The central compartment contains in its double bottom the main ballast tanks and a circular compensating tank placed between the two sets of batteries. Above the double bottom and below the axis of the vessel are located the storage batteries, and above these are carried the four torpedoes, which are 45 centimetres in diameter and 11 feet 8 inches in length. In the same compartment are a series of air flasks in which air at 2,000 lbs. to the square inch pressure is stored for the purpose of keeping pure the living spaces of the crew.[9] Footnote 9: Also for firing the torpedoes and for blowing the water out of the ballast tanks when it is desired to navigate as a surface boat. In the rear compartment is the four-cylinder gasoline engine, which is rated at from 160 to 190 actual horse-power, and at from 320 to 390 revolutions per minute. Its net weight is 1,300 lbs., its length over all is 9 feet 7 inches, and its total height above the crank-shaft centre is 5 feet 6 inches. In these engines, which gave satisfaction in the first Holland boat, the distribution of the cranks and the timing of the valves and igniters are so arranged that the operations in the four-cylinders alternate, so that while one is at the expansion stroke the other three are at the suction, compression, and exhaust strokes respectively. By this arrangement the engine is perfectly balanced and vibration is reduced to a minimum. _Submersion._—The first operation to be performed when it is desired to submerge the vessel is the admission of water into her ballast tanks. When these are full she runs “awash” with her conning tower alone above the waves. In order to completely submerge the boat she is steered below the surface by means of her pair of horizontal rudders at the stern. When at the required depth (normally from 20 to 30 feet) she is brought up on an even keel by the inclination of the horizontal rudders, which may be controlled automatically (by some such arrangement as is found in the Whitehead torpedo) or by hand. All modern submarine craft go under at an angle, and are steered below the waves by rudders: the principle of submersion on an even keel by down-haul propellers, advocated by Mr. Nordenfelt, has been abandoned by all latter-day designers, as has also the submersion of the drawing in of cylinders adopted in the Campbell-Ash boat. When running submerged the submarine is lighter than her displacement, and is only kept under by her horizontal rudders, which, of course, only act when she is moving. The great drawback to all under-water craft has been their lack of longitudinal stability, for they have a tendency to rise or sink at the slightest provocation. In our account of the Nordenfelt boats this disagreeable quality is illustrated. In order to keep the vessel at a regular depth recourse must be had to the horizontal rudders and to the two trimming tanks and the one circular compensating tank carried.