I was part way along the corridor when without warning my body rose in the air. Like a balloon I went to the low vaulted ceiling, struck it gently, rebounded, and floated diagonally back to the floor, where I landed in a heap! Heaven knows, it was startling. For those seconds I had been weightless, the impulse of my last step wafting me up, and my thud against the ceiling knocking me back again. The weird loss of weight was gone at once; I was close to the floor when I felt myself drop down to it. And I scrambled to my feet. My heart was thumping; I knew what had happened. In the base of the ship, artificial gravity controls gave us Earth's normal gravity on board. Without them, the slight mass of the _X-87_ would give a gravity pull so negligible that everything in its interior would be almost without weight. Len Smith, the young helio operator, had taken me around the little vessel just before the voyage began, explaining me its mechanisms. I remembered the room of magnetic controls, where the _X-87's_ artificial gravity was regulated. A young technician named Bentley had been there. I had spoken to him a moment. He and his partner alternated on duty there throughout all the voyage. And the artificial gravity controls now were being tampered with! For just a second or two, this particular area of the corridor here had been cut off, so that as I came to the de-magnetized area my step had tossed me to the ceiling. The floor section was normal now; I stepped out on it gingerly to test it. Why was Bentley experimenting with his controls? Surely that never went wrong by accident. If I could catch Bentley at it--force him to explain--Or was it someone else tampering with the complex gravitational mechanisms down there?... I remembered the location of the little magnetic control room; rushed to the nearest descending ladder. The lower level, down in the hull, was a metal catwalk, with side aisles leading into suspended tiny rooms. Freight storage compartments bow and stern; air renewal systems; pressure mechanisms; heating and ventilating systems. Beneath me, at the bottom of the hull, were the rooms of gravity plate-shifting mechanisms--compressed air shifters of the huge hull gravity plates by which the course of the ship through Space was controlled. I was not concerned with them--merely with the magnetic artificial gravity of the vessel's interior. The little magnet room was near it hand; its door was open, with its blue tubelight streaming out.