As often fitted, it has the disadvantage of making rather a hot stoke-hold, though with suffi cient precautions there is no reason why the ventilation should not be made perfect by taking the air through the stoke-holds, In the earlier American experiments (see Isherwood'sExperi mental Researches, vol. ii, Trials of Gunboats of Chippewa Class and Fulton) the air was in troduced into the ash-pits by pipes at the back ends.
Forced draft has also been produced by placing a fan in the uptake, and exhausting through the furnaces. This plan has the great advantage of dispensing with the elaborate furnace-fittings common to the undergrate systems: but it has the disadvantage of the diffi culty of keeping the fan in working order, owing to the high temperature in the chimney, and has not as yet come into common use; and, according to the researches of Dr. Tyndall On combustion in condensed and attenuated atmospheres, it should result in a more perfect com bustion, but how far this is realized in practice is not determined.
In regard to the economy of forced draft, an examination of Table III will show that while the mean consumption of coal in those steamers working under natural draft. is 1.5i3 lb. per indicated horse-power per hour, it is only 1.386 lb. in those fitted with forced draft. This is equivalent to an economy of 15 per cent. Part of this economy, however, may be due to the other heat-saving appliances with which the latter steamers are fitted. Such evidence as exists shows that not only is forced draft more economical as regards quantity of coal, but by its means such classes of coal may be used as would not without it be worth putting on board. It is in this direction perhaps that the greatest saying has followed its employment.
Thus far the following would appear to be a fair summary of the advantageous points attending the use of forced draft: First, it seems fairly well established that, if the boilers are well constructed and are provided with ample room to insure circulation, their steaming power may without injury be increased to about 30 or 40 per cent over that obtained on nat ural draft for continuous working, and may be about doubled for short runs: secondly, such augmentation is accompanied in normal eases by an increased consumption per indicated horse-power; but, thirdly, the same or even greater power being indicated, it may with mod erate assistance of forced draft be developed with a smaller expenditure of fuel. the grates,
etc., being properly proportioned : fourthly, forced draft enables an inferior fuel to be used ; and, fifthly. under certain conditions of weather, when with normal proportions of boiler it would be impossible to maintain steam for the ordinary speed with natural draft, the normal power may with forced draft be insured. In particular eases any or all these advantages may be a source of economy; and the first of them may render possible that. which would otherwise be impracticable.
Marine Boilers.—No particular change can be recorded in the general design of the marine boiler, but the change of material used and the great advance which has taken place in the application of tools to boiler-making can not pass without notice. As a material for the plates of boilers, iron is giving place to steel, though it seems probable that it will continue yet awhile to be the material for tubes. Furnaces are made with corrugated, ribbed, and spiral flues, with the object of giving increased strength against collapse without abnormally increasing the thickness of the plate. The increased pressures adopted in marine boilers have tended to cause a reduction in size, and as the high pressures have caused thicker scantlings, the larger boilers have become very heavy. The boilers of the R. M. S. Empress of India, which were 16 ft. 3 in. in diameter by 19 ft. 6 in. long, weighed 85 tons each, without furnace fittings or mountings of any description. (See BomEtts.) Engine.—The change from the principle of two-stage expansion to that of three and of four stages has been attended with corresponding modifications in the engine. The desire to economize in length of engine has given rise to more varieties of arrangement than any other single cause. For this purpose, combined with the aim of making them more accessible, the valves have been removed from the fore and aft center line and placed behind or in front, and worked either by one of the numerous forms of radial valve-gear, or by the link-motion and levers. It is true that by such an arrangement the length over the cylinders can be dimin ished; but as the extent to which the distances between the centers can be reduced is limited by the lengths of the shaft-bearings and the thicknesses of the cranks and couplings, little can be gained below the cylinders by this means.