The eastward-flowing jetstream rushed invisibly overhead. That river of racing air, pouring west to east at three hundred miles an hour and better, was lower than ordinary today. The ship should hit it at twenty-eight thousand feet. McCauley had to get into it without risking the sheering stresses the bottom part of it might exert. He had to get into it like a man stepping onto a moving sidewalk. He adjusted the rate of climb. At twenty thousand feet the ramjets were more effective. The ship climbed more steeply. There was a difference in the bellowing of the ramjets. The noise was still monstrous, but it was thinner. It did not have the substance of thunder at ground level. But the sound was still so tremendous that it seemed to fill all of McCauley's consciousness. It required an effort of will to see, when he was so battered and hammered at by sound. It was difficult to think. His hands were heavy, and movements of which he would ordinarily have been unconscious now required almost painful effort. Twenty-five thousand feet. McCauley glanced at the gyrocompass, computed swiftly in his head, added together his known air speed and the reported wind direction at this height, and deduced an actual course. Then he had to guess at the angle at which to hit the jetstream so that when its direction and speed were added to the ship's, the result of the several forces would be a course around the globe as nearly as possible the right one. It should pass over the most closely placed tracking stations, and it should not be immoderately far from the wide-spaced Navy ships which had been alerted for the flight and a possible unscheduled descent. He swung the ship from its circling. He aimed it up and up, south-east by a half east. The ship climbed. There was a logy wallowing when it penetrated the bottom of the jetstream. But it kept on, and presently a clock assured McCauley that he'd been in the stream long enough to gain all the extra speed it could give him. He aimed the ship's nose still higher and gave the ramjets every particle of fuel they could consume. The sky grew dark. Dark purple. Faint twinklings appeared here and there. They were the stars, visible in daylight. The ramjets' tumult was still thinner now. And little by little the rate of climb grew less. Presently the ship did not climb at all. It was as high as the ramjets could take it. Now the sunshine on its aluminum body was painfully bright, but the sky was almost black. Had there been time, he could have traced the constellations--the same constellations that people down below would not see for months, until this part of the heavens shone down on Earth's dark side.