southern half of the area would be patrolled in the
same way. The "Outer Beat" is shown by the arrows _C_, and the "Inner
Beat" by the arrows _D_. The points _+F_ show the possible positions of
armed patrols acting independently of any unit or flotilla.]

Subsequently a further message might come to the naval centre that the
same submarine had been chasing a merchantman in square "D," "E" or "F"
in the adjoining area. A concentration of fast ships, such as
destroyers, M.L.'s or coastal motor boats, could then be made so as to
intercept the raider or enclose her in a circle while other vessels
hunted her down.

In a like manner important convoys coming down the coast, or entering a
danger zone from the open sea, could be met by a local flotilla and
escorted to a _rendezvous_ with a flotilla from the adjoining area. In
this way they were passed through the submarine and mine infested seas
to and from their harbour terminus.

Almost the same methods were employed in dealing with the thousands of
German mines. But to describe that part of anti-submarine warfare here
would be to encroach on the subject of a succeeding chapter.

PATROLS

The _method_ of patrolling the areas and squares of sea was
comparatively simple, though the same cannot be said of the actual work.
The lines of patrol were called "beats," and there was usually an
"inner" and an "outer" beat for each unit or flotilla of ships. If when
a ship (or a unit) reached her allotted square, from which the line of
patrol extended, she elected to proceed on the _inner beat_, she would
generally accomplish the return journey to the point of departure on the
_outer beat_, thus covering her respective zone of patrol, but leaving
the exact route to the discretion of the commanding officer. In this way
no hostile submarine with a knowledge of the system could be sure of
when or where a patrol ship would be met. In the same way it was left to
the commander of a flotilla to either divide his ships into pairs,
single units, or to maintain them as a homogeneous fleet, so that any
combination of hostile submarines could not be made which would be sure
of being able to attack a _single_ patrol. Such an enemy combination
might encounter a single ship, but it might also walk into the arms of a
whole flotilla; or it might attack a single ship only to find itself
surrounded by a following fleet.

[Illustration: FIG. 22.--Diagram illustrating the operations of a
hydrophone