ON THE CONSERVATION OF FORCE 



189 



weighted by the water contained in them, the latter not; 

 the weight of the water acts continuously on only one side 

 of the wheel, draws this down, and thereby turns the wheel; 



FIG. 90 



the other side of the wheel offers no resistance, for it con- 

 tains no water. It is thus the weight of the falling water 

 which turns the wheel, and furnishes the motive power. But 

 you will at once see that the mass of water which turns the 

 wheel must necessarily fall in order to do so, and that 

 though, when it has reached the bottom, it has lost none of its 

 gravity, it is no longer in a position to drive the wheel, if it 

 is not restored to its original position, either by the power 

 of the human arm or by means of some other natural force. 

 If it can flow from the mill-stream to still lower levels, it 



