flotilla composed of armed motor launches. Each vessel is
given a number, and the flotilla proceeds in line-abreast along the
course shown by the dotted lines. Each vessel is one mile from the
other, and the whole line stops by signal at the point marked with a
cross. Hydrophones are put in operation, and after a period of listening
the flotilla continues on its course, as no submarine sounds are heard.
The flotilla turns to head south, and a stop is again made to listen on
the hydrophones. This time the sound of a hostile submarine is heard by
vessel No. 1, bearing S.W. This report is confirmed by vessel No. 2
hearing the same sound, bearing a few degrees farther W. The two
bearings _A_ and _B_ are then drawn on a chart, and the point where the
two lines cross is the approximate position of the invisible submarine.
The attack with depth charges is then ordered.]

The beats which were most distant from the base were given to the
largest ships. This was done because it was often impossible for the
more distant patrols to reach a place of shelter before one of the
fierce gales which swept the northern seas was upon them. Trawlers,
large steam yachts and converted merchantmen were usually employed on
squares more than one hundred miles distant from a harbour of refuge,
while motor launches kept watch and ward on the seas closer inshore.

The duration of patrols varied according to their position. Some lasted
three weeks and others only a few days or hours. When the ships returned
to their base after a spell at sea they were given a corresponding
"rest" in harbour. A three weeks' patrol meant several days'
"stand-off," while a two or three days' patrol entitled the ship to
twenty-four hours in the comparative comfort of a harbour.

It must not be imagined, however, that a stand-off meant entire idleness
or thorough rest. There were duties to perform which robbed it of much
that it was intended to give. Ships had to be coaled, provisioned,
painted or repaired. Engines had to be overhauled, sentries posted
ashore, a guard to be furnished, and every day one ship in each unit
that was in harbour had to be manned and in readiness for emergencies.

HYDROPHONE FLOTILLAS

We now come to the actual methods employed by surface craft when
attacking submarines. Although, as previously stated, much was left to
individual initiative, there were, nevertheless, certain recognised
methods.

Taking as an example the operations of a h