No one visible down there. I descended a ladder. The deck was empty. But in the silence something was moving! Footsteps moving away from me down the deck! I followed; and suddenly I was running. Chasing something I could hear, but could not see. It turned into the smoking room. I burst in. And a real sound smothered the phantom. Johnson the purser was sitting here alone in the dimness. He was smoking. I noticed that his cigar held a long, frail ash. It could not have been him I was chasing. He was sitting there quite calmly. A thick-necked, heavy fellow, easily out of breath. But he was breathing calmly now. He sat up with amazement at my wild-eyed appearance, and the ash jarred from his cigar. "Gregg! What in the devil--" I tried to grin. "I'm on my way to bed--worked all night helping Snap with those damn Earth messages." I went past him, out the door into the main interior corridor. It was the only way the invisible prowler could have gone. But I was too late now--I could hear nothing. I dashed forward into the main lounge. It was empty, dim and silent, a silence broken presently by a faint click--a stateroom door hastily closing. I swung and found myself in a tiny transverse passage. The twin doors of A 22 and A 20 were before me. The invisible eavesdropper had gone into one of these rooms! I listened at each of the panels, but there was only silence within. The interior of the ship was suddenly singing with the steward's siren--the call to awaken the passengers. It startled me. I moved swiftly away. But as the siren shut off, in the silence I heard a soft, musical voice: "Wake up, Anita--I think that's the breakfast call." And her answer: "All right, George. I hear it." CHAPTER IV _A Burn on a Martian Arm_ I did not appear at that morning meal. I was exhausted and drugged with lack of sleep. I had a moment with Snap, to tell him what had occurred. Then I sought out Carter. He had his little chart-room insulated. And we were cautious. I told him what Snap and I had learned: the Gamma rays from the moon, proving that Grantline had concentrated a considerable ore-body. I also told him the message from Grantline. "We'll stop on the way back, as he directs, Gregg." He bent closer to me. "At Ferrok-Shahn I'm going to bring back a cordon of Interplanetary Police. The secret will be out, of course, when once we stop at the moon. We have no right, even now, to be flying this vessel as unguarded as it is." He was very solemn. And he was grim when I told him of the invisible eavesdropper. "You think he overheard Grantline's message?" "I don't know," I said. "Who was it? You seem to feel it was George Prince?" "Yes." I was convinced that the prowler had gone into A 20. When I mentioned the purser, who seemed to have been watching me earlier in the night, and again was sitting in the smoking room when the eavesdropper fled past, Carter looked startled. "Johnson is all right, Gregg."