The duties of our destroyers, in these earliest days, were to hunt for submarines, to escort single ships, to pick up survivors in boats, and to go to the rescue of ships that were being attacked. For the purpose of patrol the sea was divided into areas thirty miles square; and to each of these one destroyer, sloop, or other vessel was assigned. The ship was required to keep within its allotted area, unless the pursuit of a submarine should lead it into a neighbouring one. This patrol, as I have described, was not a satisfactory way of fighting submarines. A vessel would occasionally get a distant glimpse of the enemy, but that was all; as soon as the U-boat saw the ship, it simply dived to security beneath the waves. Our destroyers had many chances to fire at the enemy but usually at very long ranges; some of them had lively scraps, which perhaps involved the destruction of U-boats, though this was always a difficult thing to prove. Yet the mere fact that submarines were seldom sunk by destroyers on patrol, either by ourselves or by the Allies, did not mean that the latter accomplished nothing. The work chiefly expected of destroyers on patrol was that they should keep the U-boats under the surface as much as possible and protect commerce. Normally the submarine sails on top of the water, looking for its prey. As long as it is beyond the merchantmen's range of vision, it uses its high surface speed of about 14 knots to attain a position ahead of the advancing vessel; before the surface vessel reaches a point where its lookout can see the submarine, the U-boat dives and awaits the favourable moment for firing its torpedo. It cannot take these preliminary steps if there is a destroyer anywhere in the neighbourhood; the mere presence of such a warship therefore constitutes a considerable protection to any merchant ship that is within sight. The submarine normally prefers to use its guns on merchant ships, for the torpedoes are expensive and comparatively few in number. Destroyers constantly interfered with these gunning operations. A long distance shot usually was sufficient to make the under-water vessel submerge and thus lose its power for doing harm. The early experiences of our destroyers with submarines were of this kind; but the work of chasing U-boats under the water, escorting a small proportion of the many cargo ships, and picking up survivors, important as it was, did not really constitute effective anti-submarine warfare. It gave our men splendid training, it saved many a merchant ship, it rescued many victims from the extreme dangers of German ruthlessness, it sank a small number of submarines, but it could never have won the war.