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CHAPTER LIX 



THE STEAM BOILER 



The Steam Power Plant. A steam power plant consists 

 essentially of two parts, the steam boiler, for generating steam 

 by the combustion of fuel; and the steam engine, which con- 

 verts into work the energy contained in the steam. It is 

 customary, however, to refer to the entire steam plant as the 

 steam engine, when the plant is small. When the boiler and 

 engine are mounted on wheels and arranged with suitable 

 gearing for propelling itself as well as for drawing loads, the 

 outfit is referred to as a traction engine. Of late years it 

 has become customary to refer to the steam traction engine as 

 the steam tractor. The subject of the steam power plant will 

 be divided into three parts, confined to as many chapters, as 

 follows: the steam boiler, the steam engine, and the steam 

 tractor. At one time the steam engine as defined above and 

 the steam tractor were the principal sources of power for 

 agricultural purposes, when large units were required. The 

 development of the internal-combustion engine and tractor 

 has been more rapid in recent years than that of the steam 

 engine and tractor. 



The Principle of the Steam Engine. The steam engine is 

 a heat engine, in that its function is to transfer the heat pro- 

 duced by the combustion of fuel, usually wood or coal, into 

 mechanical energy. It might be styled an external-combus- 

 tion engine, in that combustion takes place outside of the 

 boiler proper and the heat is absorbed by passing the hot 

 gases through tubes surrounded by water. 



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