CHAP. V. 



THE STEAM-ENGINE. 



399 



whenever a fresh supply of fuel is introduced into the furnace and 

 cools it. 



The hot gases, in escaping up the chimney, serve to improve the 

 draught. It is a loss which, within certain limits, is necessary, although 

 the direct result is neither to heat the water nor to produce steam. It 

 thus often happens that in industrial processes an innovation, which 

 seems to be an advance from one point of view, is retrograde from 

 another point of view. 



Fir;. 276. Boiler with two heiters (longitudinal section). 



A. Float and al.-irm whistle. B. Heater. C. Body of the boiler. E. Supply pipe.- F. Float to indicate 

 the level of the water H. Man-hole for cleaning S S. Safety valves. R. Damper for regulating 

 draught. U. Chimney. V. Steam-pipe.- C C. Flues. I. Water-gauge. G. Furnace. P. Furnace- 

 door. 



It is time to say a word about the chimney, which plays so large 

 a part in keeping up the draught. The higher the chimney is, its 

 diameter and the rest of the conditions of combustion remaining the 

 same, the better is the draught. It is found by experience that the 

 height of the chimney should be proportionate to the square of the 

 intensity of the draught. 



The draught, that is, the volume of air passing, depends upon 

 the height of the chimney and on the area of its cross section. 



