332 



ON THE CONSERVATION OP FORCE. 



what is called the undershot wheels in which it only acts 

 by its impact, as represented in Fig. 42. These are used 

 where the height from which the water comes is not great 

 enough to flow on the upper part of the wheel. The 

 lower part of undershot wheels dips in the flowing water 

 which strikes against their float-boards and carries them 

 along. Such wheels are used in swift-flowing streams 

 which have a scarcely perceptible fall, as, for instancej on 



Fig. 42. 



the Ehine. In the immediate neighbourhood of such a 

 wheel, the water need not necessarily have a great fall if 

 it only strikes with considerable velocity. It is the velo- 

 city of tlie water, exerting an impact against the float- 

 boards, which acts in this case, and which produces the 

 motive power. 



Windmills, which are used in the great plains of Holland 

 and North Grermaiiy to supply the want of falling water, 

 aft'ord another instance of the action of velocity. The 



