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 hydrophone flotilla of armed motor launches, the number of vessels forming the unit was usually five. When out scouting for the enemy they proceeded in line-abreast for about one sea mile, then stopped their engines and listened on their hydrophones for the noise of a submarine cruising in the vicinity. If nothing was heard the mile-long line of miniature warships advanced another mile and again stopped to listen. This manoeuvre was repeated until one or other of the ships heard the familiar sound of a U-boat. Nothing might be visible on the surface of the sea, but if this was the case and the noise came up from the ocean depths over the electrified wires of the detector, it was conclusive proof that a submarine was in the near vicinity. The M.L. first detecting the noise hoisted a signal (flag by day and coloured electric light by night), giving the direction from which the sound came (see Fig. 22). The next ship in the line to receive the sound on its instruments then hoisted a signal, also giving the bearing--_i.e._ N.N.W., E.S.E., etc. If the two coincided in regard to direction, the attack commenced. If, however, they did not agree in this important respect, the line of patrol ships advanced another mile and listened again. The flag-ship of the unit on receiving the direction from one or more ships marked the lines of sound on a chart (as in Fig. 22), and when this was substantiated by another ship the point where the two lines crossed was known to be the position of the hostile submarine, and the attack was ordered. As to the exact method of an anti-submarine attack little need be said here beyond the fact that the ships advanced at full speed, manoeuvring into a special formation which enabled them to cover about half a square mile of sea with the explosive force of their collective depth charges.