The receiving shield was glowing a trifle. Rays were bombarding it! It glowed, gleamed phosphorescent, and the audible recorder began sounding its tiny tinkling murmurs. Gamma rays! Snap sprang to the dials. The direction and strength were soon obvious. A richly radioactive ore body was concentrated upon this hemisphere of the Moon! It was unmistakable. "He's got it, Gregg! He's--" The tiny grids began quivering. Snap exclaimed triumphantly, "Here he comes! By God, the message at last!" Snap decoded it. _Success! Stop for ore on your return voyage. Will give you our location later. Success beyond wildest hopes._ Snap murmured, "That's all. He's got the ore!" We were sitting in darkness, and abruptly I became aware that across our open window, where the insulation barrage was flung, the air was faintly hissing. An interference there! I saw a tiny swirl of purple sparks. Someone--some hostile ray from the deck beneath us, or from the spider bridge that led to our little room--someone out there was trying to pry in! Snap impulsively reached for the absorbers to let in the outside light. But I checked him. "Wait!" I cut off our barrage, opened our door and stepped to the narrow metal bridge. "You stay there, Snap!" I whispered. Then I added aloud, "Well, Snap, I'm going to bed. Glad you've cleaned up that batch of work." I banged the door upon him. The lacework of metal bridges seemed empty. I gazed up to the dome, and forward and aft. Twenty feet beneath me was the metal roof of the cabin superstructure. Below it, both sides of the deck showed. All patched with moonlight. 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 in 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." I went past him, out the door into the main 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 A20 and A22 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.