Besides these two types, the U-boats and the UB's and UC's, the Germans had another type of submarine, the great ocean cruisers. These ships were as long as a small surface cruiser and were half again as long as a destroyer, and their displacement sometimes reached 3,000 tons. They carried crews of seventy men, could cross the Atlantic three or four times without putting into port, and some actually remained away from their bases for three or four months. But they were vessels very difficult to manage; it took them a relatively long time to submerge, and, for this reason, they could not operate around the Channel and other places where the anti-submarine craft were most numerous. In fact, these vessels, of which the Germans had in commission perhaps half a dozen when the armistice was signed, accomplished little in the war. The purpose for which they were built was chiefly a strategic one. One or two were usually stationed off the Azores, not in any expectation that they would destroy much shipping--the fact is that they sank very few merchantmen--but in the hope that they might divert anti-submarine craft from the main theatre of operations. In this purpose, however, they were not successful; in fact, I cannot see that these great cruisers accomplished anything that justified the expense and the trouble which were involved in building them. III This, then, was the type of warfare which the German submarines were waging upon the shipping of the Allied nations. What were the Allied navies doing to check them in this terrible month of April, 1917? What anti-submarine methods had been developed up to that time? The most popular game on both sides of the Atlantic was devising means of checking the under-water ship. Every newspaper, every magazine, every public man, and every gentleman at his club had a favourite scheme for defeating the U-boat campaign. All that any one needed for this engaging pastime was a map of the North Sea, and the solution appeared to be as clear as daylight. As Sir Eric Geddes once remarked to me, nothing is quite so deceptive as geography. All of us are too likely to base our conception of naval problems on the maps which we studied at school. On these maps the North Sea is such a little place! A young lady once declared in my hearing that she didn't see how the submarines could operate in the English Channel, it was so narrow! She didn't see how there was room enough to turn around! The fact that it is twenty miles wide at the shortest crossing and not far from two hundred at the widest is something which it is apparently difficult to grasp.