"Hark! What's that?" suddenly asked Paul, holding up his hand for silence.
They were made aware of a curious, humming, throbbing sound.
"Some speed boat," ventured Dick.
"None in sight," objected Paul, with a glance up and down the river, which at this point ran in a straight stretch for two miles or more. "You could see a boat if you could hear it as plainly as that."
"It's getting louder," announced Innis.
Indeed the sound was now more plainly to be heard.
Paul gave a quick glance upward.
"Look, fellows!" he exclaimed. "An airship!"
The sound was right over their heads now, and as all three looked up they saw, soaring over them, a large biplane, containing three figures. It was low enough for the forms to be distinguished clearly.
"Some airship!" cried Dick, admiringly.