It would have been, by Earth-routine, perhaps eleven p.m. The passengers were retiring to their sleeping cubbies. The decks now were almost deserted. I went up one of the side ladders to the superstructure roof. It glittered with starlight that came down through the glassite pressure dome which arched close overhead here. The superstructure roof was a rectangular deck space, a hundred feet long perhaps, by thirty wide. A low railing surrounded it at which one might look down upon the lower side decks. Chairs were scattered about up here, all of them unoccupied. Amidships was the little kiosk which housed the radio-helio equipment, with young Smith's sleeping cubby adjacent to it. The place was closed and locked now. Aft of it there was open deck space to where the roof-deck ended, with the stern deck-triangle a level below, where the earth-light still was red like blood. I turned forward. The chart room backed against the control turret. The chart room was dark. In the control turret I could see Capt. Mackensie at the controls, his squat, square figure etched by moonlight. Would this be a good time to try and talk with him? I started forward. The party down on the forward deck was just dispersing; I saw the boyish figure of Nina, starting for the superstructure corridor, with the giant handsome William Wilson escorting her. And then Dr. Frye came up the front ladder, went into the control turret and joined Mackensie. I turned aft; it was no time to see Mackensie now. * * * * * Suddenly I stopped, melted down into a black shadow near the helio kiosk and flattened myself on the deck. An S.S. Man, so they say, developes a sixth sense. Maybe so. Certainly I didn't see anything, nor hear anything. But I was aware that someone, or something, was up here on the silent deck with me. Perhaps it was a sense of smell; my nostrils dilated with the impression that the faint drift of artificial air up here had somehow changed its quality. Was there something artificially invisible stalking here? The Groff magnetic cloaks, so recently perfected, are closely held by Governmental orders. Even we S.S. Men seldom use them. But it is a queer thing--no matter what devices you use in crime-tracking, you may be pretty sure the criminal has them to use against you. I tried my infra-red glasses. They disclosed nothing save the glowing heat of the ventilators where the warm air was coming up. The nearest I had to an eavesdropper was a pair of low-scale phones. In a second or two I adjusted them; tuned them. The myriad blended tiny sounds of the ship's interior gave me nothing that I could identify. And then it seemed that there was a very faint hissing--something, quite near me, which should not have been here.