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 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.