to the deck. For a moment it
struggled furiously, as if an invisible claw had pinned it down. Then
it escaped, and fled whimpering to Thad's side.

He saw a new wound across its hips. Three long, parallel scratches,
from which fresh red blood was trickling.

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 The monster had withdrawn, at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.

Human vision, as he knew, utilizes only a tiny fraction of the
spectrum. The creature must be largely transparent to visible light,
as human