Perhaps in the early days of the war the German U-boats did hunt in flotillas; if so, however, they were compelled to abandon the practice as soon as the Allied submarines began to operate effectively. I have already indicated the circumstances which reduced their submarine operations to a lonely enterprise. In the open sea it was impossible to tell whether a submarine was a friend or an enemy. We never knew whether a submarine on the surface was one of our own or a German; as a result, as already said, we gave orders to attack any under-water boat, unless we had absolute knowledge that it was a friend. Unquestionably the Germans had the same instructions. It would therefore be dangerous for them to attempt to operate in groups, for they would have no way of knowing that their supposed associate was not an Allied or an American submarine. Possibly, even after our submarines had become exceedingly active, the Germans may have attempted to cruise in pairs; one explanation of the strange adventure of the _A L-2_, as said above, was that there were two U-boats in the neighbourhood; yet the fact remains that there is no well-established case on record in which they did so. This circumstance that they had to operate singly was a strategic point greatly to our advantage, especially, as I shall describe, when we began transporting American troops. CHAPTER IX THE AMERICAN MINE BARRAGE IN THE NORTH SEA I Was there no more satisfactory way of destroying submarines than by pursuing them with destroyers, sloops, chasers, and other craft in the open seas? It is hardly surprising that our methods impressed certain of our critics as tedious and ill-conceived, and that a mere glance at a small map of the North Sea suggested a far more reasonable solution of the problem. The bases from which the German submarines found their way to the great centres of shipping were Ostend and Zeebrugge on the Belgian coast, Wilhelmshaven and Cuxhaven on the German coast, and the harbour of Kiel in the Baltic Sea. From all these points the voyage to the waters that lay west and south of Ireland was a long and difficult one; in order to reach these hunting grounds the German craft had either to pass through the Straits of Dover to the south, or through the wide passage-way of the North Sea that stretched between the Shetland Islands and Norway, and thence sail around the northern coast of Ireland. We necessarily had little success in attempting to interfere with the U-boats while they were making these lengthy open-sea voyages, but concentrated our efforts on trying to oppose them after they had reached the critical areas. [Illustration: THE NORTH SEA BARRAGE Just how many German submarines were sunk in attempting to get by this barrage will never be known, for it did its work silently without any observers. It was probably a contributory cause of the mutiny which demoralized the German fleet in the autumn of 1918. Emery Walper Ltd. sc.]