This purely mechanical limitation explains why the German submarine was not a submarine in the popularly accepted meaning of that term. Yet the fact that this vessel remained for the greater part of its existence on the surface was no particular disadvantage, so long as it was called upon to contend only with surface vessels. Even with the larger part of its decks exposed the U-boat was a comparatively small object on the vast expanse of the sea. I have already made clear the great disadvantage under which destroyers and other patrolling vessels laboured in their attempts to "hunt" this type of enemy. A destroyer, small as it is, was an immensely larger object than the under-water boat, and the consequence was that the lookout on a submarine, proceeding along on the 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.