But the lights did not show again, and, after waiting for an hour or more, the boys started back to the camp. Half way down, the dull, reverberating boom of a cannon came to their ears, over the water.
"What does that mean?" asked Pat.
"It may be the gunboat Jimmie insisted would be sent for me," smiled Ned.
"You don't really think that?"
"Hardly," was the reply, "but I don't know what to make of it."
"Perhaps it was a command for the other ships to show their lights," Pat suggested.
"I hope Uncle Sam is becoming wise to the game that is being played down here," Ned said, "and has sent a gunboat to look into it."
"That's it!" cried Pat. "That's just it! If she doesn't pass the ships in the dark there'll be something doing here."
The dull boom of the gun came again, and, far out, the low lights of the gunboat showed above the water. She seemed to be passing swiftly to the north.
"She's going to pass us, all right!" Pat cried. "Now, what did she make that noise for? To warn the ships that she was coming, and to get out of the way?"