Arden jumped. She sailed upward, and as she passed through the opening, Channing caught her by one arm and stopped her flight. "At that speed you'd go right on across," he said. She looked up, and there about two hundred feet overhead she could see the opposite wall. Channing snapped on the lights. They were in a room two hundred feet in diameter and three hundred feet long. "We're at the center of the station," Channing informed her. "Beyond that bulkhead is the air lock. On the other side of the other bulkhead, we have the air plants, the storage spaces, and several cubic inches of machinery." "Inches?" asked Arden. Then she saw that he was fooling. "Come on," he said. He took her by the hand and with a kick he propelled himself along on a long, curving course to the opposite side of the inner cylinder. He gained the opposite bulkhead as well. "Now, that's what I call traveling," said Arden. "But my tummy goes _whoosh, whoosh_, every time we cross the center." Channing operated a heavy door. They went in through rooms full of machinery and into rooms stacked to the center with boxes; stacked from the wall to the center and then packed with springs. Near the axis of the cylinder, things weighed so little that packing was necessary to keep them from bouncing around. "I feel giddy," said Arden. "High in oxygen," said he. "The CO_{2} drops to the bottom, being heavier. Then, too, the air is thinner up here because centrifugal force swings the whole out to the rim. Out there we are so used to 'down' that here, a half mile above--or to the center, rather--we have trouble in saying, technically, what we mean. Watch!" He left Arden standing and walked rapidly around the inside of the cylinder. Soon he was standing on the steel plates directly above her head. She looked up, and shook her head. "I know why," she called, "but it still makes me dizzy. Come down from up there. Or I'll be sick." Channing made a neat dive from his position above her head. He did it merely by jumping upward from his place toward her place, apparently hanging head down from the ceiling. He turned a neat flip-flop in the air and landed easily beside her. Immediately, for both of them, things became right-side-up again. * * * * * Channing opened the door to the room marked: "Air Plant." He stepped in, snapped on the lights, and gasped in amazement. The room was empty! Completely empty! Absolutely, and irrevocably vacant. Oh, there was some dirt on the floor and some trash in the corners, and a trail of scratches on the floor to show that the life-giving air plant had been removed, hunk by hunk, out through another door at the far end of the room. "Whoa, Tillie!" screamed Don. "We've been stabbed! Arden, get on the -type and have ... no, wait a minute until we find out a few more things about this!" They made record time back to the office level. They found Burbank in his office, leaning back, and talking to someone on the phone.