A German submarine heard at a range of about a mile on a calm night presents a curious sound which almost defies description. Its principal constituent consists of a "clankety clank! clankety clank!" at first barely distinguishable from the low swish of the water past the face of the submerged microphone, then louder when the sound has been distinguished and the human ear is on the alert. But when this sound was heard in war there was little time for analysing or noting. It was the call to action. The microphone was hauled to the surface and the chase began, a halt being made every half-mile or so for a further period of listening on the hydrophone. If the sound was louder the commander of the pursuing vessel knew that he was on the right track, and if the sound came up from the sea more indistinct the course was changed and a run of a mile made in the opposite direction, when the vessel was again stopped and the instrument dropped overboard. Should this manoeuvre have placed the surface ship in close proximity to the submarine, one or more _depth charges_ were released, and if the explosion of these damaged the comparatively delicate hull or machinery of the under-water craft, she had either to rise to the surface and fight for her life with her two powerful deck guns, or, if badly damaged, sink helplessly to the bottom, emitting oil in large quantities from her crushed tanks. Before entering upon a description of the depth charge, however, there is more to say of the hydrophone, which has played such an important part in the defeat of the U-boats. When the advantages of this instrument had been fully demonstrated in the stern trial of war, successful efforts were made to improve upon the original crude appliances. The "water noises" were reduced and, greatest improvement of all, the hydrophone was made "directional." By this is meant that when a sound was heard its approximate direction north, south, east or west of the listening ship could be more or less accurately determined. What this improvement meant to a vessel hunting a submarine in a vast stretch of sea will be easily realised. When the sound came up the wires from the submerged microphone the operator had simply to turn a small handle in order to determine from which direction the noise was coming. If, for example, the sound was first heard away to the east, the instrument was turned to another quarter of the compass. Then, if the noise was plainer, the instrument was turned again until the sound decreased in intensity. In this way the line of maximum sound was obtained, and this showed the direction from the listening ship in which the U-boat was operating. [Illustration: FIG. 8.--Plan showing how microphones or ears _B_ are fitted in a submarine _A_ to enable it to detect the approach of surface craft.]