Magnetic, electrical and other apparatus having proved impracticable, attention was concentrated on listening devices. The British had been experimenting with various inventions of this nature, but none had proved very effective. The first successful listening device produced in America was the "C" tube, an application of the binaural principle--that is, hearing through both ears--which was developed by Dr. William D. Coolidge. Next was the "K" tube, developed at Nahant, an adaptation of the rotary compensator devised by Prof. Max Mason at New London, with microphones, enabling the device to be towed several hundred feet astern of the listening vessel. Subsequently the combined work at Nahant and New London resulted in production of the "Y" tube, "Delta," "O S," and "O K" tubes, all modified forms of the "K" tube, for installation on vessels of different types. Submarine chasers were equipped with these tubes, the first of which was developed by August, 1917, and a thorough test was made with American submarines, which were easily located. But much depended on the acuteness of the operator, and a school to train "listeners" was established at New London. Phonograph records of the sound made by various craft were prepared, and used in the school for listeners, who soon became experts in determining direction, distance, type of vessel and speed at which it was moving. "Find the submarine," was the problem when we entered the war, and this was the purpose of the listening devices. Once located, the "sub" could be destroyed or damaged by the depth-bomb. Before its advent there was no way of reaching the U-boat, once it submerged. The story is told that a British vessel chased down a "sub," which dived and remained stationary right under its pursuer. Down below them in the clear water, the Britishers could see the enemy plainly. "If we only had some sort of bomb that we could shoot down into the water, we could blow that Fritzie to Kingdom-come," an officer remarked. The general idea of the depth-bomb had long been known, and was then given its practical application. The first ones, designed by an officer in the Admiralty, were crude affairs, metal cylinders like ash-cans. They were, at first, not very reliable, but by development they became the most effective weapons used against under-water craft.