"Exactly. And he wondered if he would end up wrong. One of his last questions was: What if I don't end up in the right spot? Do you remember?"

"Well?" Harlan had no idea where this was leading.

"So he is mentally prepared for being displaced in Time. He will do something. Try to reach us. Try to leave traces for us. Remember, for part of his life he was an Eternal. That's an important thing." Twissell blew a smoke ring, hooked it with a finger, and watched it curl about and break up. "He's used to the notion of communication across Time. He is not likely to surrender to the thought of being marooned in Time. He'll know that we're looking for him."

Harlan said, "Without kettles and with no Eternity in the 20th, how would he go about communicating with us?"

"With you, Technician, with you. Use the singular. You're our expert on the Primitive. You taught Cooper about the Primitive. You're the one he would expect to be capable of finding his traces."

"What traces, Computer?"

Twissell's shrewd old face stared up at Harlan, its lines crinkling. "It was intended to leave Cooper in the Primitive. He is without the protection of an enclosing shield of physiotime. His entire life is woven into the fabric of Time and will remain so until you and I reverse the alteration. Likewise woven into the fabric of Time is any artifact, sign, or message he may have left for us. Surely there must be particular sources you used in studying the 20th Century. Documents, archives, films, artifacts, reference works. I mean primary sources, dating from the Time itself."

"Yes."

"And he studied them with you?"

"Yes."

"And is there any particular reference that was your favorite, one that he knew you were intimately acquainted with, so that you would recognize in it some reference to himself?"

"I see what you're driving at, of course," said Harlan. He grew thoughtful.

"Well?" asked Twissell with an edge of impatience.

Harlan said, "My news magazines, almost surely. News magazines were a phenomenon of the early 20's. The one of which I have nearly a complete set dates from early in the 20th and continues well into the 22nd."

"Good. Now is there any way, do you suppose, in which Cooper could make use of that news magazine to carry a message? Remember, he'd know you'd be reading the periodical, that you'd be acquainted with it, that you'd know your way about in it."

"I don't know." Harlan shook his head. "The magazine affected an artificial style. It was selective rather than inclusive and quite unpredictable. It would be difficult or even impossible to rely on its printing something you would plan to have it print. Cooper couldn't very well create news and be sure of its appearance. Even if Cooper managed to get a position on its editorial staff, which is very unlikely, he couldn't be certain that his exact wording would pass the various editors. I don't see it, Computer."

Twissell said, "For Time's sake, think! Concentrate on that news magazine. You're in the 20th and you're Cooper with his education and background. You taught the boy, Harlan. You molded his thinking. Now what would he do? How would he go about placing something in the magazine; something with the exact wording he wants?"

Harlan's eyes widened. "An advertisement!"

"What?"

"An advertisement. A paid notice which they would be compelled to print exactly as requested. Cooper and I discussed them occasionally."

"Ah, yes. They have that sort of thing in the 186th," said Twissell.

"Not like the 20th. The 20th is peak in that respect. The cultural milieu--"

"Considering the advertisement now," interposed Twissell hastily, "what kind would it be?"

"I wish I knew."

Twissell stared at the lighted end of his cigarette as though seeking inspiration. "He can't say anything directly. He can't say: 'Cooper of the 78th, stranded in the 20th and calling Eternity--'"

"How can you be sure?"

"Impossible! To give the 20th information we know they did not have would be as damaging to the Mallansohn circle as would wrong action on our part. We're still here, so in his whole lifetime in the current Reality of the Primitive he's done no harm of that sort."

"Besides which," said Harlan, retreating from the contemplation of the circular reasoning which seemed to bother Twissell so little, "the news magazine is not likely to agree to publish anything which seems mad to it or which it cannot understand. It would suspect fraud or some form of illegality and would not wish to be implicated. So Cooper couldn't use Standard Intertemporal for his message."

"It would have to be something subtle," said Twissell. "He would have to use indirection. He would have to place an advertisement that would seem perfectly normal to the men of the Primitive. Perfectly normal! And yet something that is obvious to us, once we knew what we were searching for. Very obvious. Obvious at a glance because it would have to be found among uncounted individual items. How big do you suppose it would be, Harlan? Are those advertisements expensive?"

"Quite expensive, I believe."

"And Cooper would have to hoard his money. Besides which, to avoid the wrong kind of attention, it would have to be small, anyway. Guess, Harlan. How large?"

Harlan spread his hands. "Half a column?"

"Column?"

"They were printed magazines, you know. On paper. With print arranged in columns."

"Oh yes. I can't seem to separate literature and film somehow… Well, we have a first approximation of another sort now. We must look for a half-column advertisement which will, practically at a glance, give evidence that the man who placed it came from another Century (in the upwhen direction, of course) and yet which is so normal an advertisement that no man of that Century would see anything suspicious in it."

Harlan said, "What if I don't find it?"

"You will. Eternity exists, doesn't it. As long as it does, we're on the right track. Tell me, can you recall such an advertisement in your work with Cooper? Anything which struck you, even momentarily, as odd, queer, unusual, subtly wrong."

"No."

"I don't want an answer so quickly. Take five minutes and think."

"No point. At the time I was going over the news magazines with Cooper, he hadn't been in the 20th."

"Please, boy. Use your head. Sending Cooper to the 20th has introduced an alteration. There's no Change; it isn't an irreversible alteration. But there have been some changes with a small 'c,' or micro-changes, as it is usually referred to in Computation. At the instant Cooper was sent to the 20th, the advertisement appeared in the appropriate issue of the magazine. Your own Reality has micro-changed in the sense that you may have looked at the page with that advertisement on it rather than one without that advertisement as you did in the previous Reality. Do you understand?"

Harlan was again bewildered, almost as much at the ease with which Twissell picked his way through the jungle of temporal logic, as at the "paradoxes" of Time. He shook his head, "I remember nothing of the sort."

"Well, then, where do you keep the files of that periodical?"

"I had a special library built on Level Two, using the Cooper priority."

"Good enough," said Twissell. "Let's go there. Now!"

Harlan watched Twissell stare curiously at the old, bound volumes in the library and then take one down. They were so old that the fragile paper had to be preserved by special methods and they creaked under Twissell's insufficiently gentle handling.

Harlan winced. In better times he would have ordered Twissell away from the books, Senior Computer though he was.

The old man peered through the crinkling pages and silently mouthed the archaic words. "This is the English the linguists are always talking about, isn't it?" he asked, tapping a page.