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simple lift valve, as Fig. 101; the pipe is led well up above the crown of the magazine and bent round, with the end perforated. In ships in which the crown of the magazine is near the waterline, these air escape valves are provided with a spring to

cause them to lift easily under the small pressure.

Fire Main (Figs. 102 and 103).— The fire main and its system of pipes is obviously a most important set of fittings in a war-ship. The fire main itself in a large ship is a

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Fig. 101.—Air escape to magazine. copper pipe 5 in. diameter, running all fore and aft under the protective deck. It is connected in the engineroom with the fire and bilge pumps, stop valves being fitted to shut off the fore-and-after portions of the system as desired. The fire main system can be charged from the Downton pumps if required, but the connection is fitted with a non-return valve, so that the pressure in the fire main may not affect the Downton pumps. The fire main is not rigidly connected to the protective deck, but is either supported from bulkheads or from slings connected to the deck. Branches are taken from the fire main to each stokehold, to submerged torpedo-rooms, capstan engine-room, ammunition passages, etc., with connections at each place for attaching a couple of hoses.

Rising mains are led at intervals to the upper deck, two of these are led up under protection inside the barbettes. Each rising main has a stop valve beneath the protective deck at its junction with the fire main, and these valves are placed in accessible positions so that they can be worked from below the deck if desired. They are also geared to work from above the deck. To most of the rising mains there is also a stop valve beneath the main deck, to shut off the pipe above that deck in the event of the pipe being damaged in action. Branches are taken from the rising mains for flushing w.c.'s, washing out ash-shoots, washing out barbette guns, etc. One-inch bib valves are placed on rising mains between middle and main decks, main and upper decks, and in ammunition passages for drawing off small quantities of water if desired.

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CHAPTER X.

VENTILATION.

The problem of the effective ventilation of a ship is a more difficult one than the ventilation of a building. In the case of a building, ventilation is assisted because the porous nature of the walls allows air to diffuse through, this diffusion being all the more effective as the difference of temperature within and without is greater. In the case of a ship, however, the conditions are quite different, and the impervious nature of the skin renders diffusion through impossible; ventilation must be obtained by the actual introduction of fresh air and the withdrawal of the foul air.

Ventilation can be either natural or artificial. By natural ventilation is meant supply and exhaust without the aid of fans. By artificial ventilation is meant that fans are used to draw pure air down to, or draw foul air away from, the space to be ventilated, or both. Natural ventilation is used for the ventilation of the 'tween deck spaces, and for the special cases of the coal-bunkers and spirit-room. Artificial ventilation is used for the other spaces below, as magazines, shell-rooms, store-rooms, etc.

For the 'tween decks cowls are used which can be turned to face the wind, the exhaust taking place through the hatchways, and in some cases through other cowls which can be turned away from the wind. The side-lights are also useful for ventilation purposes. For the spirit-room it is necessary to have an independent supply and exhaust, with cowls on the upper deck, as it is undesirable to have any connection between the spirit-room and any other system of ventilation which might communicate with a magazine.

Coal-bunker Ventilation.—The efficient ventilation of coalbunkers is of extreme importance, because of the gas that comes from the coal. This gas when mixed with air forms an explosive mixture, and if it is allowed to accumulate may cause serious explosions. The gases are especially liberated from the coal, if a sudden fall of the barometer occurs, or if the temperature rises.

It is worth noting in this connection that it is undesirable to take wet coal on board, because the moisture causes a rapid and dangerous generation of heat and gas. The coal should also be kept as dry as possible (thus when cleaning the main deck the coaling scuttles should be kept closed.) Temperature tubes are provided in all coal-bunkers, so that the temperature in the body of the coal may be ascertained at frequent intervals. No light except a safety lamp must be used inside a coal-bunker until it is ascertained that the bunker does not contain explosive gas. Special precautions in this respect are necessary for a few days after coaling.

In the ventilation of a coal-bunker two pipes are employed, one for the introduction of fresh air and one for the withdrawal of the foul air. The latter is led where possible up a funnel casing. These casings under ordinary circumstances will be hot, and this causes the air in the pipe to rise. This induces a current of air over the top of the bunker, and fresh air is then drawn down the supply pipe leading from the upper deck.

There are slight differences in the fittings of different ships, but the general principles will be understood by Fig. 104, which shows the ventilation of the coal-bunkers of a battle-ship. In this type of ship there are three separate series of bunkers, which are each independently ventilated.

1. The upper bunkers, behind the armour, which extend over the length of the double bottom.

2. The lower bunkers, abreast the engine- and boiler-rooms.

3. The wing bunkers, abreast the engine- and boiler-rooms (can be used for coal if desired, and in the deep load condition these are assumed to be filled).

1. Upper bunkers.—These bunkers are formed into groups, Fig. 102 shows the arrangements for the forward group of four. A trunk is taken into the funnel casing into which all the four bunkers exhaust. Each bunker has a separate supply by means of a pipe leading from the upper deck. The coaling scuttles in the main deck can also be removed and the gratings put in if desired.

2. Lower bunkers.—The supply pipes for these bunkers are taken down the ventilators in order to avoid piercing the thick decks, and the exhausts are taken up the funnel casings. A separate supply and exhaust is taken from each bunker, the supply pipe being taken to the far corner so as to get the current of air well over the top of the bunker.

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