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 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