Much research and experimental work was carried out during the year 1917 under the encouragement and supervision of the Anti-Submarine Division of the Naval Staff. Two hydrophones were invented in the early part of 1917, one by Captain Ryan, R.N., and one by the Board of Invention and Research, which could be used from ships at very slow speed and which gave some indication of the _direction_ of the sound; finally, in the summer of 1917, the ability and patience of one inventor, Mr. Nash, were rewarded, and an instrument was devised termed the "fish" hydrophone which to a considerable extent fulfilled the required conditions. Mr. Nash, whose invention had been considered but not adopted by the Board of Invention and Research before he brought it to the Anti-Submarine Division of the Naval Staff, laboured under many difficulties with the greatest energy and perseverance; various modifications in the design were effected until, in October, 1917, the instrument was pronounced satisfactory and supplies were put in hand. The next step was to fit the "fish" hydrophone in certain auxiliary patrol vessels as well as some destroyers, "P" boats and motor launches, to enter and train men to work it, and finally to organize these vessels into "submarine hunting flotillas," drill them, and then set them to their task. This work, which occupied some time, was carried out at Portland, where a regular establishment was set up for developing the "fish" hydrophone and for organizing and training the "hunting flotillas" in its use. A considerable amount of training in the use of the hydrophone was required before men became efficient, and only those with a very keen sense of hearing were suited to the work. The chances of the success of the hunting flotillas had been promising in the early experiments, and the fitting out of patrol craft and organizing and drilling them, proceeded as rapidly as the vessels could be obtained, but largely owing to the slow production of trawlers it was not until November that the first hunting flotilla fitted with the "fish" hydrophone was actually at work. The progress made after this date is illustrated by the fact that in December, 1917, a division of drifters, with a "P" boat, fitted with this "fish" hydrophone hunted an enemy submarine for seven hours during darkness, covering a distance of fifty miles, kept touch with her by sound throughout this period, and finished by dropping depth charges in apparently the correct position, since a strong smell of oil fuel resulted and nothing further could be heard of the submarine, although the drifters listened for several hours. On another occasion in the same month a division of drifters hunted a submarine for five hours. The number of hydrophones was increased as rapidly as possible until by the end of the year the system was in full operation within a limited area, and only required expansion to work, as was intended, on a large scale in the North Sea and the English Channel.