As early as the sixteenth century the possibilities of submarine navigation was the dream of the mariner, and tentative attempts at submarine boats are said to have been made even at an earlier period than this; but the first practical submarine boat capable of navigation entirely submerged for any length of time was made by David Bushnell, of Westbrook (then Saybrook), Maine, U. S. A., in 1775. Details as to the construction of the remarkable craft, are recorded in a letter written by the inventor to Thomas Jefferson in 1789, and recorded in the _Transactions of the American Philosophical Society_. In this letter Bushnell says:-- "The external shape of the submarine vessel bore some resemblance to the upper tortoise shells of equal size, joined together, the place of entrance into the vessel being represented by the opening made by the swell of the shells at the head of the animal. The inside was capable of containing the operator and air sufficient to support him thirty minutes without receiving fresh air. At the bottom, opposite to the entrance, was fixed a quantity of lead for ballast. At one edge, which was directly before the operator, who sat upright, was an oar for rowing forward and backward. At the other edge was a rudder for steering. An aperture at the bottom, with its valves, was designed to admit water for the purpose of descending, and two brass forcing-pumps served to eject the water within when necessary for ascending. At the top there was likewise an oar for ascending or descending, or continuing at any particular depth. A water-gauge or barometer determined the depth of descent, a compass directed the course, and a ventilator within supplied the vessel with fresh air when on the surface. "The vessel was chiefly ballasted with lead fixed to its bottom; when this was not sufficient a quantity was placed within, more or less according to the weight of the operator; its ballast made it so stiff that there was no danger of oversetting. The vessel, with all its appendages and the operator, was of sufficient weight to settle it very low in the water. About two hundred pounds of lead at the bottom for ballast could be let down forty or fifty feet below the vessel; this enabled the operator to rise instantly to the surface of the water in case of accident.