This illustration applies to the predicament in which the Allied navies now found themselves. When they attempted to fight the submarine they discovered that they had gone hopelessly blind. Like the sightless man, however, they still had other senses left; and it remained for them to develop these to take the place of the one of which they had been deprived. The faculty which it seemed most likely that they could increase by stimulation was that of hearing. Our men could not detect the presence of the submarine with their eyes; could they not do so with their ears? Their enemy could make himself unseen at will, but he could not make himself unheard, except by stopping his motors. In fact, when the submarine was under water the vibrations, due to the peculiar shape of its propellers and hull, and to its electric motors, produced sound waves that resembled nothing else in art or nature. It now clearly became the business of naval science to take advantage of this phenomenon to track the submarine after it had submerged. Once this feat had been accomplished, the only advantage which the under-water boat possessed over other warcraft, that 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 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.