330 



APPLIED SCIENCE 



admits steam which pushes the piston back again to its 

 original position. 



The back and forth movement, thus imparted to the piston 

 by the steam, is transmitted to the crank and then to the 

 heavy fly-wheel. The fly-wheel by means of belting or rope 

 transmits motion to the smaller wheels or pulleys which drive 

 the machines in factories. 



After moving the piston, the steam either escapes into the 

 air, as it does in the case of a steam locomotive, or passes into 

 one or more other cylinders where it exerts its force until it 

 condenses. An engine that allows steam to escape into one 

 cylinder only, is called a simple engine. If the steam expands 

 twice it is called a compound expansion, and if it expands 

 three times it is called a triple expansion. 



369. Purpose of a Governor. The governor (Fig. 170) 

 of a steam engine is a device which controls the supply of 

 steam by letting into the cylinder just 

 the right quantity. In the pipe which 

 carries the steam from the boiler to 

 the cylinder is a valve called the 

 throttle valve, by which the com- 

 munication between boiler and engine 

 may be opened or closed. A rod con- 

 nects this valve to the governor, 

 which is made to turn round by a 

 belt from the crank shaft. The faster 

 the crank shaft turns the faster the 



governor goes round. At the lower 

 FIG. 170.- Governor. . , 



end of the governor are two heavy 



balls, so hung that as the speed of the governor increases they 

 swing out farther from the center rod and as it slows do.wn 



