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70 AROUND cooled down, as in the closed stoke-hold arrangement. The single furnace of the boiler of the locomotive engine, being worked independently of other furnaces and not liable to be cooled down quickly attains a very high temperature, and consequently a very rapid combustion of the fuel is effected. These favorable conditions are wanting in the closed stoke-hold system; consequently with it a high rate of combustion can only be attained by a great waste of fuel. In this system a furnace is sustained at a higher average temperature for the quantity of fuel consumed than in any other mode of working. Several circumstances contribute to this advantageous result. The two principal are, (i) the hot air supply, and (2), the reduced quantity of air required per cent, of fuel consumed. If the temperature of the air supply is raised by the heat of the waste gases, say 2,000 degrees above the cold air supply, then the average temperature of the furnace, on this account aloue, must be within 4 or 5 per cent, of 2000 higher than with cold air supply, other things being equal. Then the manner in which the air is admitted and regulated, together with the high temperature of the furnace arising from the use of hot air, leads to the complete combustion of the fuel being effected by a smaller air supply per unit. This reduced air supply again further directly raises the tempeiature of the furnace, for every pound of air admitted beyond what is necessary to supply the oxygen required for combustion of the fuel merely reduces the furnace temperature and carries off the heat by adding to the quantity of the waste gases. These several increments of temperature each contribute to the reduction of the weight of the air of combustion required per unit of fuel, by bringing it more LAKES." quickly into the gaseous state, and so permitting of a more rapid combination of the oxygen of the air supply with the carbonaceous and other gases of the fuel wdien brought forcibly into contact therewith, as provided for by this system. The effect of the reduced air supply per unit of fuel consumed both in raising the average temperature of the furnace and in reducing the consumption of fuel per unit of evaporation are very noticeable in this system. In the case of a boiler like that of the Indiana, which has a total heating surface of 2338 square feet, and where the grate bars are 5' 3" long over extremities, and the width of the furnaces from side to side on the level of the grate bars is 3' 7", making the total grate area of the three furnaces exactly 56.5 square feet, the average temperature of the furnaces when working at a rate of combustion giving 22^ T H. P. per square foot of grate, would be about 4000 higher than it w?ould be in the same boiler, burning the same quantity of coal, if fitted on the closed stokehold system. The quantity of fuel which can be efficiently burned by this system per square foot of grate per hour is only limited b}" the arrangement for hot air supply and proportionate passages for the fire gases. There would be no difficulty whatever in arranging such a boiler as that of the Chicora to burn 70 to 80 lbs. of coal per square foot of grate per hour with comparative ease and economy. The principles on wdiich this system is based, and the mode in which it is carried out in practice having now been described, a short account of its introduction may be of interest. The arrangement, as mentioned, was designed early in 1880,