260 Our Surroundings 



The great power that locomotives exert is due to the great 

 expansive force of steam. A given quantity of water, heated 

 to steam, will occupy over 1,700 times its original space, if left 

 free to expand. If confined in a small space it necessarily exerts 

 tremendous pressure. 



While the modern locomotive is a very complicated piece of 

 machinery, the really vital parts are relatively few. There is the 

 fire box, where coal or oil is burned; the tubular boiler, a mass 

 of parallel tubes within a metal cylinder in which water is 

 heated and turned to steam; the cylinders, in which the steam's 

 energy is changed to motion, and the drive wheels, which use this 

 energy of motion to move the train. 



International Newsreel. 



A MODERN STEAM LOCOMOTIVE 



The fire is made intensely hot by a draft caused by forcing 

 used steam out through the smoke stack, thus creating a partial 

 vacuum. This draws air through the fire. Steam is generated, 

 stored in the steam chests, and admitted to the cylinders by 

 sliding valves. The amount of steam admitted is controlled 

 largely by a throttle in the engine cab. 



Steam is admitted into each cylinder in front of the piston, 

 a solid, cylindrical block capable of sliding back and forth in 

 the cylinders. The expansion of the steam forces the piston 

 sharply back within the cylinder. Then the inlet valve closes 



