Gribardsun drove a number of sharp plastic spikes into the ground to mark the outlines of the depression formed where the vessel had fallen. Four years from now, the depression might be smoothed out, and thus it would be impossible to locate.

Rachel and von Billmann took films of the spot, and then Gribardsun and Drummond Silverstein took the coordinates of the depression from three large rocks sticking out of the soil nearby.

The H. G. Wells I had been set on a wooden platform on top of a hill before being chronologically launched. The edge of the hill in the Vezere River valley, France, A.D. 2070, was forty feet away from the vessel. It had been expected that the edge of the hill in 12,000 B.C. would be even more distant. The geologists had affirmed this to be a fact. Gribardsun wondered if they had been correct but a slight displacement in space had occurred. The theoreticians said that this would not occur, but the truth was that they did not know what would happen in practice.

The process of time travel required an enormous amount of energy. The further back into time the machine went, the more the energy. This period was as far back as a machine could be sent. There was a factor, which only a few mathematicians understood, which required that the most expensive and most dangerous journey be made first. If the time travelers waited, say, eight years more before attempting to go into the Magdalenian, they would find themselves in circa 8000 B.C. The era of 12,000 B.C. would be forever out of reach. And if they waited for ten years, they would find that 4000 B.C. was as far back as they could go.

Moreover, there was a strange and unexplained limit at the other end. The first small experimental manless model had been sent back one day into time. But it had never arrived, as they knew it would not, having been present the day before. Where the model went was not known. Then another model, at great expense of materials and energy, was sent back a week. This did not appear, as the experimenters knew it would not. But they had to be sure.

At this time, the news media learned about Project Chronos, and it was suspended for a while until the public, and Congress, were satisfied that it was safe. The old science-fiction idea that tampering with time would change the course of events had to be dealt with. Stories by various writers from Wells to Silverberg and Bradbury and Heinlein, illustrating the paradox and danger of time travel, were reprinted and even dramatized. Millions of people were fearful that time travel would result in one of their ancestors being killed, and so their descendants would vanish from the face of Earth, as if the boojum were prowling it.

Jacob Moishe, leader of the project team that had invented the time-travel machines, quieted this form of protest. He showed, in a series of articles, that if time travel was going to make any changes, it had already done so, and therefore there was nothing to fear. By then the original goal of circa 25,000 B.C. was lost forever. Too much time had elapsed. The expedition would have to settle for the middle Magdalenian. The funds were restored, and a small model was sent back to one hundred years, and a search was made for it. The theory was that it had appeared in A.D. 1973 and had been picked up by someone who did not, of course, recognize it. But, since it was practically indestructible, it existed now and was probably in someone's possession. Or perhaps buried some place. Worldwide advertising failed to turn up the model.

Meanwhile, another had been sent to A.D. 1875, and the advertising for this one went around the world. None showed up. A third one was sent back at a cost that staggered Congress and the public. This one was set to bob up about A.D. 1850 within fifty feet of where the project buildings stood.

Dr. Moishe's researches had shown that, in 1850, this hilltop in Syracuse, New York, had been the scene of a mysterious and exceedingly violent explosion. He reasoned that the explosion had been caused when the model had appeared inside some solid matter, such as soil or a tree; the result of two solid objects trying to occupy the same space had been the explosion. A complete conversion of matter to energy had not occurred, of course. Otherwise, the hill and much of the surrounding countryside would have disappeared.

The model contained radioactive particles, and so, after it was sent back to 1850, the area for a mile around was scanned with geiger counters. A piece of the radioactive particle-bearing model was located and identified. Accusations of fraud were, of course, made, but Dr. Moishe had foreseen this and made foolproof arrangements. He had even gotten six congressmen and the Secretary of Science to watch the entire procedure.

One of the theories about the failure of the first two models to be found was immediately dismissed. This theory postulated that the structure of time was such that time travel was impossible within any period in which contemporaries had been living. In other words, time, to avoid a paradox, but not the pathetic fallacy, would not permit travel except in a time before anybody living in A.D. 2070 had been born. The critics pointed out, none too gently, that this would mean that somebody born before A.D. 1875 was still living and that his presence was keeping the models from appearing in A.D. 1973 and 1890. If the hypothetical person was born in, say, A.D. 1870, then he would today be 200 years old. And that was impossible, for several reasons. For one thing, a record existed of the birth date of everybody living, and the oldest person in the world was 130. She had been bora in A.D. 1940.

The theory was admittedly farfetched, if not crackpotted. Its proponent, who later committed suicide for unknown reasons, and so discredited any reputation he had for sanity, replied that anyone that old might have some reason for not wanting to be known. And it was not impossible to fake records.

John Gribardsun was thinking of this when Rachel Silverstein touched his arm. She seemed to be touching him at least ten times a day, as if she were testing to make sure that he existed. Or because she liked to touch him. He did not mind it, though he knew that Drummond disliked it. But it was up to her husband to say something about it to her, and, so far as he knew, the man had never opened his mouth about it.

'Do you think we can get the ship back up by ourselves?' she said. Her light blue eyes were bright, as if she were burning with excitement.

'I suppose so,' he said. 'But I think we could do it far more swiftly and easily if we had the strong backs of some cavemen helping us. So we won't worry about it now. After all, we have four years.'

Robert yon Billmann said something sharply. He was looking through binoculars to the northeast, across the valley. Gribardsun saw the figures that had attracted von Billmann. He lifted his own binoculars. The heads and antlers of several brownish reindeer came into view. He moved the glasses and within a minute had zeroed in on a big grayish shape. It was a wolf. Soon, he caught about a dozen with a sweep of the glasses.

The deer were well aware of the wolves. They continued to crop at the moss between liftings of the head, to sniff the air, and to eye the slinking beasts some fifty yards distant.

Presently some of the gray shapes floated behind a hill and soon appeared ahead of the herd. They disappeared again, and then those that had remained moved in slowly toward the herd. The deer waited for a minute to make sure that the wolves would not stop and suddenly, as if the leader had spoken, they bounded away. The wolves ran after them, and then, as the herd passed the hill behind which the others were, they veered away. Six wolves had run out at them. One wolf caught a doe that stumbled, and the others leaped upon it. The remaining deer got away, except for a buck that slipped when he leaped across a brook. Before he could get up again, he found two wolves tearing at his legs. These were joined by others, and the wolves quit running.