USS Alligator, the fourth United States Navy ship of that name, is the first known U.S. Navy submarine, and was active during the American Civil War (the first American submarine was during the Revolutionary War, and was operated by the Continental Army, vice Navy, in 1776 against British vessels in New York harbor). During the Civil War the Confederate States Navy would also build their own submarine, . Construction In the autumn of 1861, the Union Navy asked the firm of Neafie & Levy to construct a small submersible ship designed by the French engineer Brutus de Villeroi, who also acted as a supervisor during the first phase of the construction (de Villeroi had designed and built submarines in France and one after immigrating to the United States). The boat was about long, with a beam of and height of . "It was made of iron, with the upper part pierced for small circular plates of glass, for light, and in it were several water tight compartments". She was designed to carry 18 men. For propulsion, she was equipped with 16 hand-powered paddles protruding from the sides. On 3 July 1862, the Washington Navy Yard had the paddles replaced by a hand-cranked propeller, which improved its speed to about four knots. Air was supplied from the surface by two tubes with floats, connected to an air pump located inside the submarine; it was the first operational submarine to have an air purifying system. The boat had a forward airlock, and was the first operational submarine with the capability for a diver to leave and return while both remained submerged. Divers could affix mines to a target, then return and detonate them by connecting the mine's insulated copper wire to a battery inside the vessel. The Union Navy wanted such a vessel to counter the threat posed to its wooden-hulled blockaders by the former screw frigate Merrimack which, according to intelligence reports, the Norfolk Navy Yard was rebuilding as an ironclad ram for the Confederacy (). The Union Navy's agreement with the Philadelphia shipbuilder specified that the submarine was to be finished in not more than 40 days; its keel was laid down almost immediately following the signing on 1 November 1861 of a contract for her construction. Nevertheless, the work proceeded so slowly that more than 180 days had elapsed when the novel craft finally was launched on 1 May