Silverman was Horne's replacement. When the cruiser returned six months later bringing him, Green, to be Silverman's replacement, Silverman was completely insane. He babbled about something that had been waiting outside the bubble to kill him but his nearest to a rational statement was to say once, when asked for the hundredth time what he had seen: "Nothing--you can't really see it. But you feel it watching you and you hear it trying to get in to kill you. One time I bumped the wall and--for God's sake--take me away from it--take me back to Earth ..." Then he had tried to hide under the captain's desk and the ship's doctor had led him away. The bubble was minutely examined again and the cruiser employed every detector device it possessed to search surrounding space for light-years in all directions. Nothing was found. When it was time for the new replacement to be transferred to the bubble he reported to Captain McDowell. "Everything is ready, Green," McDowell said. "You are the next one." His shaggy gray eyebrows met in a scowl. "It would be better if they would let me select the replacement instead of them." He flushed with a touch of resentment and said, "The Bureau found my intelligence and initiative of thought satisfactory." "I know--the characteristics you don't need. What they ought to have is somebody like one of my engine room roustabouts, too ignorant to get scared and too dumb to go nuts. Then we could get a sane report six months from now instead of the ravings of a maniac." "I suggest," he said stiffly, "that you reserve judgement until that time comes, sir." * * * * * And that was all he knew about the danger, real or imaginary, that had driven two men into insanity. He would have six months in which to find the answer. Six months minus-- He looked at the chronometer and saw that twenty minutes had passed since he left the cruiser. Somehow, it seemed much longer ... He moved to light a cigarette and his metal soles scraped the floor with the same startling loudness he had noticed before. The bubble was as silent as a tomb. It was not much larger than a tomb; a sphere eighteen feet in diameter, made of thin sheet steel and criss-crossed outside with narrow reinforcing girders to keep the internal air pressure from rupturing it. The floor under him was six feet up from the sphere's bottom and the space beneath held the air regenerator and waste converter units, the storage batteries and the food cabinets. The compartment in which he sat contained chair, table, a narrow cot, banks of dials, a remote-control panel for operating the instruments mounted outside the hull, a microfilm projector, and a pair of exerciser springs attached to one wall. That was all.