Buckhart nodded silently. It certainly sounded very much like one.
“What the mischief is it doing up there on the mountain?” he asked presently.
There was no chance for Merriwell to reply. The humming increased as though the engine was speeding up, followed by a strange rustling, creaking noise unlike anything they had ever heard. Suddenly before their astonished eyes, a vast, black, shadowy shape rose slowly from the cliffs and hovered an instant in the air high above them. There was a majestic sweep of great wings, as it made a wide, half circle; then it shot northward into the darkness, gathering momentum at every instant, and a moment later the muffled hum of the engine died away in the distance.
“Thundering coyotes! What was that?” the Texan exclaimed, when he had recovered from his surprise.
“An aëroplane, I should say,” Dick returned quietly, though his voice quivered with suppressed excitement.
This new development added tremendously to the mystery with which the personality of Scott Randolph was surrounded, for it must belong to him. There could be no question of that. But why had he not spoken of it? What was it doing up on the cliffs? Above all, what did this silent, stealthy flight through the darkness mean?
“What in time is it doing up there?” Brad questioned.
“I haven’t an idea. I suppose it belongs to Randolph and that he keeps it up on the cliffs somewhere.”
Silently they turned and began to retrace their steps.
“Say, partner, mebbe that’s what he’s experimenting on,” the Texan remarked presently.