of invisibility, would be overcome; and, inasmuch as the submarine, except for this quality of invisibility, was a far weaker vessel than any other afloat, the complete elimination of this advantage would dispose of it as a formidable enemy in war.
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 cross-currents 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
