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ELEMENTARY SCIENCE 



diately admitted on the other side, by means of valves, 

 forces the piston back. Thus a powerful motion is pro- 

 duced. Thus locomotives are driven at high rates of speed 

 and draw heavy trains. You have seen the rapid motion of 

 the shaft of locomotives which runs from the cylinder to the 

 great driving-wheels. The force which turns the wheels 

 comes from that cylinder in which the piston moves rapidly 

 to and fro, the pressure of steam against it being controlled 



FIG. 33. The locomotive. 



by the engineer. You have seen the used steam as it 

 escapes, hissing, from the ends of the cylinders. Some of 

 it goes up the stack and escapes with the smoke. Study 

 Fig. 33- 



A steam-engine may be defined as a machine for convert- 

 ing heat energy into mechanical motion through the medium 

 of steam. It consists of the fire-box and boiler (where the 

 steam is generated) and the engine proper, where the 

 steam acts on a piston, producing motion. Machines in 

 which the steam acts on blades set on a rotating wheel 

 instead of on pistons are called steam-turbines, which you 

 have already noted, 



