A fact that held forth hopes of success was that water is an excellent conductor of sound--far better than the atmosphere itself. In the air there are many crosscurrents and areas of varying temperature which make sound waves frequently behave in most puzzling fashion, sometimes travelling in circles, sometimes moving capriciously up or down or even turning sharp corners. The mariner has learned how deceptive is a foghorn; when it is blowing he knows that a ship is somewhere in the general region, but usually he has no definite idea where. The water, however, is uniform in density and practically uniform in temperature, and therefore sound in this medium always travels in straight lines. It also travels more rapidly in water than in the air, it travels farther, and the sound waves are more distinct. American inventors have been the pioneers in making practical use of this well-known principle. Before the war its most valuable applications were the submarine bell and the vibrator. On many Atlantic and Pacific points these instruments had been placed under the water, provided with mechanisms which caused them to sound at regular intervals; an ingenious invention, installed aboard ships, made it possible for trained listeners to pick up these noises, and so fix positions, long before lighthouses or lightships came into view in any but entirely clear weather. For several years the great trans-Atlantic liners have frequently made Nantucket Lightship by listening for its submarine bell. From the United States this system was rapidly extending all over the world.
American inventors were therefore well qualified to deal with this problem of communicating by sound under the water. A listening device placed on board ship, which would reveal to practised ears the noise of a submarine at a reasonable distance, and which would at the same time give its direction, would come near to solving the most serious problem presented by the German tactics. Even before the United States entered the war, American specialists had started work on their own initiative. In particular the General Electric Company, the Western Electric Company, and the Submarine Signal Company had taken up the matter at their own expense; each had a research department and an experimental station where a large amount of preliminary work had been done. Soon a special board was created at Washington to study detection devices, to which each of these companies was invited to send a representative; the board eventually took up its headquarters at New London, and was assisted in this work by some of the leading physicists of our universities. All through the summer and autumn of 1917 these men kept industriously at their task; to such good purpose did they labour that by October of that year several devices had been invented which seemed to promise satisfactory results. In beginning their labours they had one great advantage: European scientists had already made considerable progress in this work, and the results of their studies were at once placed at our disposal by the Allied Admiralties. Moreover, these Admiralties sent over several of their experts to co-operate with us. About that time Captain Richard H. Leigh, U.S.N., who had been assigned to command the subchaser detachments abroad, was sent to Europe to confer with the Allied Admiralties, and to test, in actual operations against submarines, the detection devices which had been developed at the New London station. Captain Leigh, who after the armistice became my chief-of-staff at London, was not only one of our ablest officers, but he had long been interested in detection devices, and was a great believer in their possibilities.
