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." "What's the dope?" asked Billy, with great interest. "Suppose he does spot one, what good does it do?" "He's got a wireless equipment," explained Frank, "and he sends out signals to trawlers and destroyers. They come on the jump and the seaplane tells them just where the submarine is lying." "By jiminy, I think he sees one now!" exclaimed Tom Bradford, who had just come up. "See that smudge of smoke over there? That means a steamer's coming and there's another." As though by magic one boat after another hove in sight until there were four, coming from as many points in the compass and heading toward that point in the sea over which the seaplane hovered. The boys were on edge with excitement at the prospect of being in at the death and as the liner was rapidly approaching the scene of action, they had a clear view of what followed. Guided evidently by signals from the seaplane, two of the trawlers stretched a long chain between them and advanced slowly toward the other two who, with a similar chain approached from the other direction. "What do you suppose they're trying to do?" asked Billy, curiously. "The idea is, to get those chains under the submarine," explained Frank. "After they've done that, they'll crisscross them from above. If they once succeed in doing it the sub is done for. He's got to come up and surrender or else they'll slip him a depth bomb and blow him to flinders." Deftly and quickly the work went on under the direction of the skilled veterans who held command of the trawlers. Then they waited for the submarine to come up.