252 



SCIENCE OF HOME AND COMMUNITY 



gineer to regulate the amount of steam that is admitted 

 to the cylinder. When the train is starting, a large amount 

 of power is needed, and so steam is admitted during nearly 

 the whole length of the stroke of the piston. As the train 

 gets under way, the steam is admitted for a shorter and 

 shorter portion of the stroke, until when under full head- 

 way only a small amount of steam is admitted. When the 

 lever is set in the center no steam at all is admitted to the 

 cylinder ; then the locomotive goes on its own momentum. 



The most common method of controlling the motion of 

 the link is by means of a lever in the cab, which is worked 



FIG. 95. Thirty years in locomotive building. 



by hand and held in place by means of a spring clip fitting 

 into notches. In some locomotives a wheel and screw gear 

 is used in place of the lever. In still other cases the revers- 

 ing gear is operated by means of compressed air. 



Driving wheels. The motion of the piston rod is com- 

 municated to the locomotive by means of large driving 

 wheels, varying from 5 to 7 feet in diameter. In the first 

 locomotives built, a single pair of drivers was used ; but in 

 later types two and even three driving wheels are used on 

 each side, these being joined by means of a shaft so that 

 they rotate together. This gives more points of contact 



