surface, could detect the patrolling vessel long before it could be observed itself. All the submarine had to do, therefore, whenever the destroyer appeared on the horizon, was to seek safety under water, remain there until its pursuer had passed out of sight, and then rise again and resume its operations. Before the adoption of the convoy system, when the Allied navies were depending chiefly upon the patrol—that is, sending destroyers and other surface craft out upon the high seas to hunt for the enemy—the enemy submarines frequently operated in the same areas as the patrol vessels, and were only occasionally inconvenienced by having to keep under the water to conceal their presence. But let us imagine that the destroyer, in addition to its depth charges, its torpedoes, its guns, and its ability to ram, had still another quality. Suppose, for a moment, that, like the submarine, it could steam submerged, put up a periscope which would reveal everything within the radius of a wide horizon, and that, when it had picked up an enemy submarine, it could approach rapidly under the water, and discharge a torpedo. It is evident that such a manoeuvre as this would have deprived the German of the only advantage which it possessed over all other war craft—its ability to make itself unseen.
No destroyer can accomplish any such magical feat as this: indeed, there is only one kind of vessel that can do so, and that is another submarine. This illustration immediately makes it clear why the Allied submarine itself was the most destructive enemy of the German submarine. When Robert Fulton, John P. Holland, and other authorities declared that the under-water vessel could not fight its own kind, it is evident that they had not themselves foreseen the ways in which their inventions were to be used. They regarded their craft as ships that would sail the larger part of the time under the waves, coming up only occasionally to get their bearings and to take in a fresh supply of air. It was plain to these pioneers that vessels which spent practically all their time submerged could not fight each other, for the sufficient reason that they could not see each other ; a combat under these conditions would resemble a prize-fight between two blindfolded pugilists. Neither would such vessels fight upon the surface, for, even though they were supplied with guns
