500 THE STEAM-ENGINE. 



are speedily closed up. When such fuel is used, the grate-bars require to be 

 frequently raked 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 

 toward 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 downward, 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 downward 

 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. 



Between the evaporating power of the boiler, and the magnitude of surface 

 it exposes to the action of the furnace, there is a relation which, like that of 

 the grate surface, has never been ascertained by any certain or satisfactory 

 experimental investigation ; much less have the different degrees of efficiency 

 attending different parts of the boiler-surface been determined. That part of 

 the surface of the boiler immediately over and around the grate, is exposed to 

 the immediate radiation of the burning fuel, and is therefore probably the most 

 efficient in the production of steam. The tendency of flame and heated air to 

 rise, would naturally bring them in the flues into closer contact with those parts 

 of the boiler-surface which are horizontal in their position, arid which form 

 the tops of the flues, than with those which are lateral or vertical in their 

 position, and which form the sides of the flues. In a boiler constructed like 

 that already described, the flue-surface, therefore, which would be most, efficient, 

 would be the concave bottom of the boiler extending from the fire-bridge to its 

 remote end. In some boilers, especially those in which steam of high pressure 

 is produced, the form is cylindrical, the middle flue being formed into an 

 elliptical tube the greater axis of which is horizontal from end to end of the 

 boiler. It seems doubtful, however, whether in such a boiler the heat produces 

 any useful effect on the water below the flue, the water above being always at 

 a higher temperature, and therefore lighter than that below, and consequenily 

 no currents being established between the upper and lower strata of the water. 



It was considered by Mr. Watt, but we are not aware on what experi- 

 mental grounds, that from eight to ten square feet of heating surface were suf- 

 ficient to produce the evaporation of one cubic foot of water per hour. The 

 practice of engine-makers since that time has been to increase the allowance of 

 heating surface for the same rate of evaporation. Engine-builders have va- 

 ried very much in this respect, some allowing twelve, fifteen, and even eighteen 

 square feet of surface for the same rate of evaporation. It must, however, still be 

 borne in mind, that whether this increased allowance did or did not produce 

 the actual evaporation imputed to it, has not been, as far as we are informed, 

 ever accurately ascertained. The production of a given rale of evaporation 

 by a moderate heat diffused over a larger surface, rather than by a fiercer tem- 

 perature confined to a smaller surface, is attended with many practical advan- 

 tages. The plates of the boiler acted upon by the fire are less exposed to 

 oxydization, and the boiler will be proportionally more durable. 



Besides presenting to the action of the fire a sufficient surface to produce > 

 steam at the required rate, the capacity of the boiler must be proportioned to 

 the quantity of water to be evaporated. The space within the boiler is appro- 

 priated to a twofold purpose : first, to contain the water to be evaporated ; 

 secondly, to contain a quantity of ready-made steam for the supply of the cyl- 

 inder. If the space appropriated to the steam did not bear a considerable pro- 



