Brinsley Cooper entered. Excitement flushed his thin face and made it almost youthful, despite the heavy Mallansohn mustache that draped its upper lip.

(Harlan could see him through the window, hear him clearly over the room's radio. He thought bitterly: A Mallansohn mustache! Of course!)

Cooper strode toward Twissell. "They wouldn't let me in till now, Computer."

"Very right," said Twissell. "They had their instructions."

"Now's the time, though? I'll beheading out?"

"Almost the time."

"And I'll be coming back? I'll be seeing Eternity again?" Despite the straightness Cooper gave his back, there was an edge of uncertainty in his voice.

(Within the control room Harlan brought his clenched hands bitterly to the reinforced glass of the window, longing to break through somehow, to shout: "Stop it! Meet my terms, or I'll-" What was the use?)

Cooper looked about the room, apparently unaware that Twissell had refrained from answering his question. His glance fell on Harlan at the control-room window.

He waved his hand excitedly. "Technician Harlan! Come on out. I want to shake your hand before I go."

Twissell interposed. "Not now, boy, not now. He's at the controls."

Cooper said, "Oh? You know, he doesn't look well."

Twissell said, "I've been telling him the true nature of the project. I'm afraid that's enough to make anyone nervous."

Cooper said, "Great Time, yes! I've known about it for weeks now and I'm not used to it yet." There was a trace of near-hysteria in his laugh. "I still haven't got it through my thick head that it is really my show. I-I'm a little scared."

"I scarcely blame you for that."

"It's my stomach, mostly, you know. It's the least happy part of me."

Twissell said, "Well, it's very natural and it will pass. Meanwhile, your time of departure on Standard Intertemporal has been set and there is still a certain amount of orientation to be gone through. For instance, you haven't actually seen the kettle you will use."

In the two hours that passed Harlan heard it all, whether they were in sight or not. Twissell lectured Cooper in an oddly stilted manner, and Harlan knew the reason. Cooper was being informed of just those things that he was to mention in Mallansohn's memoir.

(Full circle. Full circle. And no way for Harlan to break that circle in one, last defiant Samson-smash of the temple. Round and round the circle goes; round and round it goes.)

"Ordinary kettles," he heard Twissell say, "are both pushed and pulled, if we can use such terms in the case of Intertemporal forces. In traveling from Century X to Century Y within Eternity there is a fully powered initial point and a fully powered final point.

"What we have here is a kettle with a powered initial point but an unpowered destination point. It can only be pushed, not pulled. For that reason, it must utilize energies at a level whole orders of magnitude higher than those used by ordinary kettles. Special power-transfer units have had to be laid down along the kettleways to siphon in sufficient concentrations of energy from Nova Sol.

"This special kettle, its controls and power supply, are a composite structure. For physiodecades, the passing Realities have been combed for special alloys and special techniques. The 13th Reality of the 222nd was the key. It developed the Temporal Pressor and without that, this kettle could not have been built. The 13th Reality of the 222nd."

He pronounced that with elaborate distinctness.

(Harlan thought: Remember that, Cooper! Remember the 13th Reality of the 222nd so you can put it into the Mallansohn memoir so that the Etemals will know where to look so they will know what to tell you so you can put it... Round and round the circle goes...)

Twissell said, "The kettle has not been tested past the downwhen terminus, of course, but it has taken numerous trips within Eternity. We are convinced there will be no bad effects."

"There can't be, can there?" said Cooper. "I mean I did get there or Mallansohn could not have succeeded in building the field and he did succeed."

Twissell said, "Exactly. You will find yourself in a protected and isolated spot in the sparsely populated southwestern area of the United States of Amellika..."

"America," corrected Cooper.

"America, then. The Century will be the 24th; or, to put it to nearest hundredth, the 23.17th. I suppose we can even call it the year 2317, if we wish. The kettle, as you saw, is large, much larger than necessary for you. It is being filled now with food, water, and the means of shelter and defense. You will have detailed instructions that will, of course, be meaningless to anyone but you. I must impress upon you now that your first task will be to make certain that none of the indigenous inhabitants discovers you before you are ready for them. You will have force-diggers with which you will be able to burrow well into a mountain to form a cache. You will have to remove the contents of the kettle rapidly. They will be stacked so as to facilitate that."

(Harlan thought: Repeat! Repeat! He must have been told all this before, but repeat what must go into the memoir. Round and round...)

Twissell said, "You will have to unload in fifteen minutes. After that, the kettle will return automatically to starting point, carrying with it all tools that are too advanced for the Century. You will have a list of those. After the kettle returns, you will be on your own."

Cooper said, "Must the kettle return so quickly?"

Twissell said, "A quick return increases the probabilities of success."

(Harlan thought: The kettle must return in fifteen minutes because it did return in fifteen minutes. Round and...)

Twissell hurried on. "We cannot attempt to counterfeit their medium of exchange of any of their negotiable scrip. You will have gold in the form of small nuggets. You will be able to explain its possession according to your detailed instructions. You will have native clothing to wear or at least clothing that will pass for native."

"Right," said Cooper.

"Now, remember. Move slowly. Take weeks, if necessary. Work your way into the era, spiritually. Technician Harlan's instructions are a good basis but they are not enough. You will have a wireless receiver built on the principles of the z4th which will enable you to come abreast of the current events and, more important, learn the proper pronunciation and intonation of the language of the times. Do that thoroughly. I'm sure that Harlan's knowledge of English is excellent, but nothing can substitute for native pronunciation on the spot."

Cooper said, "What if I don't end up in the right spot? I mean, not in the 23.17?"

"Check on that very carefully, of course. But it will be right. It will be right."

(Harlan thought: It will be right because it was right. Round...)

Cooper must have looked unconvinced, though, for Twissell said, "The accuracy of focus was carefully worked out. I intended to explain our methods and now is a good time. For one thing, it will help Harlan understand the controls."

(Suddenly Harlan turned away from the windows and fixed his gaze on the controls. A corner of the curtain of despair lifted. What if--)

Twissell still lectured Cooper with the anxious overprecise tone of the schoolteacher, and with part of his mind Harlan still listened.

Twissell said, "Obviously a serious problem was that of determining how far into the Primitive an object is sent after the application of a given energy thrust. The most direct method would have been to send a man into the downwhen via this kettle using carefully graduated thrust levels. To do that, however, would have meant a certain lapse of time in each case while the man determined the Century to its nearest hundredth through astronomical observation or by obtaining appropriate information over the wireless. That would be slow and also dangerous since the man might well be discovered by the native inhabitants with probably catastrophic effects on our project.

"What we did then instead was this: We sent back a known mass of the radioactive isotope, niobium-94, which decays by beta-particle emission to the stable isotope, molybdenum-94. The process has a halflife of almost exactly 500 Centuries. The original radiation intensity of the mass was known. That intensity decreases with time according to the simple relationship involved in first-order kinetics, and, of course, the intensity can be measured with great precision.

"When the kettle reaches its destination in Primitive times, the ampule containing the isotope is discharged into the mountainside and the kettle then returns to Eternity. At the moment in physiotime that the ampule is discharged, it simultaneously appears at all future Times growing progressively older. At the place of discharge in the 575th (in actual Time and not in Eternity) a Technician detects the ampule by its radiation and retrieves it.

"The radiation intensity is measured, the time it has remained in the mountainside is then known, the Century to which the kettle traveled is also known to two decimal places. Dozens of ampules were thus sent back at various thrust levels and a calibration curve set up. The curve was a check against ampules sent not all the way into the Primitive but into the early Centuries of Eternity where direct observations could also be made.

"Naturally, there were failures. The first few ampules were lost until we learned to allow for the not too major geological changes between the late Primitive and the 575th. Then, three of the ampules later on never showed up in the 575th. Presumably, something went wrong with the discharge mechanism and they were buried too deeply in the mountain for detection. We stopped our experiments when the level of radiation grew so high that we feared that some of the Primitive inhabitants might detect and wonder what radioactive artifacts might be doing in the region. But we had enough for our purposes and we are certain we can send back a man to any hundredth of a Century of the Primitive that is desired.

"You follow all this, Cooper, don't you?"

Cooper said, "Perfectly, Computer Twissell. I have seen the calibration curve without understanding the purpose at the time. It is quite clear now."

But Harlan was exceedingly interested now. He stared at the measured arc marked off in centuries. The shining arc was porcelain on metal and the fine lines divided it into Centuries, Decicenturies and Centicenturies. Silvery metal gleamed thinly through the porcelainpenetrating lines, marking them clearly. The figures were as finely done and, bending close, Harlan could make out the Centuries from 17 to 27. The hairline was fixed at the 23.17th Century mark.

He had seen similar time-gauges and almost automatically he reached to the pressure-control lever. It did not respond to his grasp. The hairline remained in place.

He nearly jumped when Twissell's voice suddenly addressed him.

"Technician Harlan!"

He cried, "Yes, Computer," then remembered that he could not be heard. He stepped to the window and nodded.

Twissell said, almost as though chiming in with Harlan's thoughts, "The time-gauge is set for a thrust back to the 23.17th. That requires no adjustment. Your only task is to pour energy through at the proper moment in physiotime. There is a chronometer to the right of the gauge. Nod if you see it."

Harlan nodded.

"It will reach zero-point backward. At the minus-fifteen-second point, align the contact points. It's simple. You see how?"

Harlan nodded again.

Twissell went on, "Synchronization is not vital. You can do it at minus fourteen or thirteen or even minus five seconds, but please make every effort to stay this side of minus ten for safety's sake. Once you've closed contact, a synchronized force-gear will do the rest and make certain that the final energy thrust will occur precisely at time zero. Understood?"

Harlan nodded still again. He understood more than Twissell said. If he himself did not align the points by minus ten, it would be taken care of from without.

Harlan thought grimly: There'll be no need for outsiders.

Twissell said, "We have thirty physiominutes left. Cooper and I will leave to check on the supplies."

They left. The door closed behind them, and Harlan was left alone with the thrust control, the time (already moving slowly backward toward zero)-and a resolute knowledge of what must be done.

Harlan turned away from the window. He put his hand inside his pocket and half withdrew the neuronic whip it still contained. Through all this he had kept the whip. His hand shook a little.

An earlier thought recurred: a Samson-smash of the temple!

A corner of his mind wondered sickly: How many Eternals have ever heard of Samson? How many know how he died?

There were only twenty-five minutes left. He was not certain how long the operation would take. He was not really certain it would work at all.

But what choice had he? His damp fingers almost dropped the weapon before he succeeded in unhinging the butt.

He worked rapidly and in complete absorption. Of all the aspects of what he planned, the possibility of his own passage into nonexistence occupied his mind the least and bothered him not at all.

At minus one minute Harlan was standing at the controls.

Detachedly he thought: The last minute of life?

Nothing in the room was visible to him but the backward sweep of the red hairline that marked the passing seconds.

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. Not hastily!

Minus twelve seconds.

Contact!

The force-gear would take over now. Thrust would come at zero time. And that left Harlan one last manipulation. The Samson-smash!

His right hand moved. He did not look at his right hand.

Minus five seconds.

Noys!

His right hand mo-ZERO-ved again, spasmodically. He did not look at it.

Was this nonexistence?

Not yet. Nonexistence not yet.

Harlan stared out the window. He did not move. Time passed and he was unaware of its passage.

The room was empty. Where the giant, enclosed kettle had been was nothing. Metal blocks that had served as its base sat emptily, lifting their huge strength against air.

Twissell, strangely small and dwarfed in the room that had become a waiting cavern, was the only thing that moved as he tramped edgily this way and that.

Harlan's eyes followed him for a moment and then left him.

Then, without any sound or stir, the kettle was back in the spot it had left. Its passage across the hairline from time past to time present did not as much as disturb a molecule of air.

Twissell was hidden from Harlan's eyes by the bulk of the kettle, but then he rounded it, came into view. He was running.

A flick of his hand was enough to activate the mechanism that opened the door of the control room. He hurtled inside, shouting with an almost lyrical excitement. "It's done. It's done. We've closed the circle." He had breath to say no more.

Harlan made no answer.

Twissell stared out the window, his hands flat against the glass. Harlan noted the blotches of age upon them and the way in which they trembled. It was as though his mind no longer had the ability or the strength to filter the important from the inconsequential, but were selecting observational material in a purely random manner.

Wearily he thought: What does it matter? What does anything matter now?

Twissell said (Harlan heard him dimly), "I'll tell you now that I've been more anxious than I cared to admit. Sennor used to say once that this whole thing was impossible. He insisted something must happen to stop it-- What's the matter?"

He had turned at Harlan's odd grunt.

Harlan shook his head, managed a choked "Nothing."

Twissell left it at that and turned away. It was doubtful whether he spoke to Harlan or to the air. It was as though he were allowing years of pent-up anxieties to escape in words.

"Sennor," he said, "was the doubter. We reasoned with him and argued. We used mathematics and presented the results of generations of research that had preceded us in the physiotime of Eternity. He put it all to one side and presented his case by quoting the man-meetshimself paradox. You heard him talk about it. It's his favorite.

"We knew our own future, Sennor said. I, Twissell, knew, for instance, that I would survive, despite the fact that I would be quite old, until Cooper made his trip past the downwhen terminus. I knew other details of my future, the things I would do.

"Impossible, he would say. Reality must change to correct your knowledge, even if it meant the circle would never close and Eternity never established.

"Why he argued so, I don't know. Perhaps he honestly believed it, perhaps it was an intellectual game with him, perhaps it was just the desire to shock the rest of us with an unpopular viewpoint. In any case, the project proceeded and some of the memoir began to be fulfilled. We located Cooper, for instance, in the Century and Reality that the memoir gave us. Sennor's case was exploded by that alone, but it didn't bother him. By that time, he had grown interested in something else.

"And yet, and yet"-he laughed gently, with more than a trace of embarrassment, and let his cigarette, unnoticed, burn down nearly to his fingers-"you know I was never quite easy in my mind. Something might happen. The Reality in which Eternity was established might change in some way in order to prevent what Sennor called a paradox. It would have to change to one in which Eternity would not exist. Sometimes, in the dark of a sleeping period, when I couldn't sleep, I could almost persuade myself that that was indeed so. -and now it's all over and I laugh at myself as a senile fool."

Harlan said in a low voice, "Computer Sennor was right."

Twissell whirled. "What?"

"The project failed." Harlan's mind was coming out of the shadows (why, and into what, he was not sure). "The circle is not complete."

"What are you talking about?" Twissell's old hands fell on Harlan's shoulders with surprising strength. "You're ill, boy. The strain."

"Not ill. Sick of everything. You. Me. Not ill. The gauge. See for yourself."

"The gauge?" The hairline on the gauge stood at the 27th Century, hard against the right-hand extreme. "What happened?" The joy was gone from his face. Horror replaced it.

Harlan grew matter-of-fact. "I melted the locking mechanism, freed the thrust control."

"How could you--"

"I had a neuronic whip. I broke it open and used its micro-pile energy source in one flash, like a torch. There's what's left of it." He kicked at a small heap of metal fragments in one corner.

Twissell wasn't taking it in. "In the 27th? You mean Cooper's in the 27th--"

"I don't know where he is," said Harlan dully. "I shifted the thrust control downwhen, further down than the z4th. I don't know where. I didn't look. Then I brought it back. I still didn't look."

Twissell stared at him, his face a pale, unhealthy yellowish color, his lower lip trembling.

"I don't know where he is now," said Harlan. "He's lost in the Primitive. The circle is broken. I thought everything would end when I made the stroke. At zero time. That's silly. We've got to wait. There'll be a moment in physiotime when Cooper will realize he's in the wrong Century, when he'll do something against the memoir, when he-" He broke off, then broke into a forced and creaky laughter. "What's the difference? It's only a delay till Cooper makes the final break in the circle. There's no way of stopping it. Minutes, hours, days. What's the difference? When the delay is done, there will be no more Eternity. Do you hear me? It will be the end of Eternity.