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. [Illustration: Courtesy of the "Scientific American" FIG. 21. Chart of an actual pursuit of a U-boat which ended in the destruction of the submarine] [Illustration: 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. [Illustration: (C) Underwood & Underwood A Paravane hauled up with a Shark caught in its jaws]