During the Revolutionary War David Bushnell built a submarine with which attempts were made to sink a British frigate lying in the Hudson River. This submarine was driven by a hand-operated screw propeller. The boat was provided with water ballast tanks, and by pressing a valve with one foot he could let in water enough to submerge the boat while with the other foot he could operate a pump to empty the tanks and bring the boat to the surface. When the boat was ballasted so that she would barely float, a vertical screw propeller was operated to raise or lower her as much as desired. A 200-pound lead weight was attached to a long cable which passed up through the bottom of the boat, and by letting out this cable the submarine could be made to rise instantly in case of an accident. FULTON’S HAND-PROPELLED SUBMARINE To Robert Fulton, however, belongs the credit of building the first submarine operating on the principle that is now universally used. His boat was also driven by a hand-operated screw propeller and was furnished with water tanks which could be filled or pumped out at will, but after the submarine was weighted until only the conning tower showed above water, she was submerged or raised by means of horizontal rudders or hydroplanes which could be tilted to any angle desired. Of course these rudders, like any other rudders, would not operate unless the boat were in motion. Such is the case with modern submarines. Like bicycles, they must keep on going or they will fall. If they are heavy, they will fall to the bottom, and if light they will “fall” to the surface. When in motion the hydroplanes will either hold them down or lift them up according to the angle to which these horizontal rudders are tipped. Robert Fulton’s _Nautilus_ had a fish-shaped hull of copper plating on iron ribs and was twenty-one feet three inches long by six feet five inches at her greatest diameter. The screw propeller was operated by two men. When the boat was on the surface a sail was raised to assist in driving the boat. This sail could be folded up like a fan when it was desired to submerge. Fulton deserves full credit for anticipating so many of the essential features of the modern submarine, but of course the _Nautilus_ was a mere toy compared to the marvelous machines which swim under the surface of the sea to-day. The German U-boats at the outbreak of the war were 150 feet long and could make only nine knots submerged and twelve knots on the surface, but later they grew to a length of 300 feet with a submerged speed of twelve knots and a surface speed of eighteen knots. The British in the meantime developed a submarine that was 340 feet long and had a displacement submerged of 2,700 tons as against 800 for the largest German U-boats. The speed of this big British boat is twenty-four knots on the surface and ten knots submerged. SUBMARINES AS SURFACE BOATS