One great obstacle to successful submarine navigation on an extended scale is the difficulty of keeping a supply of air not only for the use of the crew, but for the engines. Any really powerful engine, either steam or gas, consumes an enormous amount of air. This is not true, of course, of the storage batteries which furnish the power for running while submerged, but these, at best, are but feeble generators of energy, although Edison's recent improvements may materially improve their power. If gasoline engines could be used during submergence a far greater speed would be acquired; but this is out of the question, as such engines would consume the air supply of the little boat far too rapidly. The compromise, now adopted universally, is to use gasoline motors while running at the surface or partly submerged, when the conning-tower is open, utilizing part of their energy meanwhile to charge the storage batteries. It is evident, therefore, that no great speed can be expected of the submarine in its present state; and in point of fact the largest type is able to develop only about ten or eleven knots when submerged, and fifteen while at the surface--far below the speed of any other type of war vessel. But the experimental attacks upon the "Yankee" prove that they are dangerous fighting craft, and a recent voyage by a flotilla of Italian submersibles shows that such boats are no longer harbor-locked vessels. In 1908 the Italian flotilla in question made a voyage from Venice to Spezia, a distance of thirteen hundred miles, without assistance from auxiliary boats. About the same time a British submarine flotilla, on a three-hundred mile trip, remained submerged for forty consecutive hours. The depth of the submergence in this case was only a few feet, but great depths may be reached with relative safety. In one test a Lake boat carrying her crew sank to a depth of one hundred and thirty-eight feet, returning to the surface in a few minutes. At another time the "Octopus," without her crew, was lowered to a depth of two hundred and five feet, sustaining a pressure of fifteen thousand tons, without injury. [Illustration: A FLEET OF BRITISH SUB-MARINES MANŒUVERING AT THE SURFACE.] Such performances as these are thought-provocative, to say the least. Submarine boats that can hit the target without being detected, go on cruises unattended for more than a thousand miles, and remain submerged for more than a day and a half, must be classed as efficient engines of warfare.