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bulkheads intact. These are frequently found either to work too stiffly, so that they do not act, or to be too sensitive, so that the motion or vibration of the ship continually closes them. In the former case the watertightness of the bulkheads is completely destroyed, and in the latter great inconvenience is caused by the shutting off of the ventilation.

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FIG. 106.-Ventilation with electric motor vans.

In the above system of ventilation the exhaust is usually obtained through hatchways and doors, but for some compartments this is insufficient, and special exhaust trunks are led to the upper deck with cowls. Such spaces are compartments for auxiliary machinery, capstan engine, and steam steering engine. (These trunks can be used for escape purposes.)

The special arrangements for magazine ventilation will be referred to later.

2. With small fans driven by electric motors.-The system now adopted is to have a large number of smaller fans driven by electric motors taking the current from the electric light circuit of the ship. The principle of the system is that the vessel is divided into a number of sections, so that the main watertight bulkheads of the ship below water are not pierced,1 and the compartments in each of these sections are supplied with air by one or more motor fans. In this system automatic valves are unnecessary, and the inconvenience of heating by

steam-engines and steam-pipes is obviated.

AUTOMATIC
VALVE.

FLOAT

GAUZE

PERFORATED.

In a recent large ship there are fifteen of these fans, 24 in. in diameter. Fig. 106 shows a set of leads to and from a specimen fan. The fan draws air from a trunk opening above the upper deck, with a mushroom top. It delivers the air into an air chamber, and from this six pipes are led. All these pipes are closed by a single slide valve, worked at the valve or from the main deck. These pipes are watertight and lead-two to the barbette, two to the 12-in. shellroom, and two to the 12-in. magazine. The pipes into the magazine and shell-room have slide valves at the entrance. The top of the supply trunk can be closed at the upper deck, and a sliding shutter is also provided at the middle deck.

SLIDE VALVE.

MAGAZINE

AIR

SPACE

MAGAZINE LINING

GAUZE

PERFORATED

TOP

FIG. 107.-Ventilation exhaust
to magazine.

For ordinary compartments, as in the previous system the exhaust is obtained through hatchways and doors, but for some, special exhaust trunks are necessary, as seen above.

In either of the above systems of ventilation, spaces like

1 Some of the pipes ventilating spaces above water are led through watertight bulkheads. These have watertight slide valves at the bulkheads, but not automatic valves.

provision-rooms, etc., are not supplied with separate ventilating pipes, but they can be ventilated when required by means of a hose taken to a nozzle on an adjacent ventilation pipe.

Magazines.-Magazines require special arrangements for ventilation, because of the presence of the ammunition. It is necessary to avoid any undesirable rise of temperature, and also any excessive amount of moisture, because the ammunition is very sensitive in these respects. It must be possible that ventilation can proceed when the magazine is closed. For this purpose, in addition to the supply, special exhausts are fitted to the crown of the magazine; two of these are shown to the 12-in. magazine in Fig. 106. The detail of an exhaust is shown in Fig. 107. It has a slide valve, and in addition an automatic valve, which closes if the flat is flooded. These automatic valves can also be closed by flooding pipes leading from the main deck (Fig. 106).

In some ships the magazines have been placed at the middle line of the ship, between the boiler-rooms. Here, in addition to ordinary ventilation, a continuous current of air has to be maintained in the space between the magazine and the boilerroom, in order to keep the magazine cool.

The book of watertight compartments supplied to each ship (of which a specimen page is given at the end of Chapter IX.) gives particulars of how each compartment of the ship can be ventilated.

CHAPTER XI.

CORROSION AND FOULING.

Rust.—If bare iron or steel is allowed to remain in contact with moist air containing carbon dioxide (CO2), a chemical action goes on, by which the oxygen in the gas unites with the iron, and forms certain oxides of iron. We term the resulting compound rust. A similar action goes on if the iron or steel is immersed in fresh or salt water, owing to the carbon dioxide contained in the water. Rusting action is much hastened by heat, and where heat and moisture exist together, rusting goes on very rapidly indeed. Rust is about six times as bulky as the iron from which it is formed.

Corrosion.-Corrosion is very much accelerated by galvanic action. If iron and copper are immersed in dilute acid and metallic connection is made between them, an electric current is set up, and the energy of the current is provided at the expense of the iron. If iron and zinc are similarly immersed, it is found that the zinc wastes away. Not only do different metals act in this way, but different parts of the same plate may be sufficiently apart in the electro-motive series, owing to differences of density, etc., to give rise to an electric current if immersed in dilute acid. Iron and its rust are sufficiently different as to give rise to a current, and it is the iron which becomes corroded, so that when rust is once formed it does not cover up and protect the material, but itself is a cause of further corrosion. Rust is hydroscopic, i.e. it takes up moisture, and so dampness gets between it and the iron, and hastens the rusting and corrosion.

A black oxide is formed on steel during the process of manufacture, called mill scale. This stands in the same relation to the steel as ordinary rust, in that it is electro-negative to the steel, giving rise to an electric current, by which the steel is further corroded. This mill scale clings very tenaciously to the steel, and

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