MECHANICAL VALUE OF COAL. 89 



a greater number of pounds of fuel per square foot of grate ; thus 

 economizing space and developing the greatest horse-power from the 

 smallest expenditure of capital in purchase of boiler. 



Large consumptions per square foot of grate have not, until 

 recently, proved economical in coal per horse-power developed, due 

 j)rincipal]y to the choked air supply, natural furnace draught being 

 unable to draw in sufficient to burn all the carbouic oxide evolved 

 from the deep mass of hot carbon without the temperature in the 

 chimney being so high as to waste the heat by throwing it out into 

 the atmosphere unabsorbed, or a chimney of excessive elevation and 

 cost was used. 



Mr. Howden recently, before the Institute of Naval Architects, 

 claimed to have secured per pound of Scotch coal an evaporation of 

 10 pounds of water (from 212°), consuming 30 ])0uiids of coal per 

 sc^uare foot of grate per hour. Tliis, using a compound cylindered 

 engine, developing a horse-power per 20 pounds of steam, gives Ift 

 horse-power per square foot of grate, with but a fraction over two 

 pounds of coal per one horse-power — a good showing. 



4th. Siemens.-— His system is by the partial combustion of cheap 

 coal in detached converters (with insufficient supply of oxygen^ to 

 change this solid fuel into gaseous fuel (C^O); conveying it thence by 

 pipes to the different furnaces and boilers throughout the workshops 

 or town, whei-e combustion is completed, and it is burnt from oxide 

 to acid (CO) by the addition of one more part of oxygen. The air 

 ducts in his regenerative furnace, being so arranged that the heat 

 otherwise wasted is utilized in raising the temperature of the inflow- 

 ing air, before it reaches the mixing chamber, almost up to igniting 

 point, and the absence of open grate, and control ovei- the compara- 

 tively small amount of air required, results in the highest working 

 temperature yet attained. When solid fuel is used and intense 

 temperatures are required, comparntively small grates are necessary ; 

 therefore, in the earlier Siemens practice, it was inferi'ed that in 

 burning gaseous fuel (CjO) a small combining chamber (with close walls 

 and low roof) vvas also necessary, this having the natural result of 

 bringing the flame into close contact with the walls, as well as with 

 the material to be heated. 



Later experiments have proved the complete fallacy of this idea, 

 Mr. F. Siemens having discovered that by simple radiation I'rom 



