When the lad uttered his signal, he was startled by the result. A hundred
echoes were awakened within the cavern, and the uproar fairly deafened
him. It seemed to him that ten thousand little imps were perched all
around the cavern, with their fingers thrust in their mouths, waiting for
him to start the tumult, when they joined in, with an effect that was
overwhelming and overpowering.

"Good gracious!" he gasped, "I never heard anything like that. I thought
all the rocks were going to tumble down upon my head, and I believe some
must have been loosened."

He looked apprehensively at the dark, jagged points overhead. But they
were as grim and motionless as they had been during the many long years
that had rolled over them.

"Mickey must have heard that, if he is anywhere within twenty miles," he
concluded.

But, if such was the case, he sent back no answering signal, as was his
invariable custom, when that of his friend reached him. Fred listened long
and attentively, but caught no reply.

"I guess I'll have to try it again," he added, with a mingled laugh and
shudder. "I think these walls can stand a little more such serenading."

He threw his whole soul in the effort, and the screeching whistle that he
sent out was frightful, followed, as it was, by the innumerable echoes. It
seemed as if the walls took up the wave of sound as if it were a foot-ball
and hurled it back and forth, from side to side, and up and down, in