weighs five hundred and forty at Mesklin’s equator and nearly sixty tons at its poles.

	Its people were fun to make up too. Little, many-legged types afraid of flying away with the wind at their low-gravity equator; about at the cultural level of Marco Polo-—and one of them at least was a much sharper trader. He followed along very co-operatively when the strange beings from the sky had him, until he had them where he wanted them. Then he held out for scientific knowledge.

	Of course one doesn’t interfere with the development of a primitive by teaching it modem technology.

	"Lecture Demonstration" takes place during the formative years to College on Mesklin, when the teachers are still learning too, and Mesklin are finding that human beings are quite human.

	Whatever that may mean.

* * * * * *

	The wind wasn’t really strong enough to blow him away, but Estnerdole felt uneasy on his feet just the same. The ground was nearly bare a gray-speckled, wind-polished reddish mass which the voice from the tank had been calling a "sediment." It was dotted every few yards with wide-spreading, rubbery bushes whose roots had somehow eaten their roots into a surface which the students’ own claws could barely scratch. Of course the plants themselves could provide anchorage if necessary. The ex-sailor’s nippers were tensed, ready to seize any branch that might come in handy if he did slip. While the gusts of hydrogen sweeping up from the sea at his right wouldn’t have provided much thrust for a the feeble gravity of Mesklin’s equator—less than two percent of what was used to in the higher latitudes—made Estnerdole feel as though nothing at all could send him flying. It took a long time, as he had been warned, to get used to conditions at the World’s Rim; even the bulky metal tank crawling beside him seemed somehow unsteady.

	He was beginning to wonder whether he had been right to sign on at the College—the weird establishment where beings from the sky taught things that only the most imaginative Mesklinites had ever dreamed of. It would be fine to be able to perform miracles, of course, but the preliminaries—or what the aliens insisted were necessary preliminaries—got dreary at times.

	This walk out on the peninsula, for example. How could the aliens know what the rocks below the surface were like, or what kind would bear at the surface at a given place, and how they had been folded up to twice this long arm of dry land? And, most of all, why was any of this worth knowing? True, he had seen some of the holes drilled near the school and had even examined the cylinders of stone extracted from them; but could anyone feel even moderately sure that things were the same a few cables away? It seemed like expecting the wind to blow from the north on one hilltop merely because it was doing the same on the next one.

	Well, the alien teachers said that questions should be asked when things weren’t clear. Maybe one would help here—if only Destigmet wouldn’t cut in with his own version of the answer. That was the worst of asking questions. You couldn’t be sure that everyone else didn’t already know the correct answer...

	But Estnerdole had learned a way around that.

	"Des!" he called, loudly enough to be sure that the other students would also hear. "I’m still not straight on one thing. The human said that this exercise was to tell whether the peninsula is a cuesta or a hogback. I still don’t see the difference. It seems to me that they could be pretty much the same thing. How do you know that anything you call a cuesta isn’t a small part of a hogback, or wouldn’t be if you looked over enough ground?"

	Destigmet started to answer in a self-assured tone, "It’s simply a matter of curvature. A cuesta is a flat layer of rock which has a softer layer under it which has eroded away, while a hogback—" He paused, and Estnerdole’s self-esteem took an upward turn. Perhaps this know-it-all was going to run aground on the same problem. If Des just had to give up and ask the instructor, everything would be solved. If he didn’t, at least there would be the fun of hearing him struggle through the explanation; and if it didn’t come easily, there would be nothing embarrassing about checking with the tank driver.

	The voice which came suddenly from the vehicle was therefore an annoyance and a disappointment. Estnerdole did not blame a malicious power for interfering with his hopes, since Mesklinites have little tendency toward superstition in the mystical sense, but he was not pleased.

	"We will turn south now," the alien voice said in perfectly comprehensible, though accented, Stennish. "I haven’t said much yet, because the ground we’ve been covering is similar to that near the College. I hope you have a clear idea of what lies underneath there. You remember that the cores show about twelve meters of quartz sandstone and then over twenty of water ice, followed by several more layers of other silicate sediments. It shouldn’t have changed much out here. However, you will recall that along the center of this peninsula the surface looks white from space—you have been shown pictures. I am guessing, therefore, that erosion has removed the sandstone and uncovered the ice at the top of a long fold. This is of course extrapolation, and is therefore a risky conclusion. If I’m right, you will get some idea of what can be inferred from local measurements, and if I am wrong we will spend as much time as we can to find out why."

	"It will take us only a few minutes to reach the edge of the white strip. Any of you who don’t mind the climb may ride on the tank. Now, I want each of you to think out in as much detail as your knowledge and imagination permit just what the contact area should be like—thickness of sandstone near it, smoothness of the two surfaces, straightness of the junction, anything else which occurs to you. I know you may feel some uncertainty about making predictions on strictly dry-land matters, but remember I’m taking an even worse chance. You’re sailors, but at least you’re Mesklinites. I’m from a completely different world. There, I’m stopping; any of you who wish, climb on. Then we’ll head south."

	Estnerdole decided to ride. The top of the tank was seven or eight body lengths above the ground and acrophobia is a normal, healthy state of mind for a Mesklinite; but College students were expected to practice overriding instinct with intelligence wherever possible. There would obviously be a better view of the landscape from the top of the vehicle. Estnerdole, Destigmet, and four of the remaining ten class members made their way up the sides of the machine by way of the ladder-like grips provided to suit their pincers. The other six elected to remain afoot. They took up positions beside and ahead of the tank as it resumed motion, the flickering legs which rimmed their eighteen-inch worm-like bodies barely visible to the giant alien inside as they kept pace with him.

	Visibility dropped as night fell, and for nearly nine minutes the tank’s floodlights guided the party. Then the sun reappeared on their left. By this time the edge of the dark rock they were traversing was visible from the tank roof, only a few hundred yards ahead. The teacher slowed, and as the ground party began to draw ahead, he called its members back. "Hold on. We’re almost close enough to check predictions, and I’d like to get a few of them on record first. Has anyone seen details yet which surprised him?"

	Estnerdole remained silent; he had made no predictions he would trust, and did not expect to be surprised by anything. Destigmet also said nothing, but his friend suspected that it was for a different reason. None of the others had anything to say either, and the teacher sighed inaudibly inside his machine. It was the same old story, and he knew better than to let the silence last too long. "There’s something I didn’t guess," he finally said. "The edge of the dark rock isn’t as straight as I had expected—it looks almost wavy. Can anyone suggest why?"

	Destigmet spoke up after a brief pause. "How about the bushes? I see them growing along the edge. Could they have interfered with the erosion?"

	"Possibly. How could you check on that possibility?"

	"See whether the rock where they’re growing is any higher or lower than where they aren’t."

	"All right. Let’s see."

	And that was why the group was all together when the shell of sandstone gave way under the tank.

	The human teacher observed less of the event than his pupils. The yielding ledge freed his vehicle for a fall of some fifty feet under three times his normal gravity, and one second was not long enough for him to appreciate the situation. His safety clamps, padded and reinforced though they were, had not been designed for any such shock, though it was just as well they were there. Neither was the shell of the tank, and even the students least familiar with the alien machinery could tell that something was wrong with it. The evidence was not visual; a stink of oxygen permeated the neighborhood and for a moment sent the Mesklinites scurrying as far as they could. Even a creature which doesn’t actually breathe because it is small enough for high-pressure hydrogen to reach all its tissues by direct diffusion may have evolved a sense of smell.

	The space into which they had dropped was windy, and the oxygen quickly became imperceptible. Estnerdole crept back to the side of the motionless tank; like his fellows, he was of course uninjured. The fall had meant no more to them physically than a similar one on Phobos would have to a human being, though any fall can be expected to provide an emotional jolt to a Mesklinite.

	"Teacher! Dr. LaVerne! Can you answer us?"

	There was no response, and after a moment the sailor began to examine the machine in detail, looking for visible damage. The process was hampered by the fact that it was three quarters buried in white powder— the ammonia snow which had been blowing from the north for weeks as winter for Mesklin’s habitable hemisphere drew on. The snow formed a slope of about thirty degrees, extending into a hollow which reached east and west as far as Estnerdole could see. The cavern’s north face was walled by a nearly vertical cliff of clear, glassy material. The roof, now pierced by the hole through which the party had fallen, was rock. Sunlight slanting through the hole was reflected by the ammonia which formed the south side and illuminated the immediate area for the moment, though the light changed constantly as the beam scanned along the slope.

	The exposed portion of the tank showed no visible cracks; the oxygen must have leaked from some place below. Light was shining from the exposed windows, and Estnerdole made his way to the nearest of these by means of the climbing grips which studded the shell. Destigmet was close behind.

	Neither was really familiar with the vehicle’s interior, so neither could be sure whether the apparent chaos of objects within was normal or not. The form of the teacher was visible, motionless in the control seat. His armor, which they had seen often enough to know well, appeared intact; but the transparent front of the headpiece seemed to have colored liquid over part of its inner surface. The human being’s head could not be discerned in detail. Neither sailor was familiar with the appearance of human blood, but both had good imaginations—even though they lacked real circulatory systems of their own.

	"We’ll have to get in somehow and get him out of there," Destigmet said. "He’ll have to get back to the College somehow, and we certainly can’t carry the tank."

	"But if we break in or open the door, our air will get in too, and he can’t stand that. Shouldn’t we, or some of us, go back ourselves and bring human help?"

	"Our air is already inside—at least, his came out, and ours has much higher pressure. Either his armor saved him, or it’s too late already. Certainly if any of us can get out, one should go for help; but the rest must get to him and at least do our best to see that—well, to see whether we can do anything. Come on, everyone—dig out the door and try to get it open while we can still see. One of you climb the hill."

	The snow was loose and powdery, defeating any attempt to dig a narrow hole. The door of the tank was on the downhill side, which helped some. The bulk of the vehicle kept the entire mass of white dust from sliding down. Legs working at near-invisible speed hurled the stuff away from the metal in clouds, and as the minutes passed, the lower part of the vehicle grew more and more visible. The five minutes or so of daylight left when they started was not nearly enough to let them shift all those cubic yards of material, but enough light came from the windows to let the Mesklinites work through the night; and within two days the door was uncovered. There would have been no difficulty in opening it, but even Destigmet was a little uneasy about doing so in spite of his earlier logic. "Let’s check the window once more," he said. "Maybe—" He left the sentence unfinished and began the climb to the nearest window.

	He had scarcely started, however, when the hull of the tank shifted slightly, tilting toward the cluster of watching Mesklinites. Destigmet had never jumped in his life—the concept was alien to a being reared in nearly three hundred Earth gravities—but his reflexes did something. Suddenly he found himself over twenty yards away from the tank, close to the glassy cliff which formed the other wall of their prison.

	His fellows had also scattered, but not quite so abruptly. They were delayed mostly by bad traction, the fluffy material under their claws doing most of the initial moving. Destigmet had been on the tank.

	The latter did not complete its threatened fall, for the moment. It was resting entirely on the loose, white dust which had saved it from flattening like an egg under an elephant’s foot, and most of this had been removed from the downhill side; but it did not yet fall. The Mesklinites approached again with caution. Even they, in a place where everything’s weight seemed negligible to them, had no wish to be underneath that mass if it really did topple.

	"I thought your weight must have shifted it, but something else must be moving inside," remarked Estnerdole. "Maybe the teacher is in better shape than we thought."

	"A person’s weight doesn’t mean a thing here," returned Destigmet. "It must be him moving. Let’s get to that window."

	"If he is moving around, the climb will be pretty risky. Nothing but luck is keeping that thing from rolling the rest of the way down the slope now.

	"No matter. We have to find out. Come on." Destigmet led the way up the loose material, but before any of the Mesklinites had reached the tank, it became evident that its occupant was once more active. Its outer lights suddenly flashed on.

	Estnerdole gave a hoot of relief, and followed it with words. "Dr. LaVerne! How can we help you?"

	For several seconds there was no answer, but the tank wavered even more alarmingly. Then the door opened, and the giant figure of the armored alien appeared in the opening. It tottered a moment, then fell outward into the snow. The tank rocked away as the man’s weight left it, swung forward again in a way which would have brought Estnerdole’s heart to his mouth if he had possessed a heart, and then stopped once more.

	The Mesklinites swarmed forward with the common intent of dragging the human being away from the dangerous neighborhood, but before they reached him he started crawling under his own power. His voice came haltingly from his helmet speaker. "Stay back—all of you—I can make it—you couldn’t move me in this stuff anyway.

	Estnerdole and two of the others kept coming; with Mesklinites as with other intelligent races, some customs override selfish caution. The three tiny figures swarmed around the struggling monster, trying to speed its faltering trip away from the danger zone, but they promptly found that the teacher had been right; they couldn’t help. It was not that the five hundred kilograms of weight were too much for them—any one of them could have lifted that. The trouble was the footing. A Mesklinite’s legs end in insect-like claws, except for the flippers on the fore and aft pairs; the claws provide excellent traction on the wooden deck of a ship or the hard-packed soil which covers much of Mesklin. But a sand dune or a heap of ammonia snow is a different matter. The students’ efforts to push the huge bulk of their teacher simply drove their own bodies into the loose fluff

	LaVerne was only partly aware of their presence. He had more or less recovered from the shock of his fall, and had seen enough to evaluate the situation fairly well; but he was not really in full possession of his faculties. He knew he was on a sloping surface of loose material, and that the tank was rather likely to roll over on him at any moment; his whole attention was focused on getting out of the way. The warning to the students had been little more than reflexive like their own move to help him, and he did not follow up the order. He simply crawled as well as the situation permitted. A human observer might have had trouble deciding whether his mode of progress should have been called crawling or swimming, but he did make progress. He never was sure whether it took him five seconds or a whole minute, but presently he found himself on smooth, solid rock with the white slope safely behind him. He relaxed with a sigh, and only slowly became aware of the dozen caterpillar-like figures around him.

	With an effort he managed to lever himself to a sitting position. His students waited silently. He took in the nearly buried tank, the cliff in the opposite direction, the rock roof above with sunlight slanting through the jagged hole, and the darkness which swallowed the seemingly endless cavern to east and west. The Mesklinites were reasonably familiar with human facial expression and tried to read his, but they could make out little in the poor light through the face plate, which was partly obscured by blood from his nose. They waited for him to speak, and were not surprised when his first words formed pertinent questions. "How long did it take you to do that digging job? How long has that fellow been trying to climb the hill?"

	Destigmet answered, "Only a few days; we didn’t keep close count.

	"Hmph. He hasn’t gotten very far. I’m reminded of an animal on my own world which traps its prey in pits rather like this—loose stuff at its angle of repose. Climbing such a surface is nearly impossible. What will he do if he gets to the top? That hole is twenty of your body lengths away from the bank."

	"But, Doctor!" pointed out Estnerdole, "the stone itself is not very thick. If we can reach the top, we know the stone comes to an end only a short distance to the south. We can dig our way out from under the edge easily enough."

	"True enough. All right, the rest of you might as well try dim too. So, for that matter, might I; if I get out on the surface I can call for help instead of having to send a runner.

	"Rest first," advised Destigmet. "You can’t be in very good condition yet. You must have been hurt some, if the state of your face plate m anything."

	"All right. I want to think, anyway."

	Near-silence fell while the rest of the students began to climb. or three, starting just below the stranded tank, had little trouble as far as the vehicle; but from there on it was a different matter. The creatures were tiny, some eighteen inches long with split-cylinder bodies an inch and a half in diameter. They were light; their half-pound m weighed less than a kilogram at Mesklin’s equator. Even that weight, however, sank their tiny legs full-length into the snow. The motion of short limbs could be inferred from the clouds of white dust which sprayed backward from the small bodies. A hollow formed around each sic form, with material sifting down into it from the front and sides. hind, it built up into something approaching a level surface, and so very slowly—the Mesklinites followed their fellow uphill. Sometimes would speed up briefly as he encountered a slightly more firmly placid area; almost as often he would slide back a body length or two, spraying frantic clouds of white dust, before resuming forward motion. Every few seconds a pile of snow behind one of them would collapse and slide down hill, spreading its material out until a new approach to the angle of pose was attained.

	Minutes—long minutes—passed. Those who had used the tank as a starting point were four or five yards up the slope, not too far behind the one who had started so long before. The rest, whose slipping had started at a lower level, had made little visible progress. The little fans and rivulets of sliding snow, first behind one and then another of the dozen red and-black figures, were as hypnotic as the patterns in a bonfire; LaVerne had to wrench his attention away from them, suddenly realizing that I had more serious jobs than being a spectator.

	Slowly and painfully he hoisted himself to his feet. He could manage this at all only with the aid of ingenious lever-and-ratchet systems in the joints of his armor which let him concentrate on one part of the job at time, and rest frequently without losing what he had gained. Once up, he turned slowly around, clarifying the mental picture he had already developed of the space they had fallen into. It was not too hard to infer ho the cavern must have formed.

	As he had guessed, the layer of water ice under the sandstone ha been bared by erosion at the top of the fold which formed the peninsula.

	"True enough. All right, the rest of you might as well try climbing too. So, for that matter, might I; if I get out on the surface I can call for help instead of having to send a runner.

	"Rest first," advised Destigmet. "You can’t be in very good condition yet. You must have been hurt some, if the state of your face plate means anything."

	"All right. I want to think, anyway.

	Near-silence fell while the rest of the students began to climb. Two or three, starting just below the stranded tank, had little trouble getting as far as the vehicle; but from there on it was a different matter. The creatures were tiny, some eighteen inches long with split-cylinder bodies an inch and a half in diameter. They were light; their half-pound masses weighed less than a kilogram at Mesklin’s equator. Even that weight, however, sank their tiny legs full-length into the snow. The motion of the short limbs could be inferred from the clouds of white dust which sprayed backward from the small bodies. A hollow formed around each slender form, with material sifting down into it from the front and sides. Behind, it built up into something approaching a level surface, and slowly— very slowly—the Mesklinites followed their fellow uphill. Sometimes one would speed up briefly as he encountered a slightly more firmly packed area; almost as often he would slide back a body length or two, spraying frantic clouds of white dust, before resuming forward motion. Every few seconds a pile of snow behind one of them would collapse and slide downhill, spreading its material out until a new approach to the angle of repose was attained.

	Minutes—long minutes—passed. Those who had used the tank as a starting point were four or five yards up the slope, not too far behind the one who had started so long before. The rest, whose slipping had started at a lower level, had made little visible progress. The little fans and rivulets of sliding snow, first behind one and then another of the dozen red-and-black figures, were as hypnotic as the patterns in a bonfire; LaVerne had to wrench his attention away from them, suddenly realizing that he had more serious jobs than being a spectator.

	Slowly and painfully he hoisted himself to his feet. He could manage this at all only with the aid of ingenious lever-and-ratchet systems in the joints of his armor which let him concentrate on one part of the job at a time, and rest frequently without losing what he had gained. Once up, he turned slowly around, clarifying the mental picture he had already developed of the space they had fallen into. It was not too hard to infer how the cavern must have formed.

	As he had guessed, the layer of water ice under the sandstone had been bared by erosion at the top of the fold which formed the peninsula.

	The stone must have worn virtually to a knife edge; no wonder it had failed to support the tank’s weight once the underlying ice had gone. Ice was hard enough at Mesklin temperatures to stand mechanical erosion reasonably well, of course, but there was another factor operating here. Each year, as the giant world swung past periastron and the northern hemisphere began its summer, storms started sweeping ammonia snow from the virtually world-wide northern "ice" cap across the equator. This naturally buffered the local temperature near the freezing point of ammonia, which the Mesklinite student scientists had selected as the arbitrary control point for temperature in the scales they were developing.

	Once the protecting silica had eroded away, the solid ammonia encountered the equally solid water, and liquid resulted. Not only was some heat generated, but the solutions of the two had considerably lower freezing points than either compound alone—a fact which the present crop of students had all faced in their most elementary courses. The ice layer had melted, or dissolved, if one preferred to think of it that way, for fifty or sixty yards back from the edge of the protecting stone. Later in the season when the ammonia had evaporated, this would show a beautiful overhanging ledge extending probably for miles east and west. With luck, LaVerne would be able to see it; since the College had been set up less than half a Mesklin year before, no one had had the chance yet.

	LaVerne was not so much a scientist as a teacher. Still, he knew enough physical chemistry to wonder about the age of the peninsula—how long it would take the weight of overlying rock to squeeze the ice to the top of the fold and empty the filling from the sandwich. Maybe it had been going on for years already, and if they stayed in the cavern they could measure the creep of the south wall toward them. Maybe—

	A hoot that was almost deafening even through his helmet jerked his wandering mind back to the current realities. He knew about Mesklinite voices, of course, but no human being ever got used to their more extreme volumes. He turned as quickly as he could from the ice cliff to the slope which his students had been trying to climb. By the time he really got his eyes focused on the scene, the key events had happened; but it was obvious enough what they had been.

	The snow being kicked downhill by the climbers had been piling up against the tank. The earlier digging had left the vehicle almost without support on its downhill side, and what any thoughtful witness would have predicted had finally occurred. By the time LaVerne completed his turn, the machine was well into a full roll downhill toward him, and almost completely hidden inside a developing avalanche. The hoot, coming from several Mesklinites at once, had been stimulated by their discovery that they were involved in the slide; its upper edge was propagating rapidly toward the top of the slope and was already above the highest of the climbers.

	The man had little thought for his students just then. The rolling tank was heading straight toward him, and he could not possibly move fast enough to get out of its way. He was several yards from the bottom of the slope, but that might not be far enough. It all depended on whether the tank would reach the stone with enough energy to roll those few yards—let’s see; it looks as though it would land right side up; then onto its right side, then the top, then the left—that should bring it right to my fret. If there’s one more quarter-turn left in it, I’m flat. LaVerne wondered later how he was able to analyze the matter so calmly as the mass of metal came whispering down on him in its envelope of dusty snow.

	Actually it scarcely rolled at all, coming to rest with an ear-shattering clang on its right side. The man had a good heart—he would never have been allowed to serve on Mesklin otherwise—and was able to switch his attention back to his students almost at once; but the switch did his heart little more good than the juggernauting tank had. The Mesklinites were invisible.

	For a moment, real fear struck him—intelligent fear based on foresight, not just panic. If those people were gone, he would most certainly not get back to the college. Then little white fountains of dust began to erupt from various points near the bottom of the slope, and one after another the Mesklinites emerged. None of them had been buried deeply enough to matter. All was well.

	Except that there seemed no way to get out of the cave.

	Ideas flowed from all directions, since the Mesklinites were an imaginative lot; but none of these seemed very practical. Estnerdole suggested that the cave be explored in the east-west directions, on the chance that there might be a more usable way out. The objection to this was that not even the Mesklinites could see in the total darkness which obtained away from the area sunlit from their entry hole. Destigmet proposed cutting climbing notches in the cliff of water ice and reaching the top that way; unfortunately, ice met stone many yards from the hole, and there was no reason to hope that even the natives, insect-like as they appeared to human beings, could possibly crawl inverted along the stone ceiling. LaVerne, conditioned by a childhood on Earth, thought briefly of packing the snow to make a more reliable support and with it actually constructing steps up the slope. Fortunately for his reputation with the Mesklinites, he remembered in time that ammonia, unlike water, is denser in the solid than in the liquid phase. It does not, therefore, tend to melt under pressure; trying to make even a snowball out of the powdery stuff which had trapped them would be like trying to do it with a handful of sand.

	"All I can suggest," he said at last, "is for some or all of you to start climbing again—maybe farther apart this times so one person’s avalanche doesn’t involve everyone else. At least the tank won’t be a problem any more. It will be slow, but if even one you can get to the top, he can go back to the College and get help. can last here for days, with the air supplies in the tank, so there’s no emergency.

	"I’m afraid there is, Doctor," pointed out Estnerdole. "You can’t get into the tank. It’s lying on its right side, with the door underneath. Unless there is some outside connection you can reach to replenish the oxygen in your armor, you are rather limited in your supply."

	The man was silent for several seconds, except for a brief muttering which the students could not make out clearly. "You’re right, Es," he said at last. "It is an emergency after all, for me. Do you suppose you people are strong enough to turn the tank right side up?"

	The Mesklinites were somewhat doubtful, but clustered around to try. LaVerne, who shared the exaggerated idea of Mesklinite physical strength which was so common among human beings, was not surprised when the vehicle stirred under their efforts; indeed, he was disappointed when it lofted only a few millimeters. After some seconds it settled back where it had been, and one of the students reappeared from the narrow space underneath. "We can move it, but that’s all. We’d have to get this side up several body lengths before it would rock over the right way, and there’s nothing to stand on."

	Destigmet wriggled into view behind the speaker. "I can think of only two things to do, and you’ve already suggested one of them," he said. "The first is for someone to start climbing again. The other is for us to lift the tank once more, while you pack snow under it to hold it up and let us get a fresh purchase. Maybe we can work it up that way before you run out of air."

	"All right," agreed LaVerne. "It would be better if I had something to serve as a shovel, but let’s get at it. I’m using oxygen just standing here worrying."

	For a while it looked possible, if not really hopeful. Carrying the dusty snow in his armored hands proved impractical, but he found that he could do fairly well pushing a mass of it ahead of him as he crawled—and crawling was far easier than trying to walk. Essentially, he was sweeping rather than carrying. He managed to get what would have been several shovelfuls, if he had had a shovel, against the space at the edge of the tank where the Mesklinites had disappeared once more. At his call they strained upward again, and as quickly as he could he pushed the material into the widening space. "That’s all," he reported when he had done his best, and the students relaxed again. So did the pile of snow. LaVerne, optimistic by nature, felt sure that the tank had not settled quite back to its original position, and kept trying; but after an hour which left him more exhausted than he had ever felt in his three Earth years on Mesklin, he had to admit that the idea was qualitatively sound but quantitatively inadequate.

	During those days, the student who was trying to climb the slope had made little progress. Once he had gotten nearly a third of the way before sliding most of the way back in a smother of white dust; four or five times he had lost the fight in the first yard or two. The rest of his attempts came between those limits.

	But it finally became evident that the man’s air was not going to be the real limiting factor. Destigmet pointed out another one to him. "Some time ago, Doctor, one of your fellows taught us about a fact he claimed was very basic—the Law of Conservation of Energy If I have the terminology right, we can apply very large forces by your standards, but as that law should tell you, there is a limit to the amount of work we can do without food. None of us expected to need food in this class, and we brought none with us."

	One of the others cut in. "Won’t people from the College start looking for us anyway? This class should have been over days ago.

	LaVerne frowned invisibly behind the blood-stained face plate, which he had no means of cleaning. "They’ll be looking, but finding us will be another story," he said. "They’d expect to see the tank miles away on the smooth surface of the peninsula. When they don’t, they’ll think we got swept into the sea, or went off to the forest country for some reason. They won’t look over this area closely enough to find the hole we left, I suspect. It’s possible we’ll get out of this with their help, but don’t count on it.

	Estnerdole suddenly became excited. "Why not build a tower we can climb, with the water ice from the cliff? We can chip it out easily enough without tools, or even melt it out with the snow—no, that wouldn’t leave us any to work with, but—" His voice trailed off as more difficulties became apparent to him.

	LaVerne was pessimistic, too, after the just-completed practical demonstration of how much material would be needed even to prop up the tank. Then he brightened. "We could use the ice to get this machine upright—big chunks of it would be more practical and easier to move than the snow. Of course, even that doesn’t get us any closer to getting out of here; the tank certainly isn’t going to climb this sand hill even if I get into it. If only—" He paused, and the ensuing silence stretched out for long seconds. Even with the man hidden in his armor, the listeners got the impression that something had happened. Then he spoke again, and his tone confirmed the suspicion. "Thanks, Es. That does it. Start digging ice, gentlemen. We’ll be out of here in a couple of hours!"

	Actually, it took less than three days.

"You look bothered," remarked Thomasian, LaVerne’s department head. "Delayed shock from your narrow escape, or what?"

	"It wasn’t that narrow," replied the teacher. "I had hours of air still in the suit when the spinner picked us up, and we could have worked the tank upright to get at more if I had needed it. You’d have searched the area closely enough to find that hole sooner or later."

	"Later would probably have been too late—and the really narrow squeak I was thinking of was the fall. Fifteen meters under three gees— sooner you than me. If it hadn’t been for that snow bank, we’d have had to cut you out of the flattened remains of that tank—not that it would have been worth doing. Of course any of your students should have been able to think of tossing pieces of water ice over the slope, especially after you’d discussed with them why the cuesta was so deeply undercut. So should you, for that matter— "Hogback," LaVerne responded almost automatically "Sure, all sorts of ideas are obvious afterward. At the time, I wasn’t quite sure that this one would work, even if I did sound as enthusiastic as I could and even though I did have experience to go by Still, I was afraid it would simply melt holes in the slope; but it went fine. The liquid formed where the two ices met just soaked into the surrounding snow, spreading out and diluting the water ice until the mixture’s melting point came up to the local temperature again—and froze into a continuous mass. It was hard enough for Estnerdole to climb out and go for help in less than an hour, I’d guess; I didn’t actually time it.

	"What was the experience you could go by? And if it was so easy and safe, what’s bothering you?"

	"The same thing. A teaching problem. They claim that Mesklinite psychology is enough like ours for teaching techniques to be about the same, effectively They expect us to—er—’relate’ new facts to known experience."

	"Of course. So?"

	"So the experience in question should obviously be one familiar to the students, not just the teacher. What sparked this idea for me was the memory of sugar getting lumpy in the bowl when it gets damp. You know, I’m just a little shaky on the local biochemistry, chief—tell me: what do Mesklinites use for coffee, and what do they put in it?"

The End