A flicker of a smile crossed Davenport's solemn face. 'You suspect extraterrestrial invaders?'

'Maybe,' said the other, with no smile at all. 'But let me continue. A routine search in the neighborhood of the stranded ship revealed no signs of the skim boat. Then Luna Station reported receipt of weak signals of uncertain origin. They had been tabbed as coming from the western rim of Mare Imbrium, but it was uncertain whether they were of human origin or not, and no vessel was believed to be in the vicinity. The signals had been ignored. With the skim boat in mind, however, the search party headed out for Imbrium and located it. Jennings was aboard, dead. Knife wound in one side. It's rather surprising he hadlived as long as he did.

'Meanwhile the medico's were becoming increasingly dis turbed at the nature of Strauss's babbling. They contacted the Bureau and our two men on the Moon-one of them happened to be Ferrant-arrived at the ship.

'Ferrant studied the tape recordings of the babblings. There was no point in asking questions, for there was, and is, no way of reaching Strauss. There is a high wall between the universe and himself-probably a permanent one. However, the talk in delirium, although heavily repetitious and disjointed, can be made to make sense. Ferrant put it together like a jigsaw puzzle.

'Apparently Strauss and Jennings had come across an object of some sort which they took to be of ancient and non-human manufacture, an artifact of some ship wrecked eons ago. Apparently it could somehow be made to twist the human mind.'

Davenport interrupted. 'And it twisted Strauss's mind? Is that it?'

That's exactly it. Strauss was an Ultra-we can say "was" for he's only technically alive-and Jennings did not wish to surrender the object. Quite right, too. Strauss babbled of using it to bring about the self-liquidation, as he called it, of the undesirable. He wanted a final, ideal population of five million. There was a fight in which only Jennings, apparently, could handle the mind-thing, but in which Strauss had a knife. When Jennings left, he was knifed, but Strauss's mind had been destroyed.'

'And where was the mind-thing?'

'Agent Ferrant acted decisively. He searched the ship and the surroundings again. There was no sign of anything that was neither a natural Lunar formation nor an obvious product of human technology. There was nothing that could be the mind-thing. He then searched the skim boat and its surroundings. Again nothing.'

'Could the first search team, the ones who suspected nothing-could they have carried something off?'

They swore they did not, and there is no reason to suspect them of lying. Then Ferrant's partner---'

'Who was he?'

'Gorbansky,' said the District Head.

'I know him. We've worked together.'

'I know you have. What do you think of him ?'

'Capable and honest.'

'All right. Gorbansky found something. Not an alien artifact. Rather, something most routinely human indeed. It was an ordinary white three-by-five card with writing on it, spindled, and in the middle finger of the right gauntlet. Presumably Jennings had written it before his death and, also presumably, it represented the key to where he had hidden the object.'

'What reason is there to think he had hidden it?'

'I said we had found it nowhere.'

'I mean, what if he had destroyed it, as something too dangerous to leave intact?'

That's highly doubtful. If we accept the conversation as reconstructed from Strauss's ravings-and Ferrant built up what seems a tight word-for-word record of it-Jennings thought the mind-thing to be of key importance to humanity. He called it "the clue to an umimaginable scientific revolution." He wouldn't destroy something like that. He would merely hide it from the Ultras and try to report its whereabouts to the government. Else why leave a clue to its whereabouts?'

Davenport shook his head, 'You're arguing in a circle, chief. You say he left a clue because you think there is a hidden object, and you think there is a hidden object because he left a clue.'

'I admit that. Everything is dubious. Is Strauss's delirium meaningful? Is Ferrant's reconstruction valid? Is Jennings' due really a clue? Is there a mind-thing, or a Device, as Jennings called it, or isn't there ? There's no use asking such questions. Right now, we must act on the assumption that there is such a Device and that it must be found.'

'Because Ferrant disappeared?'

'Exactly.'

'Kidnapped by the Ultras ?'

'Not at all. The card disappeared with him.'

'Oh-I see.'

'Ferrant has been under suspicion for a long time as a secret Ultra. He's not the only one in the Bureau under suspicion either. The evidence didn't warrant open action; we can't simply lay about on pure suspicion, you know, or we'll gut the Bureau from top to bottom. He was under surveillance.'

'By whom?'

'By Gorbansky, of course. Fortunately Gorbansky had filmed the card and sent the reproduction to the headquarters on Earth, but he admits he considered it as nothing more than a puzzling object and included it in the information sent to Earth only out of a desire to be routinely complete. Ferrant-the better mind of the two, I suppose-did see the significance and took action. He did so at great cost, for he has given himself away and has destroyed his future usefulness to the Ultras, but there is a chance that there will be no need for future usefulness. If the Ultras control the Device---'

'Perhaps Ferrant has the Device already.'

'He was under surveillance, remember. Gorbansky swears the Device did not turn up anywhere.'

'Gorbansky did not manage to stop Ferrant from leaving with the card. Perhaps he did not manage to stop him from obtaining the Device unnoticed, either.'

Ashley tapped his fingers on the desk between them in an uneasy and uneven rhythm. He said at last, 'I don't want to think that. If we find Ferrant, we may find out how much damage he's done. 'I'll l then, we must search for the Device. If Jennings hid it, he must have tried to get away from the hiding place. Else why leave a clue ? It wouldn't be found in the vicinity.'

'He might not have lived long enough to get away.'

Again Ashley tapped, 'The skim boat showed signs of having engaged in a long, speedy flight and had all but crashed at the end. That is consistent with the view that Jennings was trying to place as much space as possible between himself and some hiding place.'

'Can you tell from what direction he came?'

'Yes, but that's not likely to help. From the condition of the side vents, he had been deliberately tacking and veering.'

Davenport sighed. 'I suppose you have a copy of the card with you.'

'I do. Here it is.' He flipped a three-by-five replica toward Davenport. Davenport studied it for a few moments. It looked like this: Davenport said, 'I don't see any significance here.'

'Neither did I, at first, nor did those I first consulted. But consider. Jennings must have thought that Strauss was in pursuit; he might not have known that Strauss had been put out of action, at least, not permanently. He was deadly afraid, then, that an Ultra would find him before a Moderate would. He dared not leave a clue to open. This'-and the Division Head tapped the reproduction-'must represent a clue that is opaque on the surface but clear enough to anyone sufficiently ingenious.'

'Can we rely on that?' asked Davenport doubtfully. 'After all, he was a dying, frightened man, who might have been subjected to this mind-altering object himself. He need not have been thinking clearly, or even humanly. For instance, why didn't he make an effort to reach Lunar Station? He ended half a circumference away almost. Was he too twisted to think clearly? Too paranoid to trust even the Station ? Yet he must have tried to reach them at first since they picked up signals. What I'm saying is that this card, which looks as though it is covered with gibberish, is covered with gibberish.'

Ashley shook his head solemnly from side to side, like a tolling bell. 'He was in panic, yes. And I suppose he lacked the presence of mind to try to reach Lunar Station. Only the need to run and escape possessed him. Even so this can't be gibberish. It hangs together too well. Every notation on the card can be made to make sense, and the whole can be made to hang together.'

'Where's the sense, then ?' asked Davenport.

'You'll notice that there are seven items on the left side and two on the right. Consider the left-hand side first. The third one down looks like an equals sign. Does an equals sign mean anything to you, anything in particular ?'

'An algebraic equation.'

That's general. Anything particular?'

'No.'

'Suppose you consider it as a pair of parallel lines?'

'Euclid's fifth postulate?' suggested Davenport, groping.

'Good! There is a crater called Euclides on the Moon-the Greek name of the mathematician we call Euclid.'

Davenport nodded. 'I see your drift. As for F/A, that's force divided by acceleration, the definition of mass by Newton's second law of motion---'

'Yes, and there is a crater called Newton on the Moon also.'

'Yes, but wait awhile, the lowermost item is the astronomic symbol for the planet Uranus, and there is certainly no crater-or any other lunar object, so far as I know-that is named Uranus.'

'You're right there. But Uranus was discovered by William Herschel, and the H that makes up part of the astronomic symbol is the initial of his name. As it happens, there is a crater named Herschel on the Moon-three of them, in fact, since one is named for Caroline Herschel, his sister, and another for John Herschel, his son.'

'Davenport thought awhile, then said, 'PC/2-Pressure times half the speed of light. I'm not familiar with that equation.'

'Try craters. Try P for Ptolemaeus and C for Copernicus.'

'And strike an average ? Would that signifiy a spot exactly between Ptolemaeus and Copernicus?'

'I'm disappointed, Davenport,' said Ashley sardonically. 'I thought you knew your history of astronomy better than that. Ptolemy, or Ptolemaeus in Latin, presented a geocentric picture of the Solar System with the Earth at the center, while Copernicus presented a heliocentric one with the Sun at the center. One astronomer attempted a compromise, a picture halfway between that of Ptolemy and Copernicus---'

Tycho Brahe!' said Davenport.

'Right. And the crater Tycho is the most conspicuous feature on the Moon's surface.'

'AH right. Let's take the rest. The C-C is a common way of writing a common type of chemical bond, and I think there is a crater named Bond.'

'Yes, named for an American astronomer, W. C. Bond.'

The item on top, XY2. Hmm. XYY. An X and two Y's. Wait! Alfonso X. He was the royal astronomer in medieval Spain who was called Alfonso the Wise. X the Wise. XYY. The crater Alphonsus.'

'Very good. What's SU ?'

That stumps me, chief.'

'I'll tell you one theory. It stands for Soviet Union, the old name for the Russian Region. It was the Soviet Union that first mapped the other side of the Moon, and maybe it's a crater there. Tsiolkovsky, for instance. You see, then, the symbols on the left can each be interpreted as standing for a crater: Alphonsus, Tycho, Euclides, Newton, Tsiolkovsky, Bond, Herschel.'

What about the symbols on the right-hand side?'

That's perfectly transparent. The quartered circle is the astronomic symbol for the Earth. An arrow pointing to it indicates that Earth must be directly overhead.'

'Ah,' said Davenport, 'the Sinus Medii-the Middle Bay- over which the Earth is perpetuity at zenith. That's not a crater, so it's on the right-hand side, away from the other symbols.'

'All right,' said Ashley. Thenotations all make sense, or they can be made to make sense,so there's at least a good chance that this isn't gibberish and that it is trying to tell us something. But what?So far we'vegot seven craters and a non-crater mentioned, and what doesthat mean? Presum ably, the Device can only be in one place.'

'Well,' said Davenport heavily, 'a crater can be a huge place to search. Even if we assume he hugged the shadow to avoid Solar radiation, there can be dozens of miles to examine in each case. Suppose the arrow pointing to the symbol for the Earth defines the crater where he hid the Device, the place from which the Earth can be seen nearest the zenith.'

That's been thought of, old man. It cuts out one place and leaves us with seven pinpointed craters, the southernmost extremity of those north of the Lunar equator and the northernmost extremity of those south. But which of the seven?'

Davenport was frowning. So far, he hadn't thought of anything that hadn't already been thought of. 'Search them all,' he said brusquely.

Ashley crackled into brief laughter. 'In the weeks since this has all come up, we've done exactly that.'

'And what have you found?'

'Nothing. We haven't found a thing. We're still looking, though.'

'Obviously one of the symbols isn't interpreted correctly.'

'Obviously!'

'You said yourself there were three craters named Herschel. The symbol SU, if it means the Soviet Union and therefore the other side of the Moon, can stand for any crater on the other side: Lomonosov, Jules Verne, Joliot-Curie, any of them. For that matter, the symbol of the Earth might stand for the crater Atlas, since he is pictured as supporting the Earth in some versions of the myth. The arrow might stand for the Straight Wall.'

There's no argument there, Davenport. But even if we get the right interpretation for the right symbol, how do we recognize it from among all the wrong interpretations, or from among the right interpretations of the wrong symbols ? Somehow there's got to be something that leaps up at us from this card and gives us so clear a piece of information that we can tell it at once as the real thing from among all the red herrings. We've all failed and we need a fresh mind, Davenport. What do you see here?'

'I'll tell you one thing we could do,' said Davenport reluctantly. 'We can consult someone I---Oh, my God!' He half-rose.

Ashley was all controlled excitement at once. 'What do you see?'

Davenport could feel his hand trembling. He hoped his lips weren't. He said. Tell me, have you checked on Jennings' past life?'

'Of course.'

'Where did he go to college ?'

'Eastern University.'

A pang of joy shot through Davenport, but he held on. That was not enough. 'Did he take a course in extraterro-logy?'

'Of course, he did. That's routine for a geology major.'

'All right, then, don't you know who teaches extraterrology at Eastern University?'

Ashley snapped his fingers. 'That oddball. What's-his-name -Wendell Urth.'

'Exactly, an oddball who is a brilliant man in his way. An oddball who's acted as a consultant for the Bureau on several occasions and given perfect satisfaction every time. An oddball I was going to suggest we consult this time and then noticed that this card was telling us to do so. An arrow pointing to the symbol for the Earth. A rebus that couldn't mean more clearly "Go to Urth," written by a man who was once a student of Urth and would know him.'

Ashley stared at the card, 'By God, it's possible. But what could Urth tell us about the card that we can't see for ourselves?'

Davenport said, with polite patience, 'I suggest we ask him, sir.'

Ashley looked about curiously, half-wincing as he turned from one direction to another. He felt as though he had found himself in some arcane curiosity shop, darkened and dangerous, from which at any moment some demon might hurtle forth squealing.

The lighting was poor and the shadows many. The walls seemed distant, and dismally alive with book-films from floor to ceiling. There was a Galactic Lens in soft three-dimentionality in one corner andbehind it were star charts that could be made out A map of the Moon in another corner might, however, possibly be a map of Mars.

Only the desk in the center of the room was brilliantly lit by a tight-beamed lamp. It was littered with papers and opened printed books. A small viewer was threaded with film, and a clock with an old-fashioned round-faced dial hummed with subdued merriment.

Ashley found himself unable to recall that it was late afternoon outside and that the sun was quite definitely in the sky. Here, within, was a place of eternal night. There was no sign of any window, and the clear presence of circulating air did not spare him a claustrophobic sensation.