So thereafter the chaser simply follows that surface wave until the submarine comes to the surface, as she must do sooner or later to get her bearings and look about for prey. When she does come up--she goes down for good. The hunt of the chaser has been aided in the past year or so by the depth-bomb, which did not exist in the first two and a half years of war. Equipped with this, she need not necessarily follow a surface wave all day; she simply drops the bomb down through this wave; at least she does under certain conditions. [Illustration: _From a photograph by Brown Brothers_. A SUBMARINE-CHASER.] [Illustration; _From a photograph by Brown Brothers_. A TORPEDO-DESTROYER.] This depth-bomb, by the way, is a wonderful invention, and with its perfection began the great decrease in submarine losses. The bomb is cylindrical and has in the top a well in which is fitted a small propeller. As the water comes in contact with the propeller the sinking motion causes it to revolve. As it revolves it screws down a detonator which comes in contact with the charge at ten, fifteen, twenty, or forty or more feet as designated by the hand of an indicator on the bomb. The hand of this indicator is, of course, set by the officer before the bomb is released either from a gun or from tracks along the deck. Then there have been a number of tricks; some of them Yankee tricks, some of them the creatures of the equally fruitful British tar. One day in the North Sea a British patrol-vessel came across a trawler. It resembled the ordinary British trawler, but there were points of difference, points that interested the inquisitive--and suspicious--commander of the war-vessel. Chiefly there were a lot of stores upon her deck. She flew the Norwegian flag, and her skipper said he was neutral. But the British commander decided to take a chance. He arrested the crew, placed them in irons, and manned the trawler with a crew of French and English navy men. The trawler hovered about in the same locality for three days, and then one morning, lo and behold, a periscope popped up close alongside. Seeing the waters clear of enemy ships, the U-boat came to the surface and frisked blithely up to the trawler. She was greeted by a shower of machine-gun bullets, and surrendered without ado. There was really nothing else for the surprised skipper to do. For when he had last seen that trawler she was the parent ship of the submarine flotilla operating in that vicinity. In all, before the week was over, that trawler had captured six submarines without the loss of a life, and no one injured. Thereafter the parent-ship trawler was seized whenever the British could capture one, and the same expedient was tried. But after a time the Germans became wary of approaching parent-ships until they were convinced that their parenthood was more real than assumed.