THE STEAM EXGI] 



-. . latter and countervailing advantages. 36. Condensing 

 engines. 37. Condensing apparatus. 38. Air-pump. 39. Cold water 

 pump. 40. Hot water pump. 



17. WHATEVER be the form of boiler used, its magnitude and 

 proportions, as well as those of the furnaces and their appendages, 

 must be determined by the rate at which the steam is required to 

 be produced, and in some degree also by the quality of the fuel. 



The principle upon which a chimney more or less lofty produces 

 a draft through the fuel in a fire-place in connection with it, has 

 been already explained in our Tract on " Fire." The chimney 

 connected with the furnace of a steam-boiler acts on the same 

 principle, and its dimensions and height must necessarily be pro- 

 portionate to those of the furnace, and to the quantity of fuel 1 o 

 be consumed in a given time. 



But since the evaporation produced in the boiler requires to be 

 varied with the varying work exacted from the engine ; and since 

 this evaporation will necessarily be proportionate to the rate at 

 which the fuel is consumed in the furnace, it follows that the rate 

 of combustion in the furnace should be varied with the varying 

 power to be exacted from the engine. In order, therefore, to 

 maintain this proportion between the force of the furnace and the 

 demands upon the engine, it is necessary to stimulate or mitigate 

 the furnace, as the evaporation is to be augmented or diminished. 



The activity of the furnace must depend on the current of air 

 which is drawn through the grate bars, and this will depend on 

 the magnitude of the space aflbrded for the passage of that current 

 through the flues. A plate called a damper is accordingly placed 

 with its plane at right angles to the flue, so that by raising and 

 lowering it in the same manner as the sash of a window is raised 

 or lowered, the space allowed for the passage of air through the 

 flue may be regulated. This plate might be regulated by the 

 hand, so that by raising or lowering it the draught might be 

 increased or diminished, and a corresponding effect produced on 

 the evaporation in the boiler : but the force of the fire is rendered 

 uniformly proportional to the rate of evaporation by the following 

 arrangement, without the intervention of the engineer. The 

 column of water sustained in the feed pipe (figs. 7, 8), represents 

 by its weight the difference between the pressure of steam within 

 the boiler and that of the atmosphere. If the engine consumes 

 steam faster than the boiler produces it, the steam contained in 

 .the boiler acquires a diminished pressure, and consequently the 

 column of water in the feed pipe will fall. If, on the other hand, 

 the boiler produce steam faster than the engine consumes it, the 

 accumulation of steam in the boiler will cause an increased 

 pressure on the water it contains, and thereby increase the height 

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