MECHANICS 77 



valves which hinder the moving water from returning 

 while allowing it to pass freely in the opposite direction ; 

 by this means it can be raised to a much greater height 

 than that from which it falls. It is so arranged 

 that a stream of water is made, by its descent, to 

 open and close a valve, which, each time it shuts, 

 drives a portion of the water up another tube and 

 to a higher level, where it is again retained by a 

 valve, and so on. 



In water-wheels, the liquid acts as a moving power by 

 its weight, its momentum, or by both of them combined, 

 acting on such wheels. 



In the first case, a wheel is provided at its cir- 

 cumference with troughs into which the water is 

 received near the level of the axle of the wheel ; the 

 vessels thus filled becoming heavier than those on the 

 other side, the wheel is made to revolve by mere excess 

 of weight. 



In the second case, the water may fall into the 

 troughs from a more or less considerable height above 

 the axle, so as to add the increased effect of the 

 momentum gained by it in its fall. This is called an 

 overshot wheel. An undershot one has fiat projections 

 from its circumference, while its lower portion is plunged 

 in a stream capable of turning it. 



We must now pass to the last subdivision of mechanics, 

 namely, that which relates to aeriform fluids, and which 

 is known as the science of pneumatics. 



Aeriform bodies differ greatly in nature from solids 

 and liquids. In solids, the particles of which we 

 may conveniently suppose them to consist, are stably 

 held together or, as we have seen, cohere in varying 

 degrees of tenacity. In liquids, the particles still cohere, 

 but so unstably that they glide over each other with 



