For some time, now, the boys had been dully conscious of a peculiar sound in the distance, just as one sometimes is of the ticking of a clock which he takes no distinct note of. But now this mysterious sound became more pronounced, and forced a recognition. The boys started, glanced at each other, and then each assumed a listening attitude. There was a long silence, profound and unbroken; then a deep, sullen boom came floating down out of the distance.

     "What is it!" exclaimed Joe, under his breath.

     "I wonder," said Tom in a whisper.

     "'Tain't thunder," said Huckleberry, in an awed tone, "becuz thunder--"

     "Hark!" said Tom. "Listen--don't talk."

     They waited a time that seemed an age, and then the same muffled boom troubled the solemn hush.

     "Let's go and see."