194 



HELMHOLTZ 



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

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



FIG. 94 



what is called the undershot wheel, in which it only acts 

 by its impact, as represented in FIG. 94. 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 neighborhood of such a wheel, the water 

 need not necessarily have a great fall if it only strikes with 

 considerable velocity. It is the velocity of the water, exert- 

 ing 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 Germany to supply the want of falling water, 

 afford another instance of the action of velocity. The sails 

 are driven by air in motion by wind. Air at rest could 

 just as little drive a windmill as water at rest a water-wheel. 



