MECHANICAL VALUE OF COAL. 87 



eration, although comparative figures from practice ai"e not attainable. 

 Making a clear distinction between combustion of carbon (coke) 

 and the formation of flame, he emphasizes the statement that heat is 

 increased, while smoke and light are lessened, by the bni-ning of any 

 flame in an atmosphere at high mountain elevations, or — what 

 amounts to the same thing — with air at less than normal barometric 

 pressure, and asserts that one of the first requirements for pei-fect 

 combustion is air at low tension ; and to readily attain it at ordinary • 

 levels, he dispenses with chimney draught, and substitutes fan-suction 

 at the forward end of the furnace or boiler. 



Through tubes that form the furnace grate-bars air, partially 

 warmed in its passage, is drawn by the fan-suction into the combus- 

 tion chamber, in front of a pei-f orated fire-brick wall (or septum) 

 dividing the combustion chamber from the furnace proper ; and 

 large amouiits of this warmed air are permitted to enter the chamber 

 after each supply of fresh coal, the volatile matter of which at once 

 undergoes rapid distillation on touching the incandescent coke, with 

 the natural effect of cooling down the fire ; but to avoid any addi- 

 tional cooling, and consequent formation of smoke, the supply of air 

 usually flowing into the ash-pit, and thence through the fire-grate, 

 is completely cut off; one lever, controlling the upper and lower 

 damper slides, being so connected, that when a liberal supply of air 

 is passing through the hollow grate-bars and diffusing itself forward 

 of the septum wall, the ash-pit is sealed air-tight ; the amounts of 

 air delivered being steadily varied, and eventually reversed in rela- 

 tive quantity, as the lever is drawn over, fully opening the ash-pit 

 dauiper when thei-e is nothing but coke to burn on the grate. 



Analogy, di-awn from the Bunsen burner, is used as an argument 

 for dividing and distributing the incoming warm air at numerous 

 points ; the openings in the septum wall being so arranged as to give 

 at once the most complete intermixture of the liberated hydrogen 

 and hydrocarbon gases, with the warmed air entering through the 

 numerous hollow grate-bars. 



The theory of his feedwater heater, or "Atheruious Superheater" — 

 although only indirectly connected with combustir i — is of intei'est, 

 and is based on Tyndal's statement, that dry lir freely permits 

 through itself tlie passage of radiant heat wi* lOUt loss of heat by 

 absorption ; whereas saturated or even moist air is a i-apid and effec- 

 tive absorbent of radiant heat. Conversely, the radiating power of 



