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armour of which this barbette is built varies in thickness from about 6 to about 12 in., according to the class of the ship, it being thicker in battleships than in battle-cruisers. The barbette extends, in depth, from a few feet above the level of the upper deck downwards through the various decks, and rests upon one of the horizontal armoured decks, usually at about the level of the load waterline of the ship. The lower tiers of the barbette armour are thinner than the upper ones, as they come behind the protection of the side armour of the ship. The turret structure is made to train round inside the bar- bette by a large pinion, carried in the revolving turret, which gears into a circular rack fixed to the structure of the ship. The revolving part of the turret is composed of 3 chief divisions : (i.) the armoured gunhouse, in which the guns are mounted ; (ii.) the working chamber, immediately below the gunhouse, in which the ammunition is worked; (iii.) the trunk, which extends down to the level of the magazines and shell rooms and is used for the transfer of the ammuni- tion up to the working chamber.

The power used in the British navy for working the turrets is, at present, entirely hydraulic, supplied by hydraulic pumps which are situated outside the revolving structure of the turret. The Japanese and some of the German ships follow the British system, but in other navies electrical power has been used almost entirely for working the turret machines. Electrical turrets were tried in H.M.S. In- vincible " in 1912-3, but as they were not entirely satisfactory they were converted to the hydraulic system in 1914. The hydraulic system successfully withstood the severe test imposed upon it by the war, in the very large number of turrets afloat in the British fleet, and any other system will have to be proved to be very reliable before it can be expected to supplant it. Electrical machines have, however, made great strides in efficiency and reliability in recent years, and the combination of electric motors with hydraulic trans- mission (of the type of the Williams-Janney and Hele-Shaw machines) has rendered electric power peculiarly adaptable to turret machinery. This system has been used in all recent American and German turret designs, and the increased use of electrical power for the auxiliary and possibly for the main engines of war- ships may lead to the electro-hydraulic system being generally adopted in power-worked gun mountings.

If we take the before-mentioned 3 divisions of the turret sepa- rately, the detailed construction is as follows:

(i.) The Gunhouse. -On the circular structure, which revolves on the roller path, is carried the shield of heavy armour for the protec- tion of the guns and their mountings. The front and side plates of this shield are usually from 10 to 12 in. in thickness and sometimes even heavier. The rear plate is generally from 7 to 9 in. thick and the roof and floor plates from 3 to 5 in. The floor is extended to the rear, over the circumference of the barbette, in order to make room for the machines used for the loading operations and also to balance the turret. The guns are mounted, side by side, on separate slides, each gun being capable of moving independently of the other in elevation. Two pairs of very strong side transoms are built up on the floor of the gunhouse, and these carry on their upper parts the trun- nion bearings for the gunslides. The gunslides are stiff girder struc- tures, upon the upper surfaces of which the guns (trunnionless) are carried in cradles to which they are secured by steel straps. At the fore end of the slide are built 2 cheeks which carry the trunnions, and these latter work in the trunnion bearings of the transoms. The weights of the gun, cradle and slide, are so distributed that, when the gun is loaded and run out ready for firing, the whole is practically balanced about the trunnions. The elevating cylinder is fixed to the floor of the gun well, immediately under the slide, the piston rod being connected by a link to an arm which projects downwards for the lower part of the slide. The cradle, in which the gun rests, works in the direction of the axis of the gun, upon the upper surface of the 2 side girders of the slide. The recoil is controlled by 2 cylinders, fixed to the cradle one on each side of the gun, which contain a fluid, usually a mixture of glycerine and water. The piston rods are attached to the gunslide, and the piston head has a port cut in it which works over a valve key of varying depth. The flow of the liquid from one end of the cylinder to the other is thus gradually restricted during the recoil of the gun, which is thus brought to rest.

The gun and cradle are replaced in the firing position by pneu- matic cylinders in the latest mountings, hydraulic power having been previously used for this purpose. Some automatic arrangement is always fitted for bringing the gun to rest gently, at the end of the running-out motion. The breech of the gun is operated either by hand or by an hydraulic cylinder mounted on the rear face of the gun, the power being supplied by means of telescopic pipes under the slide. A high-pressure airblast system is fitted, which automatically admits a blast of air into the gun as the breech is opened. This pre- vents the dangerous backflame that is liable to occur when firing to windward, when the breech is opened. A jet of water, under high pressure, is also blown into the gun as the breech is opened to quench any smouldering remains of the charge. An extension, called the loading arm, is bolted on to the rear end of the gunslide and carries a chain rammer and the machine for working it. The gun-loading cage, on coming up from the working chamber, is locked to the load- ing arm and moves with it as the slide is elevated. This admits of the gun being kept laid on the target whilst it is being loaded. In the rear of the gunhouse a small hydraulic crane is fitted for alter-

native loading. This crane can pick up shells from either of two positions, and place them upon a loading tray which can be secured to the loading arm when required. These two positions are the shell bins in the gunhouse behind each gun, where about 5 rounds per gun are stowed, and the shell bins in the working chamber below. The position of the officer of the turret is in the centre of the rear of the gunhouse, in an enclosed cabinet. Here the turret range-finder is mounted and the various instruments for control of fire and com- munications are situated. The gun sights are mounted on the slide trunnions, one on each side of each gun. The sighting ports are formed by 3 low hoods in the roof of the turret, the centre one con- taining the inner sights of both guns and the side positions the outer ones. Elevating and training control wheels are fitted in all the sighting positions, but the turret is usually trained from the centre, and each gun is laid for elevation at its own sighting position.

(ii.) The Working Chamber. The working chamber is a circular space, usually from 10 to 12 ft. deep, immediately below the gun- house. Its roof is formed by the gun wells, in which are the elevating cylinders and the telescopic pipes for taking pressure on to the gun- slides. Below the floor of the working chamber is a compartment known as the walking pipe space. In this are situated the swivel or walking pipes by which the duplicated supply of pressure is taken from the fixed: structure of the ship to the moving part of the turret. On the walls of the working chamber are arranged the leads of pressure piping and the groups of stop valves which control the admission of pressure to the different machines in the turret. In the working chamber are situated the 2 training engines, the presses for working the gun-loading cages, the alternative hand gear and other small machines for various purposes. There are also 2 shell bins in which about 8 shells for each gun are stowed, and holes are cut in the roof of the working chamber to enable these shells to be picked up by the crane in the gunhouse. The centre of the working chamber is occupied by the top of the trunk in which the ammuni- tion is brought up from below. At each side of the top of the trunk are waiting positions, on to which the shell and cartridges are automatically discharged from the ammunition cages working in the trunk. From these waiting positions the ammunition is transferred, by hydraulic rammers, to the gun-loading cages; these latter, when in the down position, are in rear of the waiting trays and in line with them. The gun-loading cage consists of a tray upon which the shell is carried, and above this tray and to one side of it are 2 compart- ments, each of which carries a half-charge of cordite. The cage travels on a curved guide rail, and on being hoisted to the loading arm remains attached to it during the loading operation. When attached to the loading arm, the projectile tray of the gun-loading cage is in line with the bore of the gun, and the projectile is rammed into the gun by the hydraulically worked chain rammer carried on the loading arm. After the projectile has been rammed home the rammer is withdrawn, and the two half-charges of cordite are dropped in succession on to the tray which has been vacated by the projectile, and are pushed into the gun by the rammer. When the rammer is withdrawn after placing the last half-charge, the gun- loading cage is lowered to the working chamber in readiness to receive another round of ammunition from the waiting trays. A complete system of flashproof doors and screens is fitted between the gun-loading cage and the working chamber and also around the waiting trays and the top of the trunk. As far as possible, the whole path of the cordite, from the handing room to the gunhouse, is closed, so that the effects of an explosion in the gunhouse shall not be communicated, by way of the working chamber and the revolving trunk, to the magazines.

(iii.) The Revolving Trunk. The trunk hangs from the floor of the working chamber, and, passing through circular holes in the different decks, is steadied at its lower end by lateral rollers. The lower end of the trunk is entirely free and supports no weight. At the centre of the bottom of the trunk, the electric and high-pressure air leads and also the voice pipes are passed from the fixed structure to the moving part of the turret. The trunk is divided into 5 com- partments. In 2 large ones at the sides work the main ammunition cages, the space between them being occupied by the hydraulic presses for working these cages and by high-pressure-air bottles. In the rear compartment an auxiliary cordite hoist is fitted, and the front one forms an auxiliary shell hoist, which is also used as a ladder way for giving access from the shell room to the working chamber. At the magazine level there are openings in the trunk leading into the handing room, and the cordite compartments of the main cages are opposite these openings when they are in the lowest position. In the handing room, attached to the revolving trunk, are two loading hoppers which are fitted with flashlight doors. The cartridges are placed by hand into the hoppers, from which they are discharged into the cordite compartments of the main cage when the hopper doors are opened. The shell tray of the main cage comes opposite to openings in the trunk, -at the shell-room level, to which sliding doors are fitted. Two shell bogies travel on racks round the outside of the trunk in the shell room, and are so arranged that they can be locked, either to the floor of the shell room, or to the revolving trunk. The shell are placed on the bogies by hydraulically worked grabs, and the bogies are then trained round by hand, until they are opposite the shell doors in the trunk. When the shell doors are opened, stops which hold the shell are released, and the shell rolls