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The _Hydrophone_, for use in locating submerged submarines, although
first evolved in 1915, was in its infancy, so far as supply to ships was
concerned, at the commencement of 1917. Experiments were being carried
out by the Board of Invention and Research at Harwich, and by Captain
Ryan, R.N., at Hawkcraig, and although very useful results had been
obtained and a considerable number of shore stations as well as some
patrol vessels had been fitted with hydrophones, which had a listening
range of one or two miles, all the devices for use afloat suffered from
the disadvantage that it was not possible to use them whilst the ship
carrying them was moving, since the noise of the vessel's own machinery
and of the water passing along the side prevented the noise made by
other vessels being located. What was required was a listening
instrument that could be used by a ship moving at least at slow speed,
otherwise the ship carrying the hydrophone was herself, when stopped, an
easy target for the submarine's torpedo. It was also essential, before
an attack could be delivered, to be able to locate the _direction_ of
the enemy submarine, and prior to 1917 all that these instruments showed
was the presence of a submarine somewhere in the vicinity.

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