Nedra called his name but he did not hear her. His mind was working like an exquisitely refined machine. He was counting, squeezing the fractional lost months and weeks and days and hours, the minutes and seconds of the past twelve months into a pattern, calculating time, estimating his position by the daytime pinpoints of the stars. "O'Hara, O'Hara! Our mountains have vanished--" December 20--and precisely at 12:35:01 P.M.--and for the ensuing ten seconds. Sweat poured from his body. Infinitesimal memories arranged themselves. He could not be certain. But then he could never be certain again on this side of the Curtain. To the day, yes! And to the hour and possibly quite close to the minute--but never to the second! He would have to feel his way into it, watching his milliroentgen count, and if the Curtain was there, he would have to turn back. Yet this fuel would not last him back from the Curtain to Nedra's mountains. It would be a chance--a frightful chance. "O'Hara, tell me--" "We're going through the Curtain, Nedra." And she nodded somberly. At fifty thousand feet O'Hara leveled off. As he accelerated past two thousand miles an hour he checked and re-checked his calculations of the day and hour. His problem, as he analyzed it, was to reach the Curtain before 12:35:01 P.M., and to continue swinging into it and away from it until--at the miraculous proper moment--his scintillometer reading dropped below the danger point, and then, upon the inward arc of his ellipse of flying, to smash through. It would be best to approach first in a dive from his maximum altitude, at his maximum speed. It was true that he might thus smash into the Curtain before the ten-second interval in which it did not exist, during the changing of the reactors, but at least this time he would know what he was about to do--he would not be senseless from a bolt of lightning. This was his plan. And it worked. It shot him back across the Arctic, deep into Asia, deep into the hemisphere of Delhi, Rome and Paris and the Twelve Old Men of Geneva. "You've got to have luck for anything like that," O'Hara admitted. "The luck of a Columbus. Would you have bet on us?" He laughed, then he corked the bottle. There would be no more tippling in that little flat in Bloomsbury. "I would not bet against your luck, O'Hara." "But there was more than luck to it," he answered. "For I was beginning to learn the most difficult of the arts--to think. The Father had taught me how to start learning. Ninety-nine hundredths of the best motor you and I have got is never used--ninety-nine hundredths of each of our brains lies idle. Perhaps in Washington I learned to use another hundredth. At least I had begun to learn how to learn, and once that has begun no problem can remain insoluble." "Almost the words of the Father," I pointed out.