ydrophone flotilla of armed
motor launches, the number of vessels forming the unit was usually five.
When out scouting for the enemy they proceeded in line-abreast for about
one sea mile, then stopped their engines and listened on their
hydrophones for the noise of a submarine cruising in the vicinity. If
nothing was heard the mile-long line of miniature warships advanced
another mile and again stopped to listen. This manoeuvre was repeated
until one or other of the ships heard the familiar sound of a U-boat.
Nothing might be visible on the surface of the sea, but if this was the
case and the noise came up from the ocean depths over the electrified
wires of the detector, it was conclusive proof that a submarine was in
the near vicinity.

The M.L. first detecting the noise hoisted a signal (flag by day and
coloured electric light by night), giving the direction from which the
sound came (see Fig. 22). The next ship in the line to receive the
sound on its instruments then hoisted a signal, also giving the
bearing--_i.e._ N.N.W., E.S.E., etc. If the two coincided in regard to
direction, the attack commenced. If, however, they did not agree in this
important respect, the line of patrol ships advanced another mile and
listened again.

The flag-ship of the unit on receiving the direction from one or more
ships marked the lines of sound on a chart (as in Fig. 22), and when
this was substantiated by another ship the point where the two lines
crossed was known to be the position of the hostile submarine, and the
attack was ordered.

As to the exact method of an anti-submarine attack little need be said
here beyond the fact that the ships advanced at full speed,
manoeuvring into a special formation which enabled them to cover about
half a square mile of sea with the explosive force of their collective
depth charges.

When the attack had been completed all vessels engaged resumed their
stations and waited with quick-firing guns ready in case the monster
should rise from the deep to make a dying effort to destroy her
pursuers.

The tactical methods of anti-submarine attack were, of course, numerous,
and they varied according to the speed of the surface ships engaged.
What was possible of accomplishment by fast-moving coastal motor boats
or the larger-sized M.L.'s proved impracticable for the more heavily
armed but slow-moving trawlers and drifters. The tactics of these
latter craft were often of the simplest character, and c