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--things which did not figure in the early designs of submarines--one boat could decline the combat simply by submerging. In the minds of Fulton and Holland an engagement between such craft would reduce itself to mutual attempts to ram each other under the water, and many fanciful pictures of the early days portrayed exciting deep-sea battles of this kind, in which submarines, looking like mighty sea monsters, provided with huge glaring headlights, made terrific lunges at each other. None of the inventors foresaw that, in such battles as would actually take place, the torpedo would be used, and that the submarine which was defeated would succumb to one of those same stealthy attacks which it was constantly meditating against surface craft.