Toot! toot! sounded the boat's pneumatic whistle.
"Foreman Corbett is signaling to us to wait and he'll put in for us," said Tom, coming to a halt. Soon the motor craft chugged in alongside, coming close to the wall. Tom, Harry and Mr. Prenter jumped, landing safely aboard.
"How did the enemy come to catch you napping, Corbett?" Tom inquired good-humoredly.
"They didn't catch me napping, sir," protested Foreman Corbett. "It is the strangest thing, sir—-that explosion. Why, I had had my light turned on that very part of the wall at least a dozen times in the last half-hour before the blow-out came. Our light didn't pick up a soul around there at any time. What do you suppose I did, Mr. Reade, as soon as the explosion sounded?"
"I saw you turn about and use your search light a lot," Reade answered.
"Did you notice, sir, that I turned the light right up at the sky, first-off?"
"I believe I did notice that," Tom assented.
"It seemed to me, sir, that nothing but an airship could plant a charge of high explosive on the wall in that fashion."
"I don't believe the airship theory will explain it either," said Tom, shaking his head.
"Then what theory can explain it?" asked Mr. Prenter, anxiously.