Regular scraping sounds came from the end of the deck, where no moving thing was to be seen—sounds such as might be made by the walking of feet with unsheathed claws. Something was coming back toward Thad. Something that was invisible!
Terror seized him, with the knowledge. He had nerved himself to face desperate men, or a savage animal. But an invisible being, that could creep upon him and strike unseen! It was incredible ... yet he had seen the dog knocked down, and the bleeding wound it had received.
His heart paused, then beat very quickly. For the moment he thought only blindly, of escape. He knew only an overpowering desire to hide, to conceal himself from the invisible thing. Had it been possible, he might have tried to leave the flier.
Beside him was one of the companionways amidships, giving access to a compartment of the vessel that he had not explored. He turned, leaped down the steps, with the terrified dog at his heels.
Below, he found himself in a short hall, dimly lighted. Several metal doors opened from it. He tried one at random. It gave. He sprang through, let the dog follow, closed and locked it.
Trying to listen, he leaned weakly against the door. The rushing of his breath, swift and regular. The loud hammer of his thudding heart. The dog's low whines. Then—unmistakable scraping sounds, outside.
The scratching of claws, Thad knew. Invisible claws!
He stood there, bracing the door with the weight of his body, holding the welding arc ready in his hand. Several times the hinges creaked, and he felt a heavy pressure against the panels. But at last the scratching sounds ceased. He relaxed. The monster had withdrawn, at least for a time.
When he had time to think, the invisibility of the thing was not so incredible. The mounted creatures he had seen in the hold were evidence that the flier had visited some unknown planet, where weird life reigned. It was not beyond reason that such a planet should be inhabited by beings invisible to human sight.