Harvey found him by the sounds. Brace was coldly angry. "Either you pipe down and come along, or you go on back. Nobody's stopping me. If you try any monkey business, I'll kill you, and I'm not kidding." Harvey fell in alongside him. "You're the craziest jerk I ever saw." "Pipe down before you give us away," Brace growled. The hill loomed ahead of them like a giant blot against the sky. On their left were the thick woods where the settlers gathered timber for fencing and bridging. Straight ahead was a deep stream, traversed by a single bridge. "That bridge is a likely place for a burglar alarm," Harvey said. "We'd better swim." They took off their clothes and left them on the bank. They let themselves carefully down into the water and set out, using the breast stroke to keep the oxygen apparatus out of the water. They clambered up on the other side, chilled and shivering, and continued, naked, toward the hill. The ground roughened underfoot; the rise began. There was no apparent road. Here and there stood small blighted trees, devoid of foliage, standing like posts--They weren't trees at all! Harvey gripped Brace's arm. "Keep away from those posts," he whispered. They continued upward slowly. Harvey heard a faint click. He stopped. "Hear that?" he whispered, "that click?" Brace shook his head. They stood still, listening. There was no other sound. Brace took a step forward, and stopped. "I heard it," he whispered. They sank to the ground, and stretched out on their bellies, and waited tensely. Not a sound came out of the surrounding darkness. Far ahead, high in the sky, the windows of Baker's castle dimly glowed with a yellow light. There was still a long way to go. Brace sat up and jerked a hand impatiently at Harvey. They got to their feet and moved forward. They were moving through trees now. Harvey followed Brace reluctantly. There was something funny. Some sound he was hearing--or wasn't hearing-- "Red!" It was the oxygen pack. The soft, bubbling murmur it always made was gone. "My oxygen is off! Yours, too! We've got to get back--fast!" Brace's eyes flared white in the darkness. "Come on," Harvey shouted, and rushed back through the trees. Brace came lumbering after him, protesting loudly. They raced across an open space--there were the posts again--they passed them-- Click! And now Harvey knew where the click came from. It came from inside his oxygen pack. Almost instantly, he felt the soft flow of the oxygen through the tubes. He sat down. Brace sat down heavily beside him. "Those posts," Harvey said. "He must have them all around the hill. They throw out a radio wave that shuts these things off when you go past them." He shook his head ruefully. "I should have guessed it. When I first took a look inside this oxygen pack, I noticed something that looked like a transistor, but I couldn't imagine what that could have to do with oxygen." Brace let out a long breath. "We should have kept going," he said bitterly.