Not only was the minefield that had been laid down when the North Sea was first closed continually added to, but other barrages were always being thought out and improved as necessity demanded. Thus, in 1916, twenty miles of nets had been laid parallel to the Belgian coast, and, in the winter of the same year, another had been constructed from the Goodwins to Dunkirk. This was somewhat difficult to keep in order, but the Belgian nets were renewed in 1917, and, in November and December of the same year, 4,000 mines were laid between Folkestone and Boulogne. These were of the latest type, and, with further additions, together with a system of flares and day and night patrols, developed into a barrier against which, in the end, the German submarines beat in vain--at least seventeen of these being certainly known to have fallen victims to its efficiency. Second only in naval importance to the Grand Fleet, and in even more strenuous contact with the enemy, none had more cause, perhaps, to bless the {221} Dover Patrol, of whose unadvertised work this is but the barest outline, than those 2,000,000 soldiers, for whom, each year, it acted as crossing-sweeper, on their way home to England. {222} CHAPTER X THE SEALING OF ZEEBRUGGE AND OSTEND Manifold as were the duties, and various as was the composition of the Dover Patrol, it was in the sealing of Zeebrugge and Ostend--among the last naval operations of the war--that its qualities of enterprise, courage, and ingenuity found their most notable expression. How the possession of these places advantaged the enemy has already been indicated in the last chapter; and their continual threat to our communications was a sufficient justification of the proposed attempt. But it was not the only one, as a brief consideration of the general position will show. Robbed of these two ports, or with their usefulness so impaired as to render them abortive, the enemy would be thrown back, from the offensive standpoint, upon his Frisian estuaries, some three hundred miles distant; while the duties of the Patrol would be so sensibly lightened as to release an appreciable number of mosquito-craft. There would in fact be fewer exits to watch; these would be more distant by many hours' steaming; and there would at once be placed at our disposal more forces with which to watch them.