Mr. Nordenfelt realised the superiority of submarines possessing a reserve buoyancy when submerged over those which possessed no buoyancy and all the most important of latter-day submarines fall under this division. It is quite obvious that should any accident happen, such as the entry of water, the failure of the machinery, the asphyxiation of the crew (rendering the detaching of a false keel impossible), &c., the submarine with a reserve buoyancy would at once rise to the surface, while the boat with no floatability would remain where it was and then gradually commence to sink, owing to the fact that it is almost impossible to prevent the water from finding its way, little by little, into the boat. Submarines which possess floatability when submerged have a weight which is less than their displacement and some mechanical action must be resorted to to force them below the surface. The first operation consists in introducing a certain amount of water into the tanks so that the boat is brought to the “awash” condition, with the greater part of the hull below water and only the conning tower, &c., appearing above the waves. The complete submersion of the vessel may be attained in two ways: either screws on vertical shafts are employed to “screw” the vessel below the surface, whether at rest or whilst moving; or horizontal rudders, or planes, are used to steer the boat below the surface; this latter method is only applicable to moving vessels.[3] Footnote 3: A system of moving weights was employed by Drzewiecki and other inventors. IMMERSION BY SCREWS MOUNTED ON A VERTICAL SHAFT. Just as a ship is driven backwards and forwards in the horizontal plane by means of a screw or screws mounted on a horizontal shaft, so it is possible to drive a ship up and down the vertical plane by means of one or more screws immersed in the water and mounted on a vertical shaft; the boat is by this method literally “screwed down” into the liquid. The principle of the vertical screw was adopted by Bushnell who, in the description of his submarine vessel, writes: “At the top there was likewise an oar for ascending and descending or continuing at any particular depth.... When the skilful operator had obtained an equilibrium (by means of the forcing pumps) he could row upwards and downwards 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 in the other he depressed it.” [Illustration: PROFESSOR TUCK’S SUBMARINE. (1884.) ]