290 



ON THE CONSERVATION OF FORCE. 



tion, which he would have to use if the handle were directly 

 applied to the barrel, D. In all these machines, and parts of 

 machines, we find it confirmed that in proportion as the velocity 

 of the motion increases its power diminishes, and that when the 

 power increases the velocity diminishes, but that the amount of 

 work is never thereby increased. 



In the overshot mill-wheel, described above, water acts by 

 its weight. But there is another form of mill-wheels, what is 



FIG. 42. 



called the undershot wheel, 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 instance, 

 on the Rhine. In the immediate neighbourhood of such a wheel, 

 the water need not necessarily have a great fall if it only strikes 



