To plunge and again come to the surface of the water, ballast tanks, sea valves and hand pumps provided ample facilities readily to overcome or restore excess of buoyancy. The brass cylinder with the hemispherical ends would suffice to withstand the exterior hydrostatic pressure. The required thickness of shell was a matter of computation, one readily made with certainty even in those days. The difficulty with all early submarines was motion beneath the surface. In the British plan, Fulton proposed to obtain motion by a manually operated crank turning a propeller. The boat was larger than the Nautilus, but so also would have been the crew. For short distances he could undoubtedly have driven the boat at his estimated speed. The propeller was a two-bladed affair of modern type. Fulton had now definitely abandoned the full helical or Archimedes screw that Bushnell used and which he had himself tried in his first experiments. Reference to Plate First and its description will show, however, an exceedingly interesting addition that Fulton had made in the British boat. He reasoned correctly that a propeller when not turning would cause a considerable drag to the boat when sailing, and thus reduce her speed. He, therefore, arranged that his propeller could be folded so as to lie horizontally. This he proposed to do by a hand crank and gearing operated from within the boat. On Plate Seventh it will be seen that the propeller when folded lay well above the water surface and so would not have been an impediment to the motion of the boat. When it is recalled that the propeller was not generally adopted as a means of vessel propulsion until after 1845, when the steamship Great Britain crossed the ocean between England and New York, the first vessel driven by a screw propeller to accomplish the feat, and that a propeller that could be folded or hoisted above water was not introduced until about 1850, because at that period steam was merely an auxiliary to sails, it will be seen how far ahead of his time Fulton was in the design that he made in 1804. Another radical innovation was a horizontal propeller, Marked _B_ in Plate First, attached near the bow of the boat. This propeller, also actuated by a crank from within the boat, was to give the boat vertical motion when submerged and so enable it to be kept at any depth that might be desired. This principle of the horizontal propeller is that of the helicopter, the device now being experimented with by airplane designers in order to give planes a vertical motion or permit them to hover stationary in the air. It was precisely those same results in the water that Fulton undertook to accomplish with his submarine.