McCauley growled to himself. He knew the calculations for placing the relay. The mountain beyond the horizon was an eight-hour journey by sledge--the horizon on the moon is only two miles away instead of eight. Breathing from the sledge, the men would arrive with one tank on their suits untapped, another, also untapped, to be mounted; and an extra tank for good measure. When they'd put the sledge in place and aired its beams and set up the nondirectional auxiliary antennae, they'd start back with two full tanks each and another one for reserve. They'd make better time coming back--six hours, no more. And each man had a full six hours on his back, and there were three additional hours in the extra they'd take turns carrying. It was ample margin. But now the spare tank was left behind. There was no margin. McCauley tried to lift the tank. But it had lain in the shadow of a boulder, out of the sun's fierce glare--on moon dust, radiating heat away toward the stars. It had cooled off to the temperature of a shadow, two hundred and forty degrees below zero. It was frozen. The air was liquid air. The tank was more brittle than glass was. It slipped, striking the boulder. It cracked and broke. A glistening liquid poured out and evaporated instantly. Where it fell into shadow, part of it froze and then vanished more quickly than any earthly frost. McCauley growled again. Air was precious on the moon. But there was no use crying when it was spilt. He turned around and began his journey again. He had good reason to worry now. He was a singular, slow-motion soaring figure in a polished silvery space suit. Where there was a rise in the ground, he came smoothly up from behind it, the glaring sun glowing on his space armor. Extending one leg in what might pass as a version of a choreographer's arabesque, he came down on the extended foot and stepped on it, floating gently upward and forward swiftly in a continued series of seeming flights. He went through winding passes where the sledge trail was plain in the dust below him, he soared across preposterous areas strewn with boulders the size of apartment houses. Once, going through a narrow gap in the wall of an unnamed crater--a very small one, barely two miles across--he passed a spot which showed that the two men had changed places. The one in advance had gone to the rear, and the one who'd been behind now led the way. It was just beyond the farther wall of the crater that he saw the second air tank, dropped in the trail. It could not possibly be an accident. A moon sledge has racks for carrying air tanks. It was conceivable that a tank could have slid out and been lost unnoticed. But it was starkly inconceivable that it could have happened twice. McCauley raged suddenly. He knew what had happened, he knew why it had happened, he knew who was involved. He flipped the base-frequency switch. "Holmes! Kent! Come in!" he snapped. "Grimaldi Base, come in! Holmes! Kent! Come in! Grimaldi Base, come in!"