of the plunger. To prevent the dangerous racing which would result if a pipe burst or the suction tank became empty, and which would cause the plunger of the hydraulic governor to fall to the bottom of its cylinder and so admit full steam pressure to the engines, a centrifugal speed governor is fitted (see Fig. 353). The water used in the various machines is returned to the suction tanks. To reduce the friction of slide-valves, &c., and to prevent corrosion of internal steel parts, soft soap and mineral oil are mixed with the water.

Hydraulic gun-turning engines.-Hydraulic turning engines are used for rotating the gun turntables of turrets and barbettes. The turntables are carried on a number of flanged coned rollers, which move between accurately turned roller paths, one of which is secured to the ship and the other to the underside of the turntable. The rollers are kept at the correct distances apart by two circular rings of steel plate, which carry the axis pins of the rollers. The rollers and upper roller path are made of forged steel, and the lower path of cast-steel. Bronze racks are secured to the peripheries of the turntables. Pinions on vertical shafts gear with the racks, and the turning engines are connected to the shafts at the lower ends through suitable gearing.

The engines are of the three-cylinder oscillating type, and water is admitted and exhausted from each cylinder by a slide-valve which is moved by a crank pin on the cylinder trunnion. The valve-box also contains a reversing slide-block which by its movement starts, stops, reverses, and regulates the speed of the engine. As these blocks are large and the pressure high the friction between the working faces is great and they could not well be moved by hand. A small hydraulic reversing cylinder is therefore provided for this purpose. The pistonrod is connected to a weigh-shaft, which moves the three reversing slides simultaneously when water is admitted to one end or the other of the reversing cylinder by a small slide-valve, which is moved by handwheels fitted in the turntable close to the gun sights. The gearing is so arranged that the position of the reversing slide always corresponds to a definite position of the handwheel.

A brake is fitted on the engine shaft which comes into action automatically when the water pressure at the engine falls below a certain amount. This insures that the turntable shall always be under control. Duplicate engines and gearing are provided for each turntable. These hydraulic engines work very smoothly at any speed. They are quite free from any risk of dangerous racing, even if all load is removed, as the hydraulic resistances in the engine itself increase very rapidly as the speed rises.

In some ships the engines have been fitted in the turntables, the racks being secured to the structure which supports the lower roller path. In recent vessels care has been taken to arrange the parts of the turntable, and the guns and machinery fitted in it, so that the centre of gravity of the whole is situated on the axis of revolution.

In addition to working the turning engines, the hydraulic power is also used for working a number of hydraulic machines for loading and working the guns, raising ammunition, &c. All important machines, pipes, &c., are duplicated, and in the later ships each operation can be performed by hand if necessary, though at a much slower speed. In some ships electrical motors are fitted to perform a few of the opera

tions of working the guns in case of break down of the hydraulic machinery.

Steam turret-turning engines.-The earliest turret ships were provided with steam turret-turning engines, the engine being controlled by a balanced differential valve similar to that described in Chapter XVII., which can be worked from the turret as well as at the engine. Steam-engines have also been fitted in H.M.S. 'Barfleur' and 'Centurion' for this purpose.

Air compressing machinery.-Machinery for compressing air for the purpose of charging and sometimes launching Whitehead torpedoes is now fitted in nearly all warships. The types of engines and pumps used for this purpose vary in design and arrangement, but in all of them the compression is done in two or more cylinders with successively decreasing diameters. The drawings, Figs. 355 and 356, of a two-stage compressor will serve to illustrate the principle. Fig. 356 is a vertical cross section through Fig. 355, on a larger scale, to indicate the detail more clearly. The pump has a cylinder a fitted in a casing B, through which latter, water is circulated to carry off the heat caused by the compression of the air. In this cylinder a combined piston and cylinder c, attached to the piston-rod of the engine, works. Air is drawn into the large cylinder on the down stroke through the annular valve D.

A little water and oil is admitted into the large cylinder during the suction stroke and, passing in as spray with the air, assists in carrying off the heat of compression. During the up stroke the air is compressed and forced through a number of holes past the valve E (shown unsectioned in the figure) into the moving cylinder. Only one of these small holes is shown. On the next down stroke more air enters the large cylinder through the inlet valve, and the air in the small cylinder is forced out through the delivery valve F. The air next passes through a coil of copper wire, G, in the water casing, where it is cooled. The cooling water is circulated through the casing and also through the condenser H, in which the injection water is produced by condensation in a small steam coil, by the action of the moving cylinder which, in combination with suitable valves, one of which is shown at K, acts as a pump. Each cylinder of the compressor and also the casing is provided with a safety-valve.

The compressors are arranged in sets of one, two, or three, on one bed-plate. Each can compress 10 cubic feet of air to a pressure of 1,700 lbs. per square inch in 70 minutes. When working at their maximum capacity, the machines run at about 350 revolutions per minute. Special machines for torpedo boats and destroyers run at 500 revolutions per minute.

Separator column. After leaving the cooling coils the air passes through a separator column L, where the injection water is removed. The separator column is formed of a steel tube with caps screwed on the ends. The top cap carries an inlet and outlet valve, and the lower one has a drain valve. A short length of pipe м is screwed to the inlet orifice, and extends down the inside of the tube for a portion of its length. The water and air on entry are therefore directed downwards, the water falling to the bottom and the air rising and passing out through the outlet valve to the reservoir. The drain valve requires to be opened frequently to blow out the accumulation of water. In a

recent form the action is automatic, the valve being opened by the weight of the water which collects in a cup supported by a spring, and to which a balanced piston valve is connected.

Air reservoirs and pipes.-The air is stored in reservoirs of steel tubes arranged in groups of fifty. The tubes are fitted with gunmetal caps at the ends, and are connected together by screwed unions and connecting pipes. The tubes are about 3 inches in diameter, -inch thick, and 6 feet long, and a reservoir of fifty tubes has a capacity of about 11 cubic feet. The whole of the air fittings are tested by a water pressure of 2,550 lbs. per square inch, the maximum working air pressure being 1,700 lbs. per square inch. The air is distributed to the torpedo tubes through copper pipes of -inch internal diameter, the main leads being fitted in duplicate.

In all torpedo gear and fittings the greatest reliability and accuracy are necessary, as such very high pressures and sudden impulses have to be dealt with. Every precaution must be taken to prevent leakage, and too great an amount of care cannot be exercised in the fitting of every detail of the gear.

Two

Boat-hoisting engines. In most large ships, engines arranged to drive suitable winch barrels are now fitted for the purpose of placing the torpedo and other boats into the water, or of lifting them, and landing them in position on board in fixed crutches. engines and winch barrels are necessary, one for lifting the boats out of the water, and the other for 'topping' the derrick and bringing the boats inboard. These should be quite independent of each other. The derrick is hinged on a swivel or ball joint supported by a bracket secured to the mast or signalling pole.

FIG. 356.

Ventilating engines.-In most vessels blowing engines are required for the purpose of providing a supply of fresh air for the crew spaces, as well as for the ventilation of the engine and boiler rooms. The engines are arranged to drive rotary fans, which draw air from ventilating shafts, and distribute it through the ship by ventilating trunks with openings in the several compartments. The outlets from these ventilating pipes are generally fitted with light gridiron valves, so that only such as may be required may be opened at any time. In modern warships, with closed stokeholds, blowing-fans and engines are also fitted for accelerating the draught in the boilers.

Ash-hoisting engines.-Small engines to drive winch barrels are fitted in most steamships for the purpose of lifting the filled ash buckets, &c., from the stokeholds, to enable the ashes to be thrown overboard. Some of these engines are fitted with reversing gears, so that the

buckets are both raised and lowered by steam, or friction gear may be fitted to allow the barrel to be disconnected from the engines so that the bucket may descend by its own weight, the rate of descent being regulated by a brake. Overhead rails are fitted on the upper deck, from the top of the ash tube to the shoots at the ship's sides, to facilitate the discharge of the ashes, &c. A voice-pipe or gong is fitted for signalling between stokehold and deck.

See's ash ejector. Various arrangements have been devised to obviate the necessity of raising the ashes to the deck and thence discharging them. A steam

ash ejector has been fitted in several vessels, but its low efficiency and the necessity of preserving fresh water renders such apparatus now inadmissible. Another and much more economical arrangement is See's ash ejector shown in Fig. 357. In this apparatus, which is fitted in many large passenger steamers in which the raising of ashes on deck is objectionable, the ashes are placed in a trough leading to a pipe, a jet of water at a pressure of about 200 lbs. per square inch from one of the pumps is then admitted, and scours the ashes along the pipe into the sea. A small valve is fitted to permit the entry of air into the pipe during the discharge. The apparatus is simple and efficient.

Feed-pumps. For delivering the feed-water from the feed-tanks to the

boilers, pumps of a variety of designs have been employed. For many years in the Royal Navy feed-pumping engines of similar type to the bilge-pumping engine of Fig. 340 were employed, except that the pump valves were made of metal instead of indiarubber, as in the bilge- and fire-pumps. These pumps have a constant stroke regulated by a crank shaft, and the valves are worked by eccentrics. They gave satisfaction for many years with steam of moderate pressures, even when they worked at high rates of speed. With the high steam pressures now used this design has not been so satisfactory, and it has recently given place to others, larger and slower in speed, and generally with slidevalves worked by a tappet action from the piston-rod either of its own