294 



Introduction to the Study of Science 



In diagram 3, the valve at the left is open, admitting steam 

 for the return stroke. The used steam is expelled through port 

 B into the exhaust chamber. In diagram 4, the valve cuts off 

 the supply of live steam before the stroke is more than half 



completed. The valve may 

 be regulated to cut off the 

 live steam when the piston 

 stroke is one third, one 

 fourth, or one fifth com- 

 plete. This is done to ob- 

 tain the expansion of the 

 steam to three, four, or five 

 times its original volume in 

 the cylinder. The expan- 

 sion utilizes heat energy 

 which, aside from the loss 

 in other ways, is equivalent 

 to the work performed on 

 the piston and through it 

 on the driving crank or 

 wheels of a locomotive. 



The slide valve in the 

 steam chest is operated by 

 an eccentric. The eccentric 

 is either a wheel or a crank 

 attached to the shaft of the 

 driving wheels of a loco- 

 motive. The wheel eccen- 

 tric may be easily observed 

 on a stationary engine. 

 On late types of the compound locomotive what is known as 

 the Walschaert's valve gear is generally used. This makes use 

 of a crank and because of its action is popularly called the 

 "grasshopper gear." The external features may be noted in 

 Fig. 92. 



FiQ. 93. Diagram to show the operation 

 of the steam cylinder. 



