MAN'S IMPORTANT ENERGY TRANSFORMERS 471 



power of expanding steam. He arranged a cylinder con- 

 taining a piston so that the steam would be admitted alter- 

 nately on one side and then on the other side of the piston. 

 As the expanding steam forces the piston in one direction, 

 the used steam in front of the advancing piston escapes 

 through an open valve. When the piston reaches the end 



A SIMPLE WATER WHEEL USED FOR GRINDING CORN 



of its stroke, the moving valves cut off the steam from the 

 one side and allow it to enter the other, thus driving the 

 piston back again and forcing the used steam out through 

 the escape. This continuous back and forth movement of 

 the piston can best be understood by an examination of the 

 accompanying diagram. (Figure 147.) 



In recent years inventors have made it possible to apply 



