EVAPORATING POWER FURNACE. 



rendered compatible with great power of evaporation by expe- 

 dients which shall produce, in a small furnace, an extremely 

 intense combustion, and which shall ensure the transmission to 

 the water completely, and immediately, of the heat developed in 

 such combustion. 



17. The heat developed in the combustion of fuel in a furnace 

 is propagated in two ways. A part radiates from the vivid fuel 

 in the manner, and according to nearly the same laws which 

 govern the radiation of light. These rays of heat, diverging in 

 every direction from burning fuel, strike upon all the surfaces 

 which surround the furnace. Now, as it is essential that they 

 should be transmitted immediately to the water in the boiler, it 

 follows that the furnace ought to be surrounded on every side 

 with a portion of the boiler containing water ; in short, a hollow 

 casing of metal, filled with water, ought to surround the fire- 

 place. . By this expedient, the heat radiating from the fuel, strik- 

 ing upon the metal which forms the inner surface of such casing, 

 will enter the water, and become efficient in producing 

 evaporation. 



Whatever then be the particular form given to the engine, the 

 furnace must be surrounded by such a casing. This casing is 

 called the FIRE-BOX. The bottom of it is occupied by a grate, 

 which should consist of bars sufficiently deep to prevent them 

 from being fused by the fuel which rests upon them, having 

 sufficient space between them to allow the air to enter so freely 

 as to sustain the combustion, but not such as to allow the unburnecl 

 fuel to fall through them. 



18. The limited magnitude of locomotive boilers renders the 

 construction of the extensive flues used in stationary boilers 

 impracticable ; and accordingly, in the early engines, a great 

 waste of heat was occasioned, owing to the flame and heated air 

 being permitted to issue into the chimney before their tempera- 

 ture was sufficiently reduced by contact with the flues. 



At length an admirable expedient was adopted which com- 

 pletely attained the desired end. The boiler was traversed by a 

 considerable number of small tubes of brass or copper, running 

 parallel to each other from end to end, the furnace being at one 

 end of the boiler, and the chimney at the other. The flame and 

 heated air which passed from the furnace had no other issue to 

 the chimney except through these tubes, It was thus driven, in 

 a multitude of threads, through the water. The magnitude and 

 number of the tubes was so regulated, that when the air arrived 

 at the chimney, it had given out as much of its heat as was prac- 

 ticable to the water. 



The full importance of this expedient was not appreciated until 



121 



