4. _Propulsion._--The various solutions of this problem have naturally followed the successive steps in the development of machinery. Drebbel made use of oars. Bushnell, though he speaks of ‘an oar,’ goes on to describe it as ‘formed upon the principle of the screw--its axis entered the vessel, and being turned one way rowed the vessel forward, but being turned the other way rowed it backward: it was made to be turned by the hand or foot.’ Moreover, he had a similar ‘oar’ placed at the top of the vessel, which helped it to ascend or descend in the water. The conclusion seems unavoidable that to this designer belongs the honour of having invented the screw propeller, and also of having put it into successful operation. Fulton adopted the same method of propeller and hand-winch in his _Nautilus_; but his huge vessel, the _Mute_, built in 1814 to carry 100 men, was driven by a silent steam-engine. He died during the trials of this boat, and further experiment with it seems to have been abandoned, possibly owing to the great interest excited by his first war steamer, which was building at the same time. A regrettable set-back was thus caused. For forty years no one experimented with any kind of propulsory engine. Bauer, in 1855, could devise no better method of working his propeller than a system of 7-foot wheels, turned by a pair of men running on a treadmill. At the same moment, however, a more fruitful genius was at work. A French professor, Marié-Davy, designed a submarine in which the propeller was driven by an electro-magnetic engine placed in the stern of the ship, with batteries forward. The idea was a valuable one, with a great future before it, though for the moment it achieved no visible success. A year later, in 1855, the famous British engineer, James Nasmyth, designed a ‘submerged mortar,’ which was in reality a ram of great weight and thickness, capable of being submerged level with the surface, and driven at a speed of over 10 knots by a steam-engine with a single high-pressure boiler. But in spite of the simplicity and power of this boat, it was finally rejected as being neither invisible nor invulnerable to an armed enemy; and in their desire to obtain complete submersion, the French inventors of the next few years--Hubault, Conseil, and Masson--all returned to the hand-winch method of propulsion. Riou, however, in 1861, adopted steam for one of his boats, and electric power for the other; and in 1883 the American engineer, Alstitt, built the first submarine fitted with both steam and electricity. Steam was also used in the _Plongeur_ of Bourgois and Brun, which was completed in the same year.