"And so, O'Hara--so through these endless years alone I have been forced to tinker--selective breeding, elimination of the weak, artificial insemination--tricks, all tricks, all little stratagems to stall off the immutable. Each generation slides back further than its sires. Your son--forgive me--will be not quite you, and his son will be neither you nor his mother. And your son's son will see his newborn son with horror. It is exact. But if a new race suddenly were spawned into the hemisphere, recapturing the ground lost in these generations since the Curtain--" "Father, you'd still be tinkering." "But the possibilities of this tinkering are extensive. Beyond the Curtain, fortunately, exists an inexhaustible source of uncontaminated sires." "How would you get them here?" "That is your task." "My task? I could not count upon a providential thunderstorm to get me through the Curtain again." "Providence we cannot reckon in planning the future of a hemisphere, and never have. It is true that chance--pure chance--enabled you, and Anstruther before you, and others now dead before both of you, to pass through the Curtain. But it was not Providence. Any man flying in the correct direction at the exact second that you were flying--and at any point in the Curtain--would have got through." "I'm not sure I understand you. At any point in the Curtain at that exact second?" "Within those exact ten seconds, O'Hara, hundreds of pilots in your International Patrol must ram into the Curtain every decade, yet if all of them were to hit it at the correct speed, flying the correct course, at the correct time, they would all get through. Chance enters into it only in that you happened to be doing all of these. Once each year, and for an interval of ten seconds, our Carolina reactor shuts off automatically and the Hanford or West Coast reactor takes over supplying the power for the Curtain. That has been happening now for the two hundred and seventy years since the establishment of the Curtain--the reactors were so arranged in the beginning. Completely automatic, you understand, as are the Tubes, the photosynthetrons, the East and West Coast distillation plants, the Deluges--as everything must be within this hemisphere. And there is this interval of ten seconds between the shutting off of the Carolina reactor and the generation of the Hanford reactor--ten seconds in which the Curtain does not exist!" O'Hara closed his eyes. The taste of blood, saline and hot, was in his mouth--his heart now pounding with trip-hammer force. For here, within a sentence, was the determining great secret of the two halves of the earth--"There is an interval of ten seconds in which the Curtain does not exist!"