He was using them, as a dog would have done, to scent any signs of alarm on the part of the human quarry that he was after.
At last Harry Hazelton turned sharply, for his own ears were attuned to the stillnesses of the western forests and his hearing was unusually acute.
He had just heard a sound on the wall, not far away.

Instantly the young engineer was on the alert.
Then his eyes, piercing the darkness, made out the crawling, dark form, which did not appear to be more than fifty feet away from him.
For a second or two Harry stared.

But he knew there could be no snake as broad as this crawling figure appeared to be.
"Who's there ?" Hazelton called quickly.
The writhing mass became still, flattening itself against the bed of rock.
Hazelton was not to be deceived, however.
"Who's there ?" Harry repeated.

"You had better talk up, my man!" Still no sound.

Harry started forward to investigate.