"I don't believe it," said Holmes flatly.

Burke shrugged. He found that he was tense all over, so he took some pains to appear wholly calm.

"It isn't reasonable!" insisted Holmes. "It doesn't make sense!"

"The question," observed Burke, "isn't whether it makes sense, but whether it's fact. According to the last word from Earth, they're still insisting that the ship's drive is against all reason. But we're here. And speaking of reason, would the average person look at this place and say blandly, 'Ah, yes! A fortress in space. To be sure!' Would they? Is this place reasonable?"

Holmes grinned.

"I'll go along with you there," he agreed. "It isn't. But you say its garrison was men. Look here! Have you seen a place before where men lived without writings in its public places? They tell me the ancient Egyptians wrote their names on the Sphinx and the Pyramids. Nowadays they're scrawled in phone booths and on benches. It's the instinct of men to autograph their surroundings. But there's not a line of written matter in this place! That's not like men!"

"Again," said Burke, "the question isn't of normality, but of fact."

"Then I'll try it," said Holmes skeptically. "How does it work?"

"I don't know. But put a cube about a yard from your head, and doze off. I think you'll have an odd dream. I did. I think the information you'll get in your dream will check with what you find around you. Some of it you won't have known before, but you'll find it's true."

"This," said Holmes, "I will have to see. Which cube do I try it with, or do I use all of them?"

"There's apparently no way to tell what any of them contains," said Burke. "I went back to the storeroom and brought a dozen of them. Take any one and put the others some distance away—maybe outside the ship. I'm going to talk to Keller. He'll make a lot of use of this discovery."

Holmes picked up a cube.

"I'll try it," he said cheerfully. "I go to sleep, perchance to dream. Right! See you later."

Burke moved toward the ship's air-lock.

"Pam and I have some housekeeping to do," Sandy said.

Burke nodded abstractedly. He left the ship and headed along the mile-long corridor with the turn at the end, a second level and another turn, and then the flight of steps to the instrument-room. As he walked, the sound of his footsteps echoed and reËchoed.

Behind him, Holmes set a cube in a suitable position and curled up on one of the side-wall bunks in the upper compartment of the spaceship.

"We'll go downstairs," said Sandy.

Pam parted her lips to speak, and did not. They disappeared down the stair to the lower room. Then Sandy came back and picked up the extra cubes.

"Joe said to move them," she explained.

She disappeared again. Holmes settled himself comfortably. He was one of those fortunate people who are able to relax at will. Actually, in his work he normally did his thinking while on his feet, moving about his yacht-building plant or else sailing one of his own boats. He simply was not a sit-down thinker. Sitting, he could doze at almost any time he pleased, and for a yachtsman it was a useful ability. He could go for days on snatched catnaps when necessary. Conversely he could catnap practically at will.

He yawned once or twice and settled down confidently. In five minutes or less ...

He wriggled down into an opening barely large enough to admit his body. The top clamped and sealed overhead. He fitted his feet into their proper stirrup-like holders and fixed his hands on the controls. There was violent acceleration and he shot away and ahead. Behind him the jagged shape of the fortress loomed. He swung his tiny ship. He drove fiercely for the tiny rings of red glow which centered themselves in the sighting-screen before him. He drove and drove, while the fortress dwindled to a dot and then vanished.

On either side of his ship a ten-foot steel globe clung. He checked them over, tense with the realization that he must very soon be within the practical timing-range of the new Enemy solid missiles. He made minute adjustments in the settings of the globes.

He released them together. They went swinging madly away at the end of a hair-thin wire which would sustain the tons of stress that centrifugal force gave the spheres. They spiraled toward darkness with its background of innumerable stars. The Enemy would be puzzled, this time! They'd developed missile-weapons with computing sights. In their last attack, five hundred years before, the Enemy had been defeated by the self-driving globes that had an utterly incredible acceleration. It was reported from the Cathor sector that in this current attack they had missile-weapons with a muzzle-velocity of hundreds of miles per second, which could actually anticipate a globe with a hundred-sixty-gravity drive. They could fire a solid shot to meet it and knock it down, because of some incredible computer-system which was able to calculate a globe's trajectory and meet it in space. They were smart, the Enemy!

The two globes went spinning toward the Enemy. Linked together, they spun round and round and no conceivable computer could calculate the path of either one so a projectile could hit. They did not travel in a straight line, as a trajectory in space should be. Whirling as they did around a common center of gravity, with the plane of their circling at a sharp angle to their line of flight, it was not possible to range them for gunfire. Their progress was in a series of curves, each at a different distance, which no mere calculator could solve without direction. A radar could not pick up the data a computer would need. One or the other globe might be hit, but it was far from likely.

The pilot of the one-man ship saw the blue-white flame of a hit. He flung his ship about and sped back toward the fortress. The Enemy would beat this trick, in time. Four thousand years before they'd almost won, when they invaded the Old Nation. They were getting bolder now. There was a time when a sound beating sent them back beyond the Coal-sack to lick their wounds for two thousand years or better. Lately they came more often. There'd been a raid in force only five hundred years back, and only fifteen before that ...

Holmes, obviously, had the odd dream Burke had prophesied. But Burke was up in the instrument-room by then. Keller gazed absorbedly at a vision-plate. It showed a section of the exterior surface of the asteroid—harsh, naked rock, with pitiless sunlight showing the grain and structure of the rock-crystals. Where there was shadow, the blackness was absolute. As Burke entered, Keller turned a knob. The image changed to a picture of a compartment inside the fortress. It was a part of the maze of rooms and galleries that none of the newcomers had visited. Panels and bus-bars and things which were plainly switches covered its walls. It was a power-distribution center. Keller turned the knob back, and the view of the outside of the asteroid returned.

Keller turned and blinked at Burke, and then said happily, "Look!"

He went to another vision-screen with an image of another part of the outer surface. He turned that knob, and the image dissolved into another. This was a gigantic room, lighted like more familiar places. In its center there was an enormous, gigantic machine. There were domes of metal, with great rods of silvery stuff reaching across emptiness between them. There were stairs by which one could climb to this part and that. Judging by the steps and the size of the light-tubes, the machine was the size of a four-storey house. And on the floor there were smaller machines, all motionless and all cryptic.

Keller said with conviction, "Power!"

Burke stared. Keller recovered the original view and went to still other plates. In succession, as he turned the knobs, Burke saw compartment after compartment. There was one quite as huge as the one containing the power-generating machine. It contained hemispheres bolted ten feet above the floor on many columns. There was a network of bus-bars, it seemed, overlying everything, and there were smaller devices on the floor below it.

"Gravity!" said Keller with conviction.

"Good enough," said Burke. "We've found something too, which may be useful with those machines. If we can—"

Keller held up his hand and went to one special screen. When he changed the image, the new one was totally unlike any of the others. This was a close-up. It showed a clumsy, strictly improvised and definitely cobbled metal case against a wall. It had been made by inept hands. It was remarkable to see such indifferent workmanship here. But the really remarkable thing was that the face of the box contained an inscription, burned into the metal as if by a torch. The symbols had no meaning to Burke, of course. But this was an inscription in a written language.

Keller rubbed his hands, beaming.

"It could be a message for somebody who'd come later," said Burke. "It's hard to think of it being anything else. But it wasn't placed for us to find. It should have been set up beside the ship-lock we were expected to come in by and did come in by."

"We'll see," said Keller zestfully. "Come on!"

Burke followed him. Keller seemed somehow to know the way. They went all the way back to the ship-lock, passed it, and then Keller dived off to the right, down an unsuspected ramp. There were galleries running in every direction here, crossing each other and opening upon an indefinite number of what must have been storerooms. Presently Keller pointed.

There was the case against the wall. It faced a wide corridor. It did not belong here. It was totally unlike any other artifact they had seen, because it seemed to have been made totally without skill. Yet there was an inscription—and the making of written records had appeared to be a skill the former occupants of the asteroid had not possessed. Keller very zestfully essayed to open it. He failed.

Burke said, "We'll have to use tools to get it open."

"Somebody made it," said Keller, "just before the garrison went away. They made it here!"

"Quite likely," agreed Burke. "We'll get at it presently. Now listen, Keller! I came along because a message might be useful. I think Holmes has found out something, though what it may be I can't guess. Come along with me. There've been developments and I want to hold a council of war. And I think I do mean war!"

He led the way back toward the ship. When they arrived, Holmes was awake and growling because of Burke's absence.

"You win," he told Burke. "I had a dream, and it wasn't a dream. I know something about those metal globes. They've got drives in them, and they can accelerate to a hundred and sixty gees, and I don't think I'll ride one."

Wryly, he told Burke what he'd experienced.

"I'm not too much surprised," said Burke. "I've managed two cube-experiences myself. I figure that these cubes trained men to operate things, without training their brains in anything else. They'd make illiterates into skilled men in a particular line, so anybody could do the work a highly trained man would otherwise be needed for. In one of my two cube-dreams I was a gun-pointer on one of those machines up on the third level. In the second cube-dream I was a rocket-pilot."

"No rockets in my cube," protested Holmes.

"Different period," said Burke. "Maybe, anyhow. In my dream we were using rockets to fight with, and the war was close. The enemy had taken some planets off Kandu—wherever that is!—and the situation was bad. We went out of here in rockets and fought all over the sky. But then there were supplies coming from home, and fresh fighting men turning up." He stopped abruptly. "How'd they come? I don't know. But I know they didn't come in spaceships. They just came, and they were new men and we veterans patronized them. The devil! Holmes, you say the globes have a hundred-sixty-gee drive! Nobody'd use rockets if drives like that were known!"

"To stay in the party," Sandy said suddenly, with something like defiance, "I tried a cube, too. And I was a sort of supply-officer. I had the experience of being responsible for supply and being short of everything and improvising this and that and the other to keep things up to fighting standard. It wasn't easy. The men grumbled, and we lacked everything. There was no fighting in my time, and there hadn't been for centuries. But we knew the Enemy hadn't given up and we had to be ready, generation after generation, even when nothing happened. And we knew that any minute the Enemy might throw something unexpected, some new weapon, at us."

"History-cubes," said Keller interestedly. "Different periods. Right?"

"Dammit, yes!" said Burke. "We've got accounts of past times and finished battles, but we need to know who's coming and what to do about it! Maybe the rocket-dream was earliest in time. But how could a race with nothing better than rockets ever get here? And how could they supply the building of a place like this?"

There was no answer. Facts ought to fit together. When they don't, they are useless.

"We've got snatches of information," said Burke. "But we don't know who built this fort, or why, except that there was a war that lasted thousands of years, with pauses for centuries between battles." He waved a hand irritably. "The Enemy tries to think up new weapons. They do. They try them. So far, they've been countered. But we're not prepared to fight a new weapon. Maybe the fort is set to battle old ones, but we don't know how to use it even for that! We've got to—"

"I think—" began Keller.

"I'd give plenty for a service manual on the probably useless weapons we do have," said Burke angrily. "Incidentally, Keller just found what may be an explanation of how and why this place was abandoned."

Keller said suddenly, "Where would service manuals be?"

He moved, almost running, toward the air-lock. Burke started to swear, and stopped.

"A service-and-repair manual," he snapped, "would be near the equipment it described. How many little shelves with boxes on them have we seen? They're just the right size to hold cubes! And where are they? Next to those fighting machines next to the door of the room where the ten-foot globes are! There's a shelf of them in the instrument-room! Let's find out how to fight with this misbegotten shell of a space-fort! There'll be no help coming to us, but if the Enemy's held off for thousands of years while this civilization fell apart, we might as well try to hold it together for a few minutes or seconds longer! Let's go get some real instruction-cubes!"

Keller was already gone. The others followed. Once they saw Keller in the far, far distance, hastening toward the instrument-room. Behind him, after almost running down the long corridor, Burke swung into the room where hundreds of ten-foot metal globes waited for the fortress to be remanned and to go into action again. Inside the door he found the remembered shelf, with two small boxes fastened to it. He pulled down one box and opened it. There was a black cube inside it. He thrust it upon Holmes.

"Here!" he said feverishly. "Find out how those globes work! Find out what's in them, how they drive!"

He ran. To the end of the corridor and up the ramp and past the supposed bunk-rooms and mess-halls. Up to the level where the ugly metal machines stood, each in its separate cubicle. There were little shelves inside each door. Each shelf contained a single box. Burke took one, two, and then stopped short.

"They'll be practically alike," he muttered. "No need."

He put one back. And then he felt almost insanely angry. One would need at least to be able to doze, to make use of the detailed, vivid, and utterly convincing material contained in the black cubes. And how could any man doze or sleep for the purpose of learning such desperately needed data? He'd need almost not to want the information to be able to sleep to get it!

Sandy and Pam overtook him as he stood in harried frustration with a black cube in his hands.

"Listen to me, Joe," said Sandy. "We've all taken chances, but if you get recurrent dreams from every cube you doze near—"

"When that happened to me," snapped Burke, "I was eleven years old and had one moment only. And that dream wasn't affected by the others in the cubes that came after it. And anyhow, no matter what happens to Holmes and me, we have to get these things ready for use! I don't know what we'll use them against. I don't know whether they'll be any use at all. But I've got to try to use them, so I've got to try to find out how!"

Sandy opened her mouth to speak again.

"I'm going off to fret myself to sleep," added Burke. "Holmes will be trying it too. And Keller."

"I don't think it's necessary," said Sandy.

"Why?"

"You found a sort of library of cubes. How useful would they be if one had to doze off to read them? How handy would a manual about repairing a weapon be, if somebody had to take a nap to get instructions? It wouldn't make sense!"

"Go on!" said Burke impatiently.

"Why not look in the library?" asked Sandy. "As a quartermaster officer, I think I knew that there was a reading-device for the cubes, like a projector for microfilm. It might have been taken away, but also—"

"Come along!" snapped Burke. "If that's so, it's everything! And it ought to be so!"

They hastened to the vast, low-ceilinged room which was filled with racks of black cubes. They were stacked in their places. At the far corner they found a desk and a cabinet. In the cabinet they found two objects like metal skull-caps, with clamps atop them. A cube would fit between the clamps. Burke feverishly sat a cube in position and put the skull-cap on his head. His expression was strange. After an instant he took it off and reversed the cube. He put it on. His face cleared. He lifted it off.

"I had it on backwards the first time," he said curtly. "This is better than dreaming the stuff. This lets you examine things in detail. You know you're receiving something. You don't think you're actually experiencing. We'll get this other reading-machine to Keller, so he can understand the equipment in the instrument-room. Holmes will have to wait."

Sandy said, "I can use him. Doesn't it occur to you, Joe, that we've only partly explored the top half of the fortress? We've only looked at what's between us and the instrument-room. There are all the stores—there were stores! And the generators down below. I can lead the way there now!"

"What do you know about the weapons?" demanded Burke.

"Nothing," said Sandy. "But I know something about the morale of the garrison. When grumbling began, discipline tightened up. And that worked for the men, but the women—"

"Women!" said Pam incredulously.

"They were an experiment," Sandy told her, "to see if they would content men on duty in an outpost. It'd been going on for only a few hundred years. It didn't seem to work too well. They wanted supplies that weren't exactly military, and at the time the cube I used was made, there was trouble getting even military things!"

Burke said impatiently, "I'll get one of these things to Keller. That's the most important thing. Tell Holmes not to try to sleep. Take him down to look over the supplies, if there are any. I'd guess that the garrison took most of them along. I doubt there's much left that we could use."

He made his way out of the cube-library and vanished.

Pam said uncomfortably, "Joe dreamed about a woman and is no good to you, in consequence. If there were women in this garrison, using the cubes might make anybody—"

Sandy tensed her lips.

"I don't think Joe is thinking about his old dream. Something deadly's on the way here. His mind's on that. I suspect all three of the men are concentrating on it. They're in no mood for romance."

"Don't you think I've noticed?" Pam said gloomily. "But I'm coming with you when you show him the storerooms!"

The "him" was obviously Holmes, whose attention had been so much taken up by the problems the fortress presented that Pam felt pushed much farther on the side lines than she liked. It was one thing to be present to watch and help and cheer on a man who planned to do something remarkable. But it was less satisfying when he became so absorbed that he didn't notice being watched, and couldn't be helped, and didn't need to be cheered on. Pam was disgruntled.

Then, for a considerable number of hours, absurdly trivial activities seemed to occupy all the people in the asteroid. Burke and Keller sat in the thirty by thirty-foot instrument-room, each wearing a small metal half-cap with a black cube held atop it between a pair of clamps. Their expressions were absorbed and intent, while they seemed attired for a children's halloween party. Now and again one of them exchanged one cube for another. About them there was a multiplicity of television screens, each screen presenting a picture of infinitely perfect quality. Every square foot of the outside of the asteroid could be seen on one or another of the screens. Then, besides, there were banks of screens which showed every square degree of the sky, with every star of every magnitude represented so that one could use a magnifying glass upon the screen to discover finer detail.

Once, during the hours when Burke and Keller were sitting quite still, Keller reached over and threw a switch. Nothing happened. Everything went on exactly as it had done before. He shook his head. And much later he went to one of the star-image screens. He moved an inconspicuous knob in a special fashion, and the star-image expanded and expanded until what had been a second of arc or less filled all the screen's surface. The effect of an incredibly powerful telescope was obtained by the movement of one control. Keller restored the knob to its original place and the image returned to its former scale. These were the only actions which took place in the instrument-room.

In the lower part of the asteroid, not much more occurred. The entrance to the power and storage areas was not hidden. It simply had not been entered. Sandy and Holmes and Pam went gingerly down a corridor with doors on either side, and then down a ramp, and then into huge caverns filled with monstrous metal things. There was no sign of any motion anywhere, but gigantic power-leads led from the machines to massive switchboards, whose switches were thrown by relays operated from somewhere else.

Then there were other caverns which must have contained many varieties of stores. There were great cases, broken open and emptied. There were bins with only dust at their bottoms. There were shelves containing things which might have been textiles, but which crumbled at a touch. Some thousands of years in an absolute vacuum would have evaporated any substance giving any degree of flexibility. These objects were useless. There was a great room with a singular hundred-foot-high machine in it, but there was no vibration or sound to indicate that it was in operation. This, Sandy said decisively, was the artificial-gravity generator. She did not know how it worked. It would have been indiscreet to experiment.

She led the way through relatively small corridors to areas in which there were very many small compartments. These had been for foodstuffs. But they were empty. They had been emptied when the asteroid was abandoned.

Then they came to the crudely fashioned case with the cryptic symbols on its front.

"This is the thing Joe mentioned," said Sandy. "They had writing. They'd have to, to be civilized. But this is the only writing we've seen. Why'd they write it?"

"To tell somebody something they'd miss, otherwise," Pam said.

"Who'd come down here? Why not put it at the ship-lock where people could be expected to come?"

Holmes grunted. "Asking questions like that gets nowhere. It's like asking how the garrison was supplied. There's no answer. Or how it left."

Sandy said in a surprised voice, as if saying something she hadn't realized she knew. "There were service ships. They serviced the television eyes on the outside, and they drilled at launching missiles, and so on. They were modified fighting ships, made over after ships didn't fight any more."

She hesitated, then went on.

"It's odd that I didn't think of telling Joe this! Some of the food supply came from Earth at the time my cube was made. As a quartermaster officer, I was authorized to allow hunting on Earth in case of need. So the serviceships went to Earth and came back with mammoths tied to the outside of their hulls. They had to be re-hydrated, though. Frozen though they were, they dried out in the long trip through vacuum from Earth."

Then she shivered a little.

Pam looked at her strangely. Holmes raised his eyebrows. He'd had one experience of training-cubes. Sandy'd had quite another. Holmes felt that instinctive slight resentment a man feels when he lacks a position of authority in the presence of a woman.

"In my time—in the cube's time—there was even a hunting camp on Earth. Otherwise there simply wouldn't be enough to eat! Women were clamoring to be sent to Earth to help with the food supply. To be sent to hunt for food was a reward for exemplary service."

"Which is interesting," observed Holmes, "but irrelevant. How was the asteroid normally supplied? How did the garrison leave? Where did it come from? Where did it go? Maybe the answer's in this box. If it is," he added, "it'll be in the same language as the inscription, and we can't read it."

Archaeologists on Earth would have been enraptured by any part of the fortress, but anything which promised to explain as much as Holmes had guessed the case could, would be a treasure past any price.

But the five people in the asteroid had much more immediate and much more urgent problems to think of. They went on a little farther and came to a storeroom which had been filled with something, but now held only the remains of packing-cases. They looked ready to crumble if touched.

"There used to be weapons stored here," Sandy said. "Hand-weapons. Not for the defense of the fortress, but for the—discipline police. For the men who kept the others obedient to orders."

"I'd be glad to have one operating pea-shooter," said Holmes.

Pam wrinkled her nose suddenly. She'd noticed something.

"I think—" she began, "I think—"

Holmes kicked at a shape which once was probably a case of wood or something similar. It collapsed into impalpable dust. It had dried out to absolute desiccation. It was stripped of every molecule which could be extracted by a total vacuum in thousands of years. It was brittle past imagining.

The collapse did not end with the object kicked. It spread. One case bulged as the support of another failed. The bulged case disintegrated. Its particles pressed on another. The dissolution spread fanwise until nothing remained but a carpeting of infinitely fine brown stuff. In one place, however, solid objects remained under the covering.

Holmes waded through the powder to the solid things. He brought them up. A case of hand-weapons had collapsed, but the weapons themselves kept their shape. They had transparent plastic barrels with curiously formed metal parts inside them.

"These might be looked into," said Holmes.

He stuffed his pockets. The hand-weapons had barrels and handgrips and triggers. They were made to shoot, somehow.

"I think—" began Pam again.

"Don't," growled Holmes. "Maybe Sandy remembers when this place was different, but I've had enough of it as it is. Let's go back to the ship and some fresh air."

"But that's what—"

Holmes turned away. Like the rest, he'd accepted great age, mentally, as a part of the nature of the fortress. But the collapse of emptied shipping-cases because they were touched was a shock. Where such decay existed, one could not hope to find anything useful for a modern emergency. He vanished.

Pam was indignant. She turned to Sandy.

"I wanted to say that I smelled fresh air," she protested. "And he acts like that!"

Sandy was not listening. She frowned.

"He could lose his way down here," she said shortly. "We'd better keep him in sight. I remember the way from my dream."

They followed Holmes, who did make his way back to the upper levels and ultimately to the ship without guidance. But Pam was intensely indignant.

"We could have gotten lost down there!" she said angrily when they were back in familiar territory. "And he wouldn't have cared! And I did smell fresh air! Not very fresh, but fresher than the aged and dried-out stuff we're breathing now!"

"You couldn't," said Sandy practically. "There simply couldn't be any, except in the ship where the hydroponic wall-gardens keep it fresh."

"But I did!" insisted Pam.

Sandy shrugged. They went into the ship, which Holmes had already reached and where he sat gloomily beside a black cube. He would have to sleep to get anything from it. There were only two of the freakish-seeming metal caps which made the cubes intelligible to a man awake, and Burke and Keller were using them. Holmes felt offended.

Sandy looked at a clock and began to prepare a meal. Pam, brooding, helped her.

Burke and Keller came back to the ship together. Keller looked pale. Burke seemed utterly grim.

"There's some stuff to be coded and sent back to Earth," he told Sandy. "Keller's got it written out. We know how to work the instruments up above, now. My brain's reeling a little, but I think I'll stay sane. Keller takes it in stride. And we know the trick the Enemy has."

Sandy put out plates for five.

"What is it?"

"Gravity," said Burke, evenly. "Artificial gravity. We don't know how to make it, but the people who built this fortress did, and the Enemy does. So they've made artificial-gravity fields to give their ships the seeming mass of suns, and they've set them in close orbits around each other. They'll come spinning into this solar system. What will happen when objects with the mass of suns—artificial or otherwise—come riding through between our sun and its planets? There'll be tidal stresses to crack the planets and let out their internal fires. There'll be no stability left in the sun. Maybe it'll be a low-grade nova when they've gone, surrounded by trash that once was worlds. Anyhow there'll be no humans left! And then the Enemy will go driving on toward the other solar systems that the builders of this fortress own. They can't conquer anything with a weapon like that, but they can surely destroy!"

Keller nodded distressedly. He gave Pam a number of sheets of paper, filled with his neat handwriting.

He said sorrowfully, "For Earth. In code."

Sandy served the meal she had prepared.

"It's a matter of days," said Burke curtly. "Not weeks. Just days."

He picked up a fork and began his meal.

"So," he said after a moment, with a sort of unnatural calm, "we've got to get the thing licked fast. Up in the instrument-room there are some theory-cubes—lectures on theories with which the operators of the room were probably required to be familiar. They were intended to figure out what the Enemy might come up with, so it could at least be reported before the fortress was destroyed. The trick of sun-gravity fields was suggested as possible, but it seemed preposterously difficult. Apparently, it was. It took the Enemy some thousands of years to get it. But they've got it, all right!"

"How do you know?" demanded Holmes.

"The disk with the red sparks in it," said Burke, "is a detector of gravity-fields. It sees by gravity, which is not radiation. Keller's sending instructions back to Earth telling how to make such detectors."

He busied himself with his food once more. After a moment he spoke again.

"We're going to try to get some help," he observed. "At least we'll try to find out if there's any help to be had. I think there's a chance. There was a civilization which built this fortress. Something happened to it. Perhaps it simply collapsed, like Rome and Greece and Egypt and Babylonia back on Earth. But on Earth when an old civilization died a new, young one rose in its place. If the one that built this fort collapsed, maybe a new one has risen in its stead. If so, it will need to defend itself against the Enemy just like the old culture did. It might prefer to do its fighting here, instead of in its own land. I think we may be able to contact it."

"How'll you look for them?"

Burke shrugged.

"I've some faint hope of a few directions in that sealed-up metal case with the inscription on it. I'm going to take some tools and break into it. It's a gamble, but there's nothing to lose."

He ate briskly, with a good appetite. Sandy was very silent.

Pam said abruptly, "We saw that case. And I smelled fresh air there. Not pure air like here in the ship, but not dead air like the air everywhere else."

"Near a power generator, Pam, there'd be some ozone," Holmes said patiently. "It makes a lot of difference."

"It wasn't ozone," said Pam firmly. "It was fresh air. Not canned air. Fresh!"

Holmes looked at Burke.

"Did you or Keller find out how the air's refreshed here? Did anybody throw a switch for air apparatus?"

Keller said mildly, "Apparatus, no. Air exchange, yes. I threw switches also for communication with base. Also emergency communication. Also dire emergency. Nothing happened."

"You see, Pam?" said Holmes. "It was ozone that made the air smell fresh."

Sandy was wholly silent until the meal was over. Then Holmes went moodily off with Keller, to use the cube-reading devices in the instrument-room and try to find, against all apparent probability, some clue or some communication which would enable something useful to be done. Holmes was trying hard to believe that things were not as bad as Burke announced, and not nearly so desperate that they had to try to find the descendants of a long-vanished civilization for a chance to offer resistance to the Enemy.

Keller said confidentially, just before they reached the instrument-room, "Burke's an optimist."

And at that moment, back in the little plastic spaceship, Burke was saying to Sandy, "You can come along if you like. There are a couple of things to be looked into. And if you want to come, Pam—"

But Pam touched the papers Keller had given her and said reservedly, "I'll code and send this stuff. Go ahead, Sandy."

Sandy rose. She followed Burke out of the ship. She was acutely aware that this was the first time since they had entered the ship that she and Burke could speak to each other when nobody could overhear. They'd spoken twice when the others were presumably asleep. But this was the first time they'd been alone.

When they'd passed through the door with the rounded corners, they were completely isolated. Overhead, brilliant light-tubes reached a full mile down the gallery in one direction, and half as far in the other. The vast corridor contained nothing to make a sound but themselves.

"It's this way," said Burke.

Sandy knew the way as well as he did, or better, but she accepted his direction. Their footsteps echoed and reËchoed, so that they were accompanied by countless reflections of heel-clicks along with the normal rustling and whispering sounds of walking.

They went a full quarter-mile from the ship-lock door, and came to a very large arched opening which gave entrance to a corridor slanting downward.

"Supplies came up this ramp," said Sandy.

It was a statement which should have been startling, but Burke nodded.

Sandy went on, carefully, "That cube about a supply-officer's duties was pretty explicit. Things were getting difficult."

Burke did not seem to hear. They went on and on. They came to the place where Keller had turned aside. Burke silently indicated the turning. They moved along this other gallery.

"Joe," said Sandy pleadingly. "Is it really so bad?"

"Strictly speaking, I don't see a chance. But that's just the way it looks now. There must be something that can be done. The trick is to find it. Meantime, why panic?"

"You—act queer," protested Sandy.

"I feel queer," he said. "I know various ways to approach problems. None of them apply to this one. You see, it isn't really our problem. We're innocent bystanders, without information about the situation that apparently will kill us and everybody back on Earth. If we knew more about the situation, we might find some part of it that could be tackled, changed. There may be something in this case—perhaps a message left by the garrison for the people who sent them here. I can't see why it'd be placed here, though."

He slowed, looking down one cross-gallery after another.

"Here it is."

They'd come to the clumsily-made case with the inscription on it. It was placed against the wall of a corridor, facing the length of another gallery which came from the side at this point. A little distance down the other passage, the line of doors was broken by an archway which gave upon a hewed-out compartment. The opening was wide enough to show a fragment of a metal floor. There was no sign of any contents. Other compartments nearby were empty. The placing of the inscribed box was inexplicable. But the inscription was sharply clear.

"Maybe," suggested Sandy forlornly, "it says something like 'Explosives! Danger!'"

"Not likely," said Burke.

He'd examined the box before. He'd brought along a tool suited to the job of opening it. He set to work, then stopped.

"Sandy," he said abruptly, "I think the gravity-generator's a couple of corridors in that direction. Will you look and see if there are any tools there that might be better than this? Just look for a place where tools might be stored. If you find something, call me."

She went obediently down the lighted, excavated corridor. She reached the vast cavern. Here there were myriad tube-lights glowing in the ceiling—and the gravity machine. It was gigantic. It was six storeys high and completely mysterious.

She looked with careful intentness for a place where tools might have been kept by the machine's attendants.

She saw movement out of the corner of her eye, but when she turned there was nothing. There could be no movement in the fortress unless by machinery or one of the five humans who'd come so recently. The asteroid had been airless for ten thousand years. It was unthinkable that anything alive, even a microbe, could have survived. So Sandy did not think of a living thing as having made the movement. But movement there had been.

She stared. There were totally motionless machines all about. None of them showed any sign of stirring. Sandy swallowed the ache in her throat and it returned instantly. She moved, to look where the movement had been. She glanced at each machine in turn. One might have made some automatic adjustment. She'd tell Burke.

She passed a fifteen-foot-high assembly of insulators and bright metal, connected overhead to other cryptic things by heavy silvery bars. She passed a cylinder with dials in its sides.

She saw movement again. In a different place. She spun around to look.

Something half the height of a man, with bird-legs and feet and swollen plumage and a head with an oversized beak which was pure caricature—something alive and frightened fled from her. It waddled in ridiculous, panicky haste. It flapped useless stumps of wings. It fled in terrified silence. It vanished.

The first thing that occurred to Sandy was that Burke wouldn't believe her if she told him.