He picked the strings for a moment and then began to sing softly—
"Swing low, sweet chariot
Gwine for to ca'y me home
Swing low, sweet chario—ot
Gwine for to ca'y me home."
Bang! There was a tremendous explosion close to the mill. The air was filled with a deafening din.
The boys jumped to their feet.
"That hit mighty close!" cried Frank.
"What do you suppose it was?" came from Tom. "A shell?"
"We're too far away from the German lines for that," replied Bart.
"More likely it's a bomb from an airship," said Frank. "Let's take a squint outside and see."
They rushed out and their first glance was toward the sky. But there was nothing visible there, nor could they hear the whirring of motors that was the invariable accompaniment of air raids.
But when they searched around the mill they were more successful, for the bright moonlight revealed a freshly dug hole in the ground that formed a veritable crater.