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. was 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 fire-grate was 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 66.6 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 25 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.

Howden's system.-The development of the air-heating principle in this country is due principally to Mr. Howden, of Glasgow. The 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,

E

FIG. 21.

FIG. 22.

gradually increasing in temperature by contact with them, and being delivered through channels formed on either side of the smoke-box to closed spaces in connection with the ashpit, and also in the furnace frame. Valves are fitted to admit a certain proportion of this heated air under pressure to the ashpit, and also a certain proportion to the furnaces over the fires, the proportions being ascertained from experience. The front of the ashpit is closed as in the closed ashpit system. The details of the apparatus are shown in Figs. 21 and 22. Special strips of twisted plate are also placed in each boiler tube, called 'retarders,' which cause the gases traversing these tubes to take a spiral course and be retained in contact with the boiler tubes for a longer period than they otherwise would. Mr. Howden's grate is a very short one, so that the ratio of heating surface to fire-grate is considerably more than usual. Howden's system is fitted in a very large number of vessels of the mercantile marine, and also in several ships of the American Navy, satisfactory results being obtained.

Ellis & Eaves' system. This combination of draught by exhaust fans in the uptakes with air-heating appliances was introduced at a later period than the Howden system. In this plan the air-heating tubes are arranged in nests of horizontal tubes of considerable length

on top of the boiler. Unlike Mr. Howden's arrangement, the escaping hot gases are passed along on the outsides of the tubes on their way to the fan in the uptake, while the air for combustion enters the ends of the tubes from the open stokehold and passes through them (see Fig. 23). The heated air is conducted, as before, by channels at the sides of the smoke-boxes to the ashpits, and also to spaces around the furnace frames. A certain amount of air is admitted above the fires as well as through the ashpits. The front of the ashpit is closed, as in the last system mentioned. This plan has been fitted to several vessels of the mercantile marine, and satisfactory results have been reported.

FIG. 23a.

of a boiler,

plain tube.

With the fittings as arranged by Messrs. Brown, and also in large numbers of boilers not working on this system, a special form of boiler tube is used, known as the 'Serve' tube (Fig. 23a). It consists of an ordinary tube with the addition of several internal projecting ribs, which conduct an additional quantity of heat from the escaping gases to the surrounding water. These tubes add to the efficiency but the weight of tubes is about double that of an ordinary

CHAPTER VI.

PETROLEUM AS FUEL.

Petroleum fuel.-During the last few years a more or less successful attempt has been made to adopt petroleum as a fuel for marine boilers. The automatic and uniform supply of fuel to furnaces appears from many abortive trials of mechanical stokers to be recognised as impracticable with coal, but is easily carried out with oil fuel, such as petroleum. The natural petroleum is distilled, and gives off at first some very inflammable vapours, which are allowed to escape; at a higher temperature, the oils used for lamps come over; at still higher temperatures, the mineral oils used for lubrication are obtained. The residue is called 'astatki,' and is a heavy viscous oil amounting to about one-sixth that of the original petroleum, which by its considerable weight and evaporative power, the comparatively high temperatures below which it does not give off any vapour or burst into flame, and the little danger there is in its preservation and handling, is the most suitable for employment as liquid fuel in marine boilers. Many other oils have, however, been tried, such as blast furnace oil, tar oil, &c.

Petroleum consists practically only of carbon and hydrogen, so that its calorific value is very great. One analysis gives 85 per cent. carbon, 13 per cent. hydrogen, and 2 per cent. oxygen, giving a calorific value of about 20,000 British thermal units per pound, or at least one-third more than the best Welsh coal, the exact increase being doubtful. Some trials have indicated that for an equal value as regards production of steam it occupies only one-half the space required for coal. The specific gravity is 9.

The first attempts at burning liquid fuel, about the year 1875, were to run it in the liquid form in channels formed in the grate, but the combustion was slow, irregular, and produced much smoke.

The means now employed with much greater success consist in securing an intimate mixture of air with the oil, by injecting it through an orifice under pressure, and to reduce this stream of petroleum to a finely-divided condition of oil spray, by breaking up the issuing jet of oil by means of a similar jet of steam or compressed air under a rather greater pressure, which jet of steam or compressed air may be either oblique or concentric with the oil jet, generally the latter. The resultant petroleum spray mixes intimately with the air, of which a sufficient supply is also admitted, and when the relative discharge of the two jets is properly regulated, it burns with a clear, smokeless flame. Pumps are furnished to draw the oil from the bunkers into a reservoir, and to discharge it to the nozzles at the boilers.

Petroleum burners.-A large variety of designs for these have been