Construction was begun at once and the boat was finished in May, 1801. She was a remarkably modern-looking craft, and a great improvement on everything that had gone before. She was the first submarine to have a fish-shaped, metal hull. It was built of copper plating on iron ribs, and was 21 feet 3 inches long and 6 feet 5 inches in diameter at the thickest point, which was well forward. A heavy keel gave stability and immediately above it were the water-ballast tanks for submerging the vessel. Two men propelled the boat when beneath the surface by turning a hand-winch geared to the shaft of a two-bladed, metal propellor. (Fulton called the propellor a “fly,” and got the idea of it from the little windmill-shaped device placed in the throat of an old-fashioned fireplace to be revolved by the hot air passing up the chimney and used to turn the roasting-spit in many a French kitchen for centuries past.) The third member of the crew stood in the dome-shaped conning-tower and steered, while Fulton himself controlled the pumps, valves, and the diving-planes or horizontal rudders that steered the submarine up and down. Instead of forcing his boat under with a vertical-acting screw, like Bushnell and Nordenfelt (see pages 16 and 62), Fulton, like Holland, made her dive bow-foremost by depressing her nose with the diving-planes and shoving her under by driving her ahead. Fulton was also the first to give a submarine separate means of propulsion for above and below the surface. Just as a modern undersea boat uses oil-engines whenever it can and saves its storage batteries for use when submerged, Fulton spared the strength of his screw by rigging the _Nautilus_ with a mast and sail. By pulling a rope from inside the vessel, the sail could be shut up like a fan, and the hinged mast lowered and stowed away in a groove on deck. Later a jib was added to the mainsail, and the two combined gave the _Nautilus_ a surface speed of two knots an hour. She is the only submarine on record that could go faster below the water than above it, for her two-man-power propellor bettered this by half a knot. [Illustration: The _Nautilus_, Invented by Robert Fulton. A-B, Hull; C-D, Keel; E-E, Pumps; F, Conning Tower; G, Bulkhead; H, Propellor; I, Vertical Rudder; L, Horizontal Rudder (diving-plane); M, Pivot attaching horizontal to vertical rudder; N, Gear controlling horizontal rudder; O, “Horn of the _Nautilus_;” P, Torpedo; Q, Hull of vessel attacked; X, Anchor; Y, Mast and sail for use on surface. ]