a level as circumstances will allow, in order to increase the efficiency of the watertight arrangements.

Special provision is made for quickly closing from the hold, the doors in the large spaces forming the engine and boiler rooms, and these doors should be kept well up from the inner bottom. Vertical doors are capable of more rapid and certain closing than horizontal doors, and are fitted for all important doors in the hold space. The quick closing is now effected by fitting a quadruple-threaded screw of coarse pitch for the door, while for ordinary vertical closing doors the screw is a double-threaded one.

Sluice-valves. Small sluice-valves are usually fitted in the lower parts of the watertight bulkheads to allow the water to be drained from one compartment to another if required. These valves should be arranged to shut with a right-hand motion, and the rods, &c., for working them should be carried to the same height, and to the same positions, as the gear for working the watertight doors. Cocks are sometimes provided in certain parts instead of sluice-valves.

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Double bottom. The safety of most ships in the Royal Navy is very much increased by the construction of a 'double bottom.' This consists of an inner watertight skin, at some distance from the outer skin, extending for about two-thirds to three-fourths of the total length of the ship. The distance between the inner and outer skins at the bottom of the ship is generally about three or four feet, depending on the size of the ship; and above the turn of the bilge, the inner skin is continued by vertical bulkheads carried up above the water line, and forming what are called the wing passages.' The double bottom and wing passages are divided into many small compartments by longitudinal and transverse bulkheads.

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In H.M.S. Majestic,' for example, there are 46 compartments in the hold space, 48 in the double bottom, and 36 in the coal bunkers and wings, making a total of 130 watertight compartments in the ship. To enable the surfaces of the double bottoms, wing passages, &c., to be examined, cleaned, and painted, two manholes are made in the inner skin for each of the several compartments. The manhole doors are screwed down over the holes, generally with redlead putty between the faces, so as to make watertight joints. duplicate manhole facilitates ventilation.

The

Considerable damage may be done to the outer skin of a ship fitted with a double bottom without endangering her safety, for unless the inner skin be broken no water can enter the hold. The compartments in the double bottom are also so small that the filling of several of them would have but comparatively little effect on the immersion or the trim of the ship.

Water ballast.-In some cases the double-bottom compartments have been utilised for the carrying of water ballast, and in this way they have proved useful as the ship became lighter from any cause.

Double-bottom suction pipe.-The ordinary bilge pumping engines in the ship are connected with a suction pipe, having branches leading into each compartment of the ship below the armoured deck, except the wing spaces, but inclusive of those in the double bottom, for the purpose of pumping them out. The large ship's hand pumps are also connected to this suction pipe. This suction pipe is in large ships 6 inches in

BB

diameter in wake of the double bottom, and tapers slightly from this part to the ends of the vessel. It may also be utilised for filling the compartments when required, by taking out the non-return valve at the pump and opening the sea suction. There should be a separate suction cock or valve on the branch pipe leading to each com.part

ment.

Main drain pipes. Drain cisterns.-To facilitate the pumping out of the hold of a ship divided into several compartments two large galvanised steel drain pipes, one on each side of the ship, are fitted, with branch connections leading to the several compartments of the hold, and drain valves, which may be opened or closed, as required, by rods carried to a high level. Each drain pipe has a cistern in the engine room and foremost boiler room, so that there are four cisterns in all, and they are connected across the ship in pairs by a pipe of the same size as the main drain pipes. In recent large battleships the main drain pipes are 15 ins. diameter throughout the length of the machinery spaces, and one of the drain pipes is continued beyond these spaces as far as the double bottom extends, but gradually reduced in diameter.

Instead, therefore, of leading suction pipes from the pumping machinery to each compartment of the ship, it is only necessary to connect one suction pipe from each of the pumping engines to the drain cisterns, with which any particular compartment may be connected by opening the drain valves. This especially applies to the compartments outside the engine and boiler rooms; for although the latter are also drained into the main drain pipe, it is generally desirable to fit, in addition, an independent suction pipe from the principal auxiliary steam pumps to the engine room or stokehold bilge.

For larger bodies of water-that the ordinary steam pumps are unable to deal with-arrangements are provided so that the drain cisterns overflow into the engine-room bilge, where, when the level reaches the circulating pump bilge suction, it can be dealt with by the circulating pumps. For convenience the main drain pipe was, in the older ships, carried through the double bottom, but it was not in connection with it, and was fitted only for the drainage of compartments in the hold of the ship.

In modern ships the drain pipes are not run through the double bottom, various accidents to the outer skin of the vessel and consequent fracture of the drain pipe having shown the danger of such a lead. They are now always placed above the inner bottom. Arrangements are made for flushing the main drain pipes near the fore and after ends direct from the sea, and chains are sometimes fitted for clearing them when choked.

Fire arrangements. We have now described the pumping and watertight appliances necessary to promote safety in case of the ship springing a serious leak, and pass on to consider the precautions necessary to provide against the danger of fire.

Flooding cocks.-To the most dangerous parts of the ship, such as the magazines, shell- and spirit rooms, &c., flooding pipes are led, connected with the sea by Kingston or other valves and sea-cocks in the ordinary manner, so that in case of fire, the compartment may be flooded with water from the sea. Special precautions have to be taken

to prevent these cocks being tampered with; and, as a rule, there is between the ordinary sea-cock and the compartment an additional cock or valve, which is locked, and can only be opened by the officer entrusted with the key.

Arrangements are also made in warships so that when they are in dry dock these parts may then be flooded from the shore water pipes, by means of branch connections on the flooding system, fitted for this purpose.

Fire-main. All the independent steam pumps, except the main engine circulating pumps, that may be used for pumping out the ship in case of a leak, are also fitted with sea suctions, to deliver the water into a pipe called the fire-main, which is carried fore-andaft in the ship, with branches leading to different parts as required. At various points in the tire-main, delivery valves with suitable screwed nozzles are fitted, to which hoses may be connected to direct the water on any required spot. Non-return valves should be fitted at the junctions of the delivery pipes from the several pumping engines with the fire-main, and the pumps should have sufficient power to produce the necessary pressure in the main.

For dealing with small fires, small bib-valves are fitted to enable water to be quickly drawn off in buckets, to avoid the delay which might be caused by rigging a hose.

CHAPTER XXVIII.

AUXILIARY MACHINERY AND FITTINGS.

Ix modern steamships, especially in those of the Royal Navy, the auxiliary machinery and fittings are of great importance. In H.M.S. 'Powerful,' for example, there are ninety-nine different auxiliary steamengines for various purposes in addition to the main engines of the ship. In the battleships of the Majestic' class there are 72 auxiliary steam-engines and 32 hydraulic turning engines, lifts, bollards, &c. Those vessels are really huge floating war machines, where all the principal operations for working, steering, and fighting are performed by steam or hydraulic power with little manual labour, so that the vessel's efficiency will depend largely on the condition of the machinery department.

The enumeration of the various kinds of work on board ships that are now done by steam power would be sufficient to show the importance of this part of the duty of the engineer. In addition to the main propelling engines, steam power is used for ventilating the ship and supplying air to the boilers; weighing the anchor; steering; pumping; working turrets, and loading, training, and working the guns; compressing air for charging and launching torpedoes; putting torpedo and other boats into and lifting them out of the water; distilling fresh water; producing electricity for lighting the vessel and for search lights; for refrigerating purposes; for actuating the machines supplied in the workshop; lifting coal into the vessel, &c.

The development of electrical science has now rendered possible the transmission in a very efficient and economical manner of the power generated in the dynamo to electric motors for various purposes in different parts of the ship, and this is taken advantage of to work ventilating fans, &c., in confined spaces, where the heat of steam pipes would be objectionable. It also reduces the complication, as the necessary wire leads can be run along in places where steam and exhaust pipes would be most difficult. Such motors will probably be much more extensively used on board ship in the future than they are at present.

Steering engine. Figs. 343 and 344 show an arrangement of steam steering engine which has been largely fitted and found efficient. The shaft A, leading to the tiller, is driven by the engine through a system of toothed gearing, which reduces the speed of revolution to a sufficient extent to obtain the necessary turning moment to work the rudder readily, when the ship is moving at full speed.

The speciality in this engine is the fitting by which it is stopped when the helm is moved to the required angle, the rudder being held