Burke said, "In a way, a telephone system transposes sound-waves from one place to another. Sound-waves aren't carried along wires. They're here, and then suddenly they're there. But there has to be a sending and receiving station at each end. When the fortress here was 'cut off' from home it could be that its supply-system broke down." "Its air-system didn't," said Holmes. "It hadn't used up its emergency air-supply. We're breathing it!" "Anyhow we could try to find even a broken-down transposer," said Sandy. "You try," said Burke. "Keller's been looking for something for me in the cubes. I'll stay here and help him look." Sandy examined the weapon he'd given her. "Pam says she's smelled fresh air, down below where there can't be any. Mr. Keller thought he saw movements in the inside vision-plates, where there can't be any. I still believe I saw something alive in the gravity-machine room, where such a thing is impossible. We're going to look, Pam and I." Holmes lumbered to his feet. "I'll come, too. And I'll guarantee to defend you against anything that has survived the ten thousand years or so that this place was without air. My head's tired, after all those cubes." He led the way. Burke watched as the two girls followed him and closed the door behind them. "What have you found, Keller?" "A cube about globes," said Keller. "Very interesting." "Nothing on communication with base?" Keller shook his head. Burke said evenly, "I figured out three chances for us--all slim ones. The first was to find the garrison when the radio summons didn't and get it or its descendants to help. I found the garrison--on Earth. No help there. The second chance was finding the civilization that had built this fortress. It looks like it's collapsed. There's been time for a new civilization to get started, but it's run away. The third chance is the slimmest of all. It's hooking together something to fight with." Keller reached out over the array of cubes that had been experienced by Holmes and himself while using the helmets from the cube-library. One cube had been set aside. Keller put it in place on the extra helmet and handed it to Burke. "Try it," said Keller. Burke put the helmet on his head. * * * * * _He was in this same instrument-room, but he wore a uniform and he sat at an instrument-board. He knew that there were drone service-boats perhaps ten thousand miles out, perhaps a hundred. They'd been fitted out to make a mock attack on the fortress. Counter-tactics men devised them. There was reason for worry. Three times, now, drones pretending to be Enemy ships had dodged past the screen of globes set out to prevent just such an evasion. Once, one of the drones had gloatingly touched the stone of the fortress' outer surface. This was triumph for the counter-tactics crew, but it was proof that an Enemy ship could have wiped out the fortress and all its garrison a hundred times over._