The End of Eternity - By Isaac Asimov Page 0,61

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