The troubles of the submarine-commander were multiplying. All over the world inventors were plotting his destruction. As long as we depended upon our eyes to ferret him out, the sea was a safe refuge, provided he dived deep enough, but when we began to use our ears as well, he found himself in a very serious predicament. Although light is badly broken up in its passage through water, sound-waves will travel through water much better than in air. The first listening-devices used were crude affairs and did not amount to much, particularly when the U-boats muffled their motors and engines so that they were virtually noiseless. But the French invented a very sensitive sound-detector. It consisted of a lot of tiny diaphragms set in a big hemisphere. There were two of these hemispheres, one at each side of the boat. When sound-waves struck these hemispheres, the diaphragms would respond. At the focus of each hemisphere there was a megaphone receiver; one of these carried the sound to the operator's right ear and the other to his left. He would turn a megaphone around until he found the diaphragm that produced the loudest sound. This gave him the direction of the sound-wave. Then the boat would be steered in that direction. He knew that it was aimed properly when the sound coming to his right ear was just as loud as that which came into his left ear.
A still better hydrophone was developed by a group of American inventors. The details of this cannot yet be disclosed, but we know that it was adopted at once by our allies. A very sensitive receiver was used which could detect a U-boat miles away and determine its direction accurately. Under ideal conditions the range of the device was from fifteen to twenty-five miles, but the average was from three to eight miles. If two or more boats fitted with sound-detectors were used, they could determine the position of the U-boat perfectly. One drawback was that the vessel would have to stop so that the noise of its own engines would not disturb the listener, but this was largely overcome by trailing the detector a hundred feet or more from the stem of the ship. The sounds were then brought in by an electric cable to the listener in the ship.
These sound-detectors were placed on Allied submarines as well as surface vessels and they were actually tried out on balloons and dirigibles, so that they could follow a U-boat after it had submerged too deeply to be followed by sight.
Courtesy of the "Scientific American"
Fig. 21. Chart of an actual pursuit of a U-boat which ended in the destruction of the submarine
Section of a captured Mine-laying U-boat, showing how the mines were laid
Many U-boats were chased to their doom by the aid of the American hydrophone. Fig. 21 illustrates a very dramatic chase. The full line shows the course of the U-boat as plotted out by hydrophones and the broken line the course of the submarine-chasers. The dots represent patterns of depth bombs dropped upon the U-boat. Try as he would, the Herr Kommandant could not shake off his pursuers. At one time, as the listeners stopped to take observations, they heard hammering in the U-boat as if repairs were being made. The motors of the submarine would start and stop, showing clearly that it was disabled. More depth bombs were dropped and then there was perfect silence, which was soon broken by twenty-five revolver-shots. Evidently the crew, unable to come to the surface, had given up in despair and committed suicide.
