ctions. Each square covered a few leagues of sea and was
known by an alphabetical sign. In this way the waters of the submarine
danger zone were divided into areas, with their bases and protective
fleets, and squares with their respective squadrons or ships.

[Illustration: FIG. 20.--Diagram showing division of sea into
anti-submarine patrol areas.]

Each square of sea was covered once or twice daily by its own patrol
ship or flotilla. Where the danger was less the patrol was not so
frequent and the squares were almost indefinite in size, but where the
chances of successful operations were exceptional, as in the Straits of
Dover, additional offensive measures were employed (see under _Mine
Barrages_).

This, then, was the chess-board on which the game of submarine warfare
was played. To facilitate communication between the different patrols
spread over the squares of sea, wireless was fitted in many ships, and
war signal stations were erected on prominent points of land. Each base
was able to communicate by wireless with any of its ships out on patrol
duty, and was also connected by land-line telegraph, telephone and
wireless with _naval centres_.

These latter were head intelligence offices, usually situated at the
great bases of the battle fleets. In this way any concentration of
hostile surface warships noticed by the patrols (sometimes submarines
were employed, especially in the Heligoland Bight) could be communicated
in a few minutes to the admirals commanding the Grand Fleet, the Battle
Cruiser Squadron or other large fighting organisations.

At the naval centres the movements of hostile submarines were recorded
on charts. If, for example, it was reported from a patrol boat that the
U16 had torpedoed a ship in square "C," area 41, at 10 A.M. (G.M.T.[6])
on 4th August, and the patrol had arrived on the scene too late to be of
any service, a warning could be wirelessed to hundreds of vessels on the
seas surrounding the scene of outrage to keep a careful look-out for
the U16.

[Illustration: FIG. 21.--Diagram showing how an area is covered by
patrols. _A_. Unit or flotilla of ships may proceed out from the base on
course indicated by arrows _B_, which would be called the "Northern
Inner Beat," and return to harbour on course _A_, "Northern Outer Beat."
Other units of ships would simultaneously follow the course _E_. These
and adjacent squares of sea would be covered daily by one or more ships
of each unit. The