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  First War  Bushnell's Submersible Torpedo Boat, 1776. Cutaway by Lt. Col. F.M. Barber, 1885, based on Bushnell's description. (detail) Submarines were first used as offensive weapons in naval warfare during the American War of Independence (1775-1783). Invented by Yale student David Bushnell, the one-man boat "Turtle" was made of an upright walnut-shaped piece of wood (see photo). Underwater, the boat was powered by a driver turning a propeller. The plan was for the Turtle to approach a British warship underwater, load the hull with gunpowder using a screw device operated from inside the ship, and then leave the ship before a timer would detonate the charge. In a real attack, however, Turtle was unable to drive the screws through the copper cladding of the hull.    A precursor to the conning tower, with glass-covered portholes, allowed observation from inside the ship. The Nautilus dived by taking up water in ballast tanks, and a horizontal "rudder",...