Twissell was shaking Harlan's shoulders. The old man's voice urgently called his name.

"Harlan! Harlan! For Time's sake, man."

Harlan emerged only slowly from the slough. "What are we to do?"

"Certainly not this. Not despair. To begin with, listen to me. Forget your Technician's view of Eternity and look at it through a Computer's eyes. The view is more sophisticated. When you alter something in Time and create a Reality Change, the Change may take place at once. Why should that be?"

Harlan said shakily, "Because your alteration has made the Change inevitable?"

"Has it? You could go back and reverse your alteration, couldn't you?"

"I suppose so. I never did, though. Or anyone that I heard of."

"Right. There is no intention of reversing an alteration, so it goes through as planned. But here we have something else. An unintentional alteration. You sent Cooper into the wrong Century and now I firmly intend to reverse that alteration and bring Cooper back here."

"For Time's sake, how?"

"I'm not sure yet, but there must be a way. If there were no way, the alteration would be irreversible; Change would come at once. But Change has not come. We are still in the Reality of the Mallansohn memoir. That means the alteration is reversible and will be reversed."

"What?" Harlan's nightmare was expanding and swirling, growing murkier and more engulfing.

"There must be some way of knitting the circle in Time together again and our ability to find the way to do it must be a high-probability affair. As long as our Reality exists, we can be certain that the solution remains high-probability. If at any moment, you or I make the wrong decision, if the probability of healing the circle falls below some crucial magnitude, Eternity disappears. Do you understand?"

Harlan was not sure that he did. He wasn't trying very hard. Slowly he got to his feet and stumbled his way into a chair. "You mean we can get Cooper back--"

"And send him to the right place, yes. Catch him at the moment he leaves the kettle and he may end up in his proper place in the 24th no more than a few physiohours older; physiodays, at the most. It would be an alteration, of course, but undoubtedly not enough of one. Reality would be rocked, boy, but not upset."

"But how do we get him?"

"We know there's a way, or Eternity wouldn't be existing this moment. As to what that way is, that is why I need you, why I've fought to get you back on my side. You're the expert on the Primitive. Tell me."

"I can't," groaned Harlan.

"You can," insisted Twissell.

There was suddenly no trace of age or weariness in the old man's voice. His eyes were ablaze with the light of combat and he wielded his cigarette like a lance. Even to Harlan's regret-drugged senses the man seemed to be enjoying himself, actually enjoying himself, now that battle had been joined.

"We can reconstruct the event," said Twissell. "Here is the thrust control. You're standing at it, waiting for the signal. It comes. You make contact and at the same time squeeze the power thrust in the downwhen direction. How far?"

"I don't know, I tell you. I don't know."

"You don't know, but your muscles do. Stand there and take the controls in your hand. Get hold of yourself. Take them, boy. You're waiting for the signal. You're hating me. You're hating the Council. You're hating Eternity. You're wearying your heart out for Noys. Put yourself back at that moment. Feel what you felt then. Now I'll set the clock in motion again. I'll give you one minute, boy, to remember your emotions and force them back into your thalamus. Then, at the approach of zero, let your right hand jerk the control as it had done before. Then take your hand away! Don't move it back again. Are you ready?"

"I don't think I can do it."

"You don't think-- Father Time, you have no choice. Is there another way you can get back your girl?"

There wasn't. Harlan forced himself back to the controls, and as he did so emotion flooded back. He did not have to call on it. Repeating the physical movements brought them back. The red hairline on the clock started moving.

Detachedly he thought: The last minute of life?

Minus thirty seconds.

He thought: It will not hurt. It is not death.

He tried to think only of Noys.

Minus fifteen seconds.

Noys!

Harlan's left hand moved a switch down toward contact.

Minus twelve seconds.

Contact!

His right hand moved.

Minus five seconds.

Noys!

His right hand mo-ZERO-ved spasmodically.

He jumped away, panting.

Twissell came forward, peering at the dial. "Twentieth Century," he said. "Nineteen point three eight, to be exact."

Harlan choked out, "I don't know. I tried to feel the same, but it was different. I knew what I was doing and that made it different."

Twissell said, "I know, I know. Maybe it's all wrong. Call it a first approximation." He paused a moment in mental calculation, took a pocket computer half out of its container and thrust it back without consulting it. "To Time with the decimal points. Say the probability is 0.99 that you sent him back to the second quarter of the 20th. Somewhere between 19.25 and 19.50. All right?"

"I don't know."

"Well, now, look. If I make a firm decision to concentrate on that part of the Primitive to the exclusion of all else and if I am wrong, the chances are that I will have lost my chance to keep the circle in time closed and Eternity will disappear. The decision itself will be the crucial point, the Minimum Necessary Change, the M.N.C., to bring about the Change. I now make the decision. I decide, definitely--"

Harlan, looked about cautiously, as though Reality had grown so fragile that a sudden head movement might shatter it.

Harlan said, "I'm thoroughly conscious of Eternity." (Twissell's normality had infected him to the point where his voice sounded firm in his own ears.)

"Then Eternity still exists," said Twissell in a blunt, matter-of-fact manner, "and we have made the right decision. Now there's nothing more to do here for the while. Let's get to my office and we can let the subcommittee of the Council swarm over this place, if that will make them any happier. As far as they are concerned, the project has ended successfully. If it doesn't, they'll never know. Nor we."

Twissell studied his cigarette and said, "The question that now confronts us is this: What will Cooper do when he finds himself in the wrong Century?"

"I don't know."

"One thing is obvious. He's a bright lad, intelligent, imaginative, wouldn't you say?"

"Well, he's Mallansohn."

"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.

"Yes. English," muttered Harlan.

Twissell put the volume back. "Heavy and clumsy."

Harlan shrugged. To be sure, most of the Centuries of Eternity were film eras. A respectable minority were molecular-recording eras. Still, print and paper were not unheard of.

He said, "Books don't require the investment in technology that films do."

Twissell rubbed his chin. "Quite. Shall we get started?"

He took another volume down from the shelf, opening it at random and staring at the page with odd intentness.

Harlan thought: Does the man think he's going to hit the solution by a lucky stab?

The thought might have been correct, for Twissell, meeting Harlan's appraising eyes, reddened and put the book back.

Harlan took the first volume of the 19.25th Centicentury and began turning the pages regularly. Only his right hand and his eyes moved. The rest of his body remained at rigid attention.

At what seemed aeonic intervals to himself Harlan rose, grunting, for a new volume. On those occasions there would be the coffee break or the sandwich break or the other breaks.

Harlan said heavily, "It's useless your staying."

Twissell said, "Do I bother you?"

"No."

"Then I'll stay," muttered Twissell. Throughout he wandered occasionally to the bookshelves, staring helplessly at the bindings. The sparks of his furious cigarettes burned his finger ends at times, but he disregarded them.

A physioday ended.

Sleep was poor and sparse. Midmorning, between two volumes, Twissell lingered over his last sip of coffee and said, "I wonder sometimes why I didn't throw up my Computership after the matter of my-- You know."

Harlan nodded.

"I felt like it," the old man went on. "I felt like it. For physiomonths, I hoped desperately that no Changes would come my way. I got morbid about it. I began to wonder if Changes were right. Funny, the tricks emotions will play on you.

"You know Primitive history, Harlan. You know what it was like. Its Reality flowed blindly along the line of maximum probability. If that maximum probability involved a pandemic, or ten Centuries of slave economy, a breakdown of technology, or even a-a-let's see, what's really bad-even an atomic war if one had been possible then, why, by Time, it happened. There was nothing to stop it.

"But where Eternity exists, that's been stopped. Upwhen from the 28th, things like that don't happen. Father Time, we've lifted our Reality to a level of well-being far beyond anything Primitive times could imagine; to a level which, but for the interference of Eternity, would have been very low probability indeed."

Harlan thought in shame: What's he trying to do? Get me to work harder? I'm doing my best.

Twissell said, "If we miss our chance now, Eternity disappears, probably through all of physiotime. And in one vast Change all Reality reverts to maximum probability with, I am positive, atomic warfare and the end of man."

Harlan said, "I'd better get on to the next volume."

At the next break Twissell said helplessly, "There's so much to do. Isn't there a faster way?"

Harlan said, "Name it. To me it seems that I must look at every single page. And look at every part of it, too. How can I do it faster?"

Methodically he turned the pages.

"Eventually," said Harlan, "the print starts blurring and that means it's time for sleep."

A second physioday ended.

At 10:22 A.M., Standard Physiotime, of the third physioday of the search Harlan stared at a page in quiet wonder and said, "This is it!"

Twissell didn't absorb the statement. He said, "What?"

Harlan looked up, his face twisted with astonishment. "You know, I didn't believe it. By Time, I never really believed it, even while you were working out all that rigmarole about news magazines and advertisements."

Twissell had absorbed it now. "_You've found it!_"

He leaped at the volume Harlan was holding, clutching at it with shaking fingers.

Harlan held it out of reach and slammed the volume shut. "Just a moment. You won't find it, even if I showed you the page."

"What are you doing?" shrieked Twissell. "You've lost it."

"It's not lost. I know where it is. But first--"

"First what?"

Harlan said, "There's one point remaining, Computer Twissell. You say I can have Noys. Bring her to me, then. Let me see her."

Twissell stared at Harlan, his thin white hair in disarray. "Are you joking?"

"No," said Harlan sharply, "I'm not joking. You assured me that you would make arrangments- Are you joking? Noys and I would be together. You promised that."

"Yes, I did. That part's settled."

"Then produce her alive, well, and untouched."

"But I don't understand you. I don't have her. No one has. She's still in the far upwhen, where Finge reported her to be. No one has touched her. Great Time, I told you she was safe."

Harlan stared at the old man and grew tense. He said, chokingly, "You're playing with words. All right, she's in the far upwhen, but what good is that to me? Take down the barrier at the 100,000th-"

"The what?"

"The barrier. The kettle won't pass it."

"You never said anything of this," said Twissell wildly.

"I haven't?" said Harlan with sharp surprise. Hadn't he? He had thought of it often enough. Had he never said a word about it? He couldn't recall, at that. But then he hardened.

He said, "All right. I tell you now. Take it down."

"But the thing is impossible. A barrier against the kettle? A temporal barrier?"

"Are you telling me you didn't put one up?"

"I didn't. By Time, I swear it."

"Then-then--" Harlan felt himself grow pale. "Then the Council did it. They know of all this and they've taken action independently of you and-and by all of Time and Reality, they can whistle for their ad and for Cooper, for Mallansohn and all of Eternity. They'll have none of it. None of it."

"Wait. Wait." Twissell yanked despairingly at Harlan's elbow. "Keep hold of yourself. Think, boy, think. The Council put up no barrier."

"It's there."

"But they can't have put up such a barrier. No one could have. It's theoretically impossible."

"You don't know it all. It's there."

"I know more than anyone else on the Council and such a thing is impossible."

"But it's there."

"But if it is--"

And Harlan grew sufficiently aware of his surroundings to realize that there was a kind of abject fear in Twissell's eyes; a fear that had not been there even when he first learned of Cooper's misdirection and of the impending end of Eternity.