Thus the war developed the fact that the most deadly enemy of the submarine is the submarine itself. Underwater warfare is evidently a disease in which like cures like. In a way this is the most astonishing lesson of the naval operations. It is particularly interesting, because it so completely demolishes all the ideas on this subject with which we entered the war. From that day in history when the submarine made its first appearance, the one quality which seemed to distinguish it from all other kinds of warship was that it could not be used to fight itself. Writers were fond of pointing out that battleship could fight battleship, that cruiser could fight cruiser, that destroyer could fight destroyer, but that submarine could not fight submarine. This supposed quality, which was constantly emphasized, was what seemed to make the introduction of this strange vessel such a dangerous thing for the British Empire. For more than a hundred years the under-water boat was a weapon which was regarded as valuable almost exclusively to the weaker sea powers. In the course of the nineteenth century this engine of sea fighting made many spectacular appearances; and significantly it was always heralded as the one effective way of destroying British domination at sea. The inventor of the modern submarine was an undergraduate of Yale named David Bushnell; his famous _Turtle_, according to the great British authority, Sir William White, formerly Chief Naval Constructor of the British navy, contained every fundamental principle of "buoyancy, stability, and control of depth" which are found in the modern submarine; "it cannot be claimed," he said in 1905, "that any new principle of design has been discovered or applied since Bushnell.... He showed the way to all his successors.... Although alternative methods of fulfilling essentials have been introduced and practically tested, in the end Bushnell's plans in substance have been found the best." The chief inspiration of Bushnell's work was a natural hostility to Great Britain, which was at that time engaged in war with his own country; his submarine, invented in 1777, was intended to sink the British warships which were then anchored off the American coast, break the communications of Great Britain with her revolting colonies, and in this way win our liberty. Bushnell did not succeed in this ambitious enterprise for reasons which it is hardly necessary to set forth in this place; the fact which I wish to emphasize is that he regarded his submarine as an agency which would make it possible for the young United States, a weak naval power, to deprive Great Britain, the dominant sea power, of its supremacy. His successor, Robert Fulton, was inspired by a similar ambition. In 1801 Fulton took his _Nautilus_ into the harbour of Brest, and blew a merchant vessel into a thousand pieces; this dramatic experiment was intended to convince Napoleon that there was one way in