GINE) perfect for surface propulsion, its low cost of running also being much in its favor. A development of 5,000 horse power by 1915 was a great advance over two- and four-cycle engines. The Fiat firm advanced the high .speed Diesel greatly and Britain gradually took up the work; all boats since and includ ing the E and F class have Diesel engines. Diesel engines have been used on all United States Holland boats for the last ten years. The driving power for under-water activity remains electricity, derived from batteries placed below the main floor of the machinery space. On later types of submarines, batteries are placed in a lead-lined compartment to prevent corrosion of hull by sulphuric acid fumes. But in recent types the Diesel engine charges the bat teries and does away with these dangers. Krupps of Kiel and M.A.N. of Nuremberg developed, at the German government's expense, engines best adapted for their 900 tons submerged displace ment submarines and (1915) two Diesels of about 900 to 950 brake horse power were in stalled on each and found to act admirably. Many other navies were supplied with them. The M.A.N. Diesels constructed at Augsburg four-cycle, 800 horse power and the Nurem berg M.A.N. two-cycle motors of 900 horse power. The Diesel engines of the British sub marines were built by the Vickers company and are four-cycle; those for E class developed 800 horse power in eight cylinders. The Fiat two-cycle 1,300 horse-power motor has been used by both Italian and British submarines, having six cylinders and 400 revolutions per minute. The Sulzer Brothers (Swiss) two cycle reversible 600 horse power were used at that time (1915) by the United States and Japan. The Polar (Stockholm, Sweden) four cycle motor was used by the Swedish navy.
Tank System.—The submerging from sur face to underwater conditions is brought about by reducing the vessel's buoyancy by means of filling her tanks with seawater. The tanks of the single hull construction are, of course, built inside the vessel, whereas in the case of double hulls the intervening space between the hulls is the tank space. But the disposition and dimensions of these water-ballast tanks is one of the most important problems of submarine building. The safety as well as the efficiency
of the vessel hinges entirely on the absolutely correct calculations made in designing the sub marine. The history of failures and accidents is generally that of faulty tank distribution. With full tanks the vessel must lie on a level keel under the water. The tankage space has to be divided into a dozen or more compart ments so that each unit shall be strictly self contained in its action and each tank must be completely filled before submerging. This set of amain ballast tanks" has to reduce the buoy ancy, leaving only 25 per cent. In about the middle of the ship's length is another tank known as the acentral,"midship,° °buoyancy° or °auxiliary° tank; this must never be com pletely filled after submerging. Next come the °trimming° tanks located fore and aft, used in maintaining the vessel on an even keel when getting ready to submerge. All submarines have the above tanks. To them are added, generally, ucompensating" tanks, each to compensate for the consumption used up in operating. Thus we have fresh water, the torpedo, the fuel and the lubricating oil compensation tanks; all needed to maintain the original weight. A °safety" tank is installed in some submersibles for use in case of emergency (accidents, etc.) ; this is quickly emptied by air pressure. But in recent constructions each tank is made capable of being emptied by air pressure, mak ing the special one unnecessary. Emptying the tanks quickly brings the submarine speedily to the surface in case of accident.
Steering Gear.— Steering can he performed on top of the conning tower, in the conning tower or in the ucentral station," properly called central operating compartment. Small sub marines are steered by hand power, the larger by electricity, but in the latter case, as on other ships, emergency handgear is provided for use in case of electric trouble. An indicator along side the steering gear shows the rudder's po sition and direction is found by repeating is pass when a gyroscopic master compass s in stalled, otherwise the steering station has its own compass if magnetic or has an image re flected from the master compass.