The submarine may be said to have remained in this essentially elementary stage until 1775, when David Bushnell, an American, launched a little one-man submarine after five years of planning and preparation. The shape of the vessel resembled a walnut held upright, the torpedo being carried outside near the top. At the bottom was an aperture fitted with a valve for admitting water, while a couple of pumps were provided for ejecting it. About 200 lb. of lead served as ballast, which could be lowered by ropes for the purpose of giving immediate increase of buoyancy should emergency require it. “When the skilful operator had obtained an equilibrium,” Bushnell writes, “he could row upward and downward, or continue at any particular depth, with an oar placed near the top of the vessel, formed upon the principle of the screw, the axis of the oar entering the vessel; by turning the oar one way he raised the vessel, by turning it the other way he depressed it.” A similar apparatus, worked by hand or foot, whichever was the more convenient, propelled the submarine forward or backward. The rudder could also be utilized as a paddle. Bushnell provided his little wooden craft with what he called a crown and we should designate a conning-tower. In this there were several glass windows. Neither artificial light nor means of freshening the air was carried, though the submarine could remain submerged for thirty minutes before the condition of the atmosphere made it necessary to ascend sufficiently near the surface to enable the two ventilator pipes to be brought into action. The water-gauge and compass were rendered discernible by means of phosphorus. The torpedo—Bushnell termed it a magazine—was an oak box containing 150 lb. of gunpowder and a clockwork apparatus which was set in operation immediately the affair was unshipped. It was attached to a wooden screw carried in a tube in the brim of the ‘crown.’ Having arrived beneath an enemy vessel, the screw was fixed in the victim’s hull from within the submarine, and the ‘U-boat’ made off. At the time required the mechanism fired what to all intents and purposes was a gun-lock, and the torpedo blew up.