THE STEAM-ENGINE. 499 



however free from counteracting effects. In ordinary furnaces the feed can 

 only be introduced by opening the fire-doors, and during the time the fire-doors 

 are opened a volume of cold air rushes in, which passing through the furnace 

 is carried through the flues to the chimney. Such is the effect of this in 

 lowering the temperature of the flues, that in many cases the loss of heat oc- 

 casioned is greater than any economy of fuel obtained by the complete con- 

 sumption of smoke. Various methods, however, may be adopted by which 

 fuel may be supplied to the grate without opening the fire-doors, and without 

 disturbing the supply of air to the fire. A hopper built into the front of the 

 furnace, with a moveable bottom or valve, by which coals may be allowed to 

 drop in from time to time upon the front of the grate, would accomplish this. 



In order to secure the combustion of the gases evolved from the coals 

 placed in the front of the grate, it is necessary that a supply of atmospheric air 

 should be admitted with them over the burning fuel. This is effected by small 

 apertures or regulators, provided in the fire-doors, governed by sliding-plates, 

 by which they may be opened or closed to any required extent. 



A patent has recently been granted to Mr. Williams, one of the directors 

 of the city of Dublin steam navigation company, for a method of consuming 

 the unburnt gases which escape from the grate, and are carried through the 

 flues. This method consists in introducing into the flue tubes placed in a 

 vertical position, the lower ends of which being inserted in the bottom of the 

 flue are made to communicate with the ash-pit, and the upper ends of which 

 are closed. The sides and tops of these tubes are pierced with small holes, 

 through which atmospheric air drawn from the ash-pit issues in jets. The 

 oxygen supplied by this air immediately combines with the carbtiretted hydro- 

 gen, which having escaped from the furnace unburnt is carried through the 

 flues at a sufficient temperature to enter into combination with the oxygen ad- 

 mitted through holes in the tubes. A number of jets of flame thus proceed 

 from these holes, having an appearance similar to the flame of a gas-lamp. 



It is evident that such tubes must be inefficient unless they are placed in 

 the flues so near the furnace, that the temperature of the unburnt gases shall 

 be sufficiently high to produce their combustion. 



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 deter- 

 mined 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 descrip- 

 tions 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 



