Secondary only to this are the areas that feed ships into it, or into which the ships that pass through it are dissipated on their way to the several ports--the Mediterranean, the Bay of Biscay, the English Channel, St. George’s Channel, the Irish Sea. It is in these, when it is driven from the main funnel point of traffic, that the submarine must do its work. The defeat of the submarine, when at large, turns upon three factors: (1) the under-water offensive--that is, mine-fields, that will tend to keep it within certain areas; (2) the efficiency with which ships liable to attack are protected by convoy; and (3) the skill and persistence with which submarines, once on their hunting grounds, are in turn hunted. To maintain a cross-Channel barrage, the enemy surface craft must be handicapped in every possible way. The second and third factors of anti-submarine war make heavy demands on material, on personnel, and on skill, judgment, and organization. Here the decisive material factor is the number of destroyers available for both forms of work. When it comes to a close-quarters fight, no craft that has a speed of less than thirty knots, that cannot maintain itself in any weather, that does not possess a large cruising radius, can be of the first efficiency. The larger petrol-driven submarine-chasers and the many special craft which are built for various purposes in connection with the defensive campaign, all have their field of utility. But for the final power to rush swiftly on to a submarine if it is momentarily seen afloat, and for covering the area into which it can submerge itself, while the destroyer approaches with depth bombs, the destroyer, if only from its superior speed, stands supreme as the enemy of the U-boat. From the very earliest days of the submarine work it has, then, been axiomatic that every measure which will put a larger number of destroyers at our disposal should be taken at almost any cost. How does the work at Zeebrügge and Ostend help us, both in this respect and in a mining policy?