The type was extremely successful, and one can only be thankful that such boats were not on the enemy's side. They would have been the very devil to deal with on the trade routes, and would have caused us to reconsider very hurriedly our whole system of anti-submarine defence. Of course, four 6-inch would be better than one 12-inch from a German point of view, especially as a destroyer would be likely to attack unscathed under the fire of one big gun, but our type was intended for use against such things as enemy cruisers, and not for sinking merchant ships. By the end of the war the enemy had arrived at the stage of submarine design where one says, "We've got a type that works--let us stick to it, and just add improvements." We passed that stage before the war and are now in the confident (with reservations!) state of feeling that we can turn out anything required to combine the properties of under-water and surface craft. If submarines continue as weapons of war, they will improve very considerably, and the range of possibility of future types is so great that any prophecy now would be rash. It must be remembered, however, that the depth of water in which a boat is intended to operate limits her size. It is not only that the distance from her keel to periscope is (in the case of a big boat) some 50 feet, but also that the great length of a big boat's hull means that even a slight fore-and-aft inclination as she dives will add enormously to her draught; a very long boat in twenty fathoms of water (the North Sea average) would have to be careful not to get off an even keel, as it might happen that in the presence of the enemy her bow or stern would touch bottom, with the result of causing the whole boat to bounce up to the surface. A submarine submerged is in a state of equilibrium, in that she has little or no tendency to rise or sink if her motors are stopped and the boat left to herself. I am afraid in this history that I continue to speak of submarines as if everybody knew a good deal about them. I use technical expressions and words as if I was dealing with things like motor-cars. I will try and explain more clearly what I mean by "bouncing to the surface," and will do so in the idea that there are probably readers who know as much about diving boats as I do of bimetallism.