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.]

With the perfection of this invention the hydrophone section of the
naval service came into being. Special courses in the detection of
submarine sounds were instituted for officers and also for seamen
listeners. The actual movements of a submarine under water at varying
distances from a hydrophone were recorded by a phonograph, and records
made so that the sounds might be reproduced at will for the education of
the ear. Surgeons with aural experience estimated the physical
efficiency in this respect of would-be volunteers for the
hydrophone-listening service, and vessels were formed into special
hydrophone flotillas, whose duties consisted of listening in long lines
for submarines and when a discovery was made attacking them in the most
approved tactical formation, with the aid of depth charges and guns.

A considerable measure of success attended these arrangements, and the
author spent many cold hours listening at night for the sound of the
wily submarine. On more than one occasion an exciting chase resulted.

It must, however, be pointed out that there is one great drawback to the
successful use of the hydrophone. It exists in the necessity for the
listening ship to stop before the hydrophone is hoisted outboard, it
being quite impossible to hear anything beyond the roar of the engines
of the carrying ship so long as they are in motion. Furthermore, all
progress through the water must have ceased and the listening ship have
become stationary before artificial sounds, such