Yet, terribly destructive as these U-boats were, the number which were operating simultaneously in this and in other fields was never very large. The extent to which the waters were infested with German submarines was another particularly ludicrous and particularly prevalent misapprehension. Merchant vessels constantly reported that they had been assailed by "submarines in shoals," and most civilians still believe that they sailed together in flotillas, like schools of fish. There is hardly an American doughboy who did not see at least a dozen submarines on his way across the Atlantic; every streak of suds which was caused by a "tide rip," and every swimming porpoise, was immediately mistaken for the wake of a torpedo; and every bit of driftwood, in the fervid imagination of trans-Atlantic voyagers, immediately assumed the shape of a periscope. Yet it is a fact that we knew almost every time a German submarine slunk from its base into the ocean. The Allied secret service was immeasurably superior to that of the Germans, and in saying this I pay particular tribute to the British Naval Intelligence Department. We always knew how many submarines the Germans had and we could usually tell pretty definitely their locations at a particular time; we also had accurate information about building operations in Germany; thus we could estimate how many they were building and where they were building them, and we could also describe their essential characteristics, and the stage of progress they which had reached at almost any day. It was not the simplest thing to pilot a submarine out of its base. The Allies were constantly laying mines at these outlets; and before the U-boat could safely make its exit elaborate sweeping operations were necessary. It often took a squadron of nine or ten surface ships, working for several hours, to manoeuvre a submarine out of its base and to start it on its journey. For these reasons we could keep a careful watch upon its movements; we always knew when one of our enemies came out; we knew which one it was, and not infrequently we had learned the name of the commander and other valuable details. Moreover, we knew where it went, and we kept charts on which we plotted from day to day the voyage of each particular submarine. "Why didn't you sink it then?" is the question usually asked when I make this statement--a question which, as I shall show, merely reflects the ignorance which prevails everywhere on the underlying facts of submarine warfare.