The United States Navy developed depth-bomb tactics vastly superior to any before in use. Instead of half a dozen bombs, our destroyers carried fifty. The old method of releasing from the stern was superseded by the "Y" gun, which hurled the huge charges with greater accuracy and less risk to the vessel firing. Instead of dropping one or two, the depth-charge barrage was devised, bombs being fired in "patterns" all around the vicinity of the submerged boat, as well as over the spot where it was believed to be. That was one reason the destroyers proved such a terror to the "subs," which, as a rule, on sighting one of these swift warships ducked or ran away. [Illustration: THE NAVAL CONSULTING BOARD AND THE NAVY DEPARTMENT CHIEFS Front row, left to right: Assistant Secretary of the Navy Roosevelt, Hiram Maxim, Thomas A. Edison, Secretary Daniels, Peter Cooper Hewitt, William LeRoy Emmett, Arthur Becket Lamb.] Gunfire, tellingly effective against submarines as long as they were on the surface, was ineffective the moment they submerged, as the ordinary sharp-nose shells were deflected and ricocheted as they struck the water. Our ordnance experts had already devised a non-ricochet shell, a "flat nose" projectile which could be fired with considerable accuracy at a target under water. The first contract for this type of projectile was placed June 19, 1917, and deliveries began the next month. Rapidity in firing was increased by a twin-gun produced for destroyers, two barrels on a single mount, both aimed at one time and firing alternately. [Illustration: SECRETARY DANIELS AND THOMAS A. EDISON Inset, Secretary Daniels and Mr. Edison with Mr. William L. Saunders and Professor Max Mason, inventor of a submarine detection device, at a test experiment at New London.] Thus we had bombs and projectiles and quick-firing guns which would "get" the undersea enemy, once it was located. The paravane, an English invention, proved of great value in protecting ships from mines. Its "wings," spread out in the water, picked up mines; and its wires bore them away from the ships, where they could be exploded without danger to the vessel. Mines played a big part in naval warfare. The Germans sowed the seas with them, and if the Allied mine-sweepers had not been so energetic and skillful, they might have been as destructive to shipping as the U-boats were. Our Bureau of Ordnance led in mine development, and the new mine, called "Mark VI," which it produced in 1917, was decidedly superior to any of its predecessors, and was the type used by us in the North Sea Barrage. Better guns for aeroplanes was a vital need. Machine-guns were made more effective; but for anti-submarine warfare there was needed something of larger caliber, with sufficient power to penetrate the hull plating of the U-boat. An aeroplane "cannon," the Davis non-recoil gun, was produced.