The boys looked at each other in alarm. Will sprang to his feet.
“That sound!” he cried. “I heard it one night when I was camping in the woods.”
“Where?” asked Jack.
“About ten miles from here. It’s the same noise.”
“We must solve this mystery!” exclaimed Jack. “I believe it must be——”
But he did not finish the sentence, for from the air above them sounded the call of a voice:
“To the left! To the left!” was the cry. “There’s the camp fire we saw before.”
They all sprang to their feet and looked up into the dark sky. Surely that was some vast shape hovering over them! And then the throbbing and the rush of wings died away.