At noon, he had done a good thousand miles. At mid-afternoon, sixteen hundred. Two and a half hours of visibility left, and more than four hundred miles to go. "I can do those on my head," muttered Wofforth to himself, and then, far in the distance, the flat rim of the horizon was flat no longer. It had sprung up jagged, full of points and bulges. Speeding toward it, he steered by the line around his waist while he cut his engine. He came close at fifty miles an hour, almost a crawl. Some ancient volcanic action had thrown up those mountains, like a rank of close-drawn sentries. The sled could not cross them anywhere. Still reducing speed, Wofforth drew close to a notch, but the notch gave into a crater, a great shallow saucer two miles in diameter and filled with shadows below, so that Wofforth could not gauge its depth. Opposite, another notch--perhaps once the crater had been a lake, with water running in and out. If he had come there at noon, he could have seen the bottom, and perhaps-- "But it isn't noon." Wofforth was talking to himself again. His voice sounded thin and petulant in his own ears. "By noon tomorrow, the heat will be out of this suit." He stopped the sled, unlashed himself and trudged to the notch. He stood in it, looking down, then across. The little bright jewel of the sun, sagging toward the horizon, showed him the upper reaches of the crater's interior, pitched at an angle of perhaps fifty degrees. Even if it had been noon, it would have been no use. The sled could never climb a slope like that. Then he looked again, this way and that. He nodded inside his helmet. He might as well try. Returning to the sled, he started the engine and lashed himself fast again. He steered away from the crater, and around. He made a great looping journey of twenty miles or so across the plain, building speed all the time. As he rounded the rear curve of his course, he was driving along at two hundred and sixty miles an hour, and he had to apply pressure to the boom with both hand and knees to point the sled back straight for the notch. Straightening his humming vehicle into a headlong course, he leaned forward and sighted between the upcurved runners. "Now!" he urged himself, and watched the break in the crater wall rush toward him. It greatened, yawned. He leaped through, and with a groaning gasp of prayer he dragged the boom over to steer the sled right. * * * * * It worked, as he had not dared hope. The runners bounced, bit. Then he was racing around the inside of the great cup's rim, like a hurtling bubble on the inner surface of a whirlpool's funnel. Two miles across, three miles and more on the half diameter--the engine laboring up to three hundred miles an hour, centrifugal force holding it there--