(149.) The magnitude of the grate and ash-pit must be determined by the rate at which the evaporation is required to be conducted in the boiler and the quality of the fuel. It must be a matter of regret, that the proportions of the various parts of steam-engines, with their boilers and furnaces, have not been determined by any exact or satisfactory experiments; and those who project and manufacture the engines themselves, are not less in ignorance on those points than others. With coals of the common quality a certain average proportion must exist between the necessary magnitude of the grate-surface and the quantity of water to be evaporated in a given time in the boiler. But what that proportion is for any given quality of fuel, is at present unascertained. Each engine-maker follows his own rule, and the rule thus followed is in most cases a matter of bare conjecture, unsupported by any experimental evidence. Some engine-makers will allow a square foot of grate-surface for every cubic foot of water per hour, which is expected to be evaporated in the boiler; others allow only half a square foot: and practice varies between these limits. Bituminous coals which melt and cake, and which burn with much flame and smoke, must be spread more thinly on the grate than other descriptions of fuel, otherwise a considerable quantity of combustible gases would be dismissed into the flues unburnt. Such coals therefore, other circumstances being the same, require a larger portion of grate-surface; and the same may be said of coals which produce clinkers in their combustion, and form lumps of vitrified matter on the grate, by which the spaces between the grate-bars are speedily closed up. When such fuel is used, the grate-bars require to be frequently raked [Pg262] out, otherwise the spaces between them being obstructed, the draught would become insufficient for the due combustion of the fuel. To facilitate the raking out of the grate, the bars are placed with their ends towards the fire-door: they are usually made of cast-iron, from two to two inches and a half wide on the upper surface, with intervals of nearly half an inch between them. The bars taper downwards, their under surfaces being much narrower than their upper, the spaces between them thus widening, to facilitate the fall of the ashes between them. The grate-bars slope downwards from the front to the back. The height of the centre of the bottom of the boiler, above the front of the grate, is usually about two feet, and about three feet above the back of it. The concave bottom of the boiler, however, brings its surfaces at the slide closer to the grate.