It did not seem possible that there could be a living soul behind those dark, silent walls; but it had looked that way before, and the opening door had revealed a bright glow of cheerful comfort. Consequently the two hastened confidently to the entrance and Dick knocked loudly on the steel door.
The sound reverberated in a hollow manner which seemed loud enough to wake the dead, and they waited expectantly for a response. But none came. Their keen ears could detect no sound of footsteps within; the massive door remained closed.
After five minutes of patient waiting, Dick was raising his hand to knock again when Buckhart gave a sudden exclamation.
“By George, pard! I’ll bet we can knock here all night without his coming. Don’t you remember what he said about shutting himself in after we were gone, and paying no attention to anybody or anything?”
“Yes, I remember that, all right,” Dick answered; “but I thought that, coming so soon after our departure, he would guess who it was and come down to——”
He broke off abruptly and looked swiftly upward.
“Listen!” he exclaimed in a low voice.
In the silence which followed there came faintly to their straining ears an odd, muffled humming. For a moment they both thought it was one of the pieces of machinery in Randolph’s laboratory, but very soon they reached the conclusion that it was much farther away than that. It seemed to come, in fact, from high up among the cliffs which towered above the house.
Dick looked at his friend significantly.
“It’s a gasoline engine,” he whispered.