"Yes," said Bart. "Then the Germans made everything tight and went below, leaving their prisoners on the deck. The U-boat sailed along the surface for a few hours and then slowly sank leaving their captives to drown. If that wasn't brutal, cold-blooded murder, there never was any in the history of the world."
"I hope this submarine was the one that did the trick," said Frank. "Perhaps drowning didn't seem such a rich joke to them when their turn came."
From that time on, the vigilance aboard ship was redoubled, for although the general opinion was that it was only a chance meeting, no one knew but what this U-boat was simply one of a fleet whose companions might look for better luck where their comrade had failed.
But nothing more was seen of the undersea terror until they were approaching the French coast and then the boys were witnesses of an exciting game that held them breathless.
"Look at that speck up there in the sky," exclaimed Frank.
"Biggest bird I ever saw," remarked Billy.
"That's no bird," declared Bart, after a prolonged inspection through a pair of glasses that he produced from his kit. "That's an aeroplane."
"An aeroplane!" exclaimed Billy. "So far away from shore as this? You're dreaming."
"You can see for yourselves," replied Bart, as he handed the glasses around. "Take a squint at it and you'll see that that bird never wore feathers."
"It must be a seaplane," announced Frank. "It's been launched from the deck of some vessel and now it's hovering up there like a hawk, looking for submarines. It's a funny thing, but they say that those seaplane pilots can look right down through the water and see a submarine when it can't be seen from the deck of a ship."