398 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK IT. 



The object of these arrangements is easy to understand. It is 

 to utilize as far as possible the heat arising from the fire, whether 

 this is accomplished by the contact, or direct action of the flame, or by 

 the gases of combustion, which, although not luminous, contain not- 

 withstanding an enormous quantity of heat. This heat, therefore, 

 would be entirely lost if the gases as they left the fire were allowed 

 to escape immediately into the open air. 



It is the same idea which led to the invention of the heaters. The 

 original boilers were hemispherical on their lower side, thus presenting 

 but little surface to the action of the fire considering the mass of water 



to be vaporised. To increase 

 the heating surface of the 

 boilers was one of the first 

 improvements which the con- 

 structors of steam-engines 

 (Watt being the first) at- 

 tempted to make. The object 

 is simply to economize the 

 fuel a problem the solution of 



which, after much successful 

 research and progress having 

 been made, is still the desi- 

 deratum in those industries 

 which employ steam power. 



It seems, after what \ve 

 have just said, that if the 

 gases of combustion, when 



they arrive at the base of the chimney, could be cooled down to 

 the temperature of the external air, all the heat would be utilized, 

 since the heat of the fuel could have been entirely extracted. But 

 this unfortunately is impossible ; or at least, if this result could 

 be obtained, the draught and the renewal of air necessary for 

 the continuation of the combustion would cease, or would be 

 at least considerably diminished. If the coal burnt badly, and 

 the heat of the furnace were not sufficiently intense, the hydro- 

 carbon gases, which are disengaged in great abundance from the 

 fuel, could not themselves be completely burnt. It is these that 

 form the thick and black smoke which comes out so profusely 



FIG. 275. Boiler with two heaters (cross section). 



