From the rather complete description of this vessel contained in one of Dr. Bushnell's letters, it appears to have been propelled by a screw propeller to obtain forward or reverse motion. It was ballasted in such a manner as to give the vessel great inherent stability. It had water ballast tanks which could be filled to give the vessel negative buoyancy, if desired, or to reduce the positive buoyancy so much that the vessel could be readily drawn under water by another screw propeller which was operated by a vertical shaft extending through a stuffing box into the vessel. This submarine carried a mine on its back, and provision was made to enable the operator inside the submarine to attach the mine to the bottom of a ship at anchor. This vessel was regulated in such a way that the mine could be exploded by a clockwork mechanism after the submarine had reached a safe distance from the vessel. With this submarine a mine was placed under the bottom of the English frigate _Eagle_, anchored in New York Bay, but the mine drifted clear before the clockwork mechanism caused it to explode, otherwise the frigate would undoubtedly have been destroyed. General Washington complimented Dr. Bushnell on having so nearly succeeded in his attempt to sink the ship. [Illustration: SKETCH OF THE CONFEDERATE SUBMARINE "HUNLEY" Made after she was recovered and hoisted on the dock years after the war. (Drawing by R. S. Skerrett.)] This submarine was unquestionably a successful model. It had one important feature that many designers have failed to appreciate, and that was great inherent stability. Great stability in a submarine means the carrying out of the now popular maxim "Safety First." Sufficient static stability is a guarantee that during all the manoeuvring evolutions of a submarine she will always remain right side up and not dive into the bottom unless the hull is punctured or flooded at one end or the other. Bushnell's model was not suited to high speed, but high speed was not essential in the days of the sailing ship. If this design had been developed further, so that several men could have been used to operate the propeller, it should have given a good account of itself. Robert Fulton's boat, to which I also have made reference in the foregoing chapter, differed from Bushnell's in its method of submerged control, which was by vertical and horizontal rudders at the stern. It also carried a collapsible mast on which a sail could be spread for surface navigation.