He was laughing as he picked himself up. But it was a brief laugh. We knew what had happened: the artificial gravity controls in the base of the ship, which by magnetic force gave us normality aboard, were being tampered with! For just this instant, this particular small section of this corridor had been cut off. The slight bulk of the _Planetara_, floating in space, had no appreciable gravity pull on Snap's body, and the impulse of his step as he came to the unmagnetized area of the corridor had thrown him to the ceiling. The area was normal now. Snap and I tested it gingerly. He gripped me. "That never went wrong by accident, Gregg! Someone--" We rushed to the nearest descending ladder. In the deserted lower room the bank of dials stood neglected. A score of dials and switches were here, governing the magnetism of different areas of the ship. There should have been a night operator, but he was gone. Than we saw him lying nearby, sprawled, face down on the floor! In the silence and dim, lurid glow of the fluorescent tubes, we stood holding our breaths, peering and listening. No one here. The guard was not dead. He lay unconscious from a blow on the head. A brawny fellow. We had him revived in a few moments. A broadcast flash of the call buzz brought Dr. Frank from the chart room. "What's the matter?" "Someone was here," I said hastily, "experimenting with the magnetic switches. Evidently unfamiliar with them--pulling one or another to test their workings and so see their reactions on the dials." We told him what had happened to Snap in the corridor; the guard here was no worse off for the episode, save a lump on the head by an invisible assailant. We left him nursing his head, sitting belligerent at his post, alert to any danger and armed now with my heat-ray cylinder. "Strange doings this voyage," he told us. "All the crew knows it. I'll stick it out now, but when we get back home I'm done with this star travelin'. I belong on the sea anyway." We hurried back to the upper level. We would indeed have to plan something at this chart room conference. This was the first tangible attack our adversaries had made. We were on the passenger deck headed for the chart room when all three of us stopped short, frozen with horror. Through the silent passenger quarters a scream rang out! A girl's shuddering, gasping scream. Terror in it. Horror. Or a scream of agony. In the silence of the dully vibrating ship it was utterly horrible.... It lasted an instant--a single long scream; then was abruptly stilled. And with blood pounding my temples and rushing like ice through my veins, I recognized it. Anita! IX "Good God, what was that?" Dr. Frank's face had gone white. Snap stood like a statue of horror.