airtight and filled with slightly compressed air by means of blowing fans, has been generally fitted in vessels of the Royal Navy where forced draught has been required. This system was first adopted by the designers of torpedo boats, and very high powers were obtained from their boilers when worked under air pressure. In these original boats only one boiler was fitted, so that the application of the system was more simple than in the case of vessels containing a number of boilers.

The latest method of applying this system in the Royal Navy is shown in Figs. 77 and 78. The stokeholds are enclosed for the purpose of being placed under air pressure, by fitting vertical screen plates carried down between and at the ends of the boilers to meet the front boiler bearers. These screen plates are worked around the fronts of the boilers to enclose the smoke-boxes, so as to keep the stokeholds cool, and are carried back sufficiently far at the sides of the boilers to clear the water gauges. In this figure the top of the enclosure is formed by the steel, or protective deck. In many cases, especially of the older vessels, where there is a considerable space between the tops of boilers and the deck, the top of the airtight enclosure is formed by fitting a horizontal ceiling, extending from the coal bunker bulkhead to the front of the boiler, at about the level of the top of the boiler, as illustrated in Figs. 19 and 20. The stokeholds are thus made into closed airtight chambers of comparatively small dimensions. The screens are shown in thick lines.

Débris deck. The airtight ceilings of the stokeholds, when fitted separately, usually form portions of the 'débris' or 'splinter' deck generally fitted over the openings in the machinery department to protect the steam pipes and fittings from injury from fragments of shot, shell, or other débris. This débris deck then serves for carrying the fans for producing air pressure in the stokeholds.

In

Air-locks. In order to provide for passage to and from the stokeholds when under pressure, air-locks are fitted. These consist of small airtight chambers fitted with two-hinged doors opening against the air pressure, as shown in Fig. 78, and also in Figs. 19 and 20. passing through, one door only is open at a time, which makes it possible to enter or leave the stokehold without allowing much air to escape, and so reduce the pressure in the stokehold. Air-locks are necessary at all places at which communication is made between the compartments under pressure and any other part of the ship.

In the stokeholds of most of the fast cruisers no special horizontal ceilings are required to be fitted, as the deck of the ship answers the purpose. All that is necessary is to carry vertical screen plates around the boilers from the deck to the boiler bearers, so as to isolate the stokeholds. The other fittings are similar to those in the armour-clad ships, modified in detail as required to suit the different arrangements of the ships.

Advantages of closed stokeholds.-Forced draught in general possesses the important advantage for warships that a great reduction can be made in the space and weight required for the boilers; the extra power necessary for full-speed working, instead of being obtained by the provision of additional boilers, which would occupy much space and weight, although but seldom required, is provided by

the addition of fans and screens, which are comparatively inexpensive and involve very little additional space and weight.

The openings in the deck for the boiler rooms may be reduced to the minimum required for the supply of air to the fans, for the funnels, and for convenient access to the boiler rooms. The supply of air to the boiler rooms being entirely provided by the fans, the power of the ship is practically independent of the wind, which is a matter of importance, especially in the Tropics, and the power developed can be easily regulated by the speed at which the fans are driven.

The first ships in the Royal Navy to which this system was applied were the sloop Satellite' and the turret ship 'Conqueror' in 1882. During the four hours' full-power trial of the Satellite' with natural draught, 10-15 I.H.P. were developed per square foot of fire-grate. With an air pressure in the stokeholds equal to 1 to 2 inches of water, 16.9 I.H.P. per square foot of firegrate were obtained, being an increase of 66.5 per cent. In the Conqueror,' also, the gain in power with a mean air pressure of 13 inches over that obtained with natural draught was 68.6 per cent.

Large numbers of vessels have since been fitted with boilers on this system, and where not carried to extreme limits it has given satisfaction. As an example of the results obtained, that of the 'Sanspareil may be mentioned. This vessel was tried in 1888, and with a grate surface of 722 square feet and total heating surface of 19,980 square feet, developed 14,483 I.H.P. for four hours with 2 inches of air pressure, or 20 I.H.P. per square foot of grate surface.

Since this period, however, experience has shown the desirability of reducing the amount to which the boilers are forced, and the last Admiralty specifications for water-tank boilers provide a total heating surface of not less than 2-5 square feet per I.H.P. at natural draught power, and 12 to 12 I.H.P. per square foot of grate, while the forced draught power is limited to 20 per cent. beyond the natural draught power.

Air heating systems. It was seen on page 40 that the amount of heat passing up the funnel and wasted is very considerable, and various plans are in operation to reduce it. The most extensively used consists in heating the air passing to the furnaces for combustion, by the escaping hot gases. This is effected by fitting in the uptakes of the boilers a series of thin tubes, through or around which the air for combustion is made to pass, and on the other side of which are the hot gases on their way to the funnel.

The combination of this system with the closed ashpit method of producing the draught and other modifications of detail, is known as Howden's system. The combination of air heating with the induced draught caused by fitting fans in the uptake, produces Ellis & Eaves' system, introduced and developed by Messrs. Brown, of Sheffield.

The

Howden's system. The development of the air-heating principle in this country is due principally to Mr. Howden, of Glasgow. air-heating appliances in his system consist of a considerable number of thin vertical tubes arranged in a chamber immediately over the smoke-box, and through which tubes the escaping hot gases pass. The air for combustion is delivered by the fans through a pipe, enters this chamber at the middle, and proceeds past these tubes on either side,

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