There was no answer. He called again and again. Then he called Grimaldi Base. Again no answer. He was out of radio contact with all humanity on the moon--even his own base ten miles away--though by switching frequencies he could raise Earth Relay a quarter million miles farther away. The men with the moon sledge might only be behind a mountain wall or anywhere in any direction below the horizon, but radio communication on the moon is limited to line-of-sight because there is no air and hence no layer of ions to bounce radio signals down behind obstacles or around the moon's curvature. McCauley started off again, fuming. Moon gait is a highly specialized form of travel. In one-sixth gravity a man can cover ten miles an hour over rough ground if he knows the trick of the gait and the trail is marked. He travels in slow-motion giant steps, with something of the effect of an extremely deliberate ballet. He begins with a leap up and forward, and he rises slowly and deliberately while soaring ahead. At mid-leap he is six feet higher than at take-off. Then he descends slowly and with dignity, touches ground and strides at the same time, and bounds up and ahead once more. There are long seconds between steps and long yards between strides. When a person is used to it, moon gait is almost restful. Some people even find it familiar. They've dreamed of such effortless half flight in their sleep. Now, though he was disturbed, McCauley made two miles with no other known cause for worry than the lateness of the two men who'd placed the relay and the prospective killing he'd had on his mind before. He passed between precipices and over dust-strewn stone and through winding defiles. The two men should be back.... Then he spotted something. Abruptly he raised his arms and extended both feet before him. He came down to the ground and stopped short. Then--not soaring this time--he walked back to an object on the trail. It was an air tank, exactly like the two tanks at the back of his own space suit. It had been dropped from the moon sledge. It would hold air for one man for three hours. Men driving a moon sledge would wear one tank on their space suits for safety, and they'd shed one for lightness. They'd breathe from the much larger tanks on the sledge itself while they traveled. Spare and extra tanks like this would ride on the sledge. It was not easy to imagine that it had dropped. One man would go on ahead of the sledge and one would follow. It was hard to believe that the second man would not notice the loss of an air tank. Air tanks were life. True, a sledge party always had more air than was needed for any expected journey--a good margin for emergency--but this tank could cut the margin for this journey seriously.