ON THE CONSERVATION OF FORCE. 327 



side turned to the observer, and empty on the other side. 

 Thus the former are weighted by the water contained in 

 them, the latter not ; the weight of the water acts con- 

 tinuously on only one side of the wheel, draws this down, 

 and thereby turns the wheel ; the other side of the wheel 

 offers no resistance, for it contains no water. It is thus 

 the weight of the falling water which tm'ns the wheel, 

 and furnishes the motive power. But you will at once see 

 that the mass of water which turns the wheel must neces- 

 sarily 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 Avheel, 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 may be used to work other wheels. But when it has 

 reached its lowest level, the sea, the last remainder of 

 the moving force is used up, which is due to gravity — 

 that is, to the attraction of the earth, and it cannot act 

 by its weight until it has been again raised to a high level. 

 As this is actually effected by meteorological processes, 

 you will at once observe that these are to be considered as 

 sources of moving force. 



Water-power was the first inorganic force which man 

 learnt to use instead of his own labour or of that of domes- 

 tic animals. According to Strabo, it was known to King 

 Mithridates, of Pontus, who was also otherwise celebrated 

 for his knowledge of nature ; near his palace there was a 

 water-wheel. Its use was first introduced among the 

 Eomans in the time of the first Emperors. Even now we 

 find water-mills in all mountains, valleys, or wherever 

 there are rapidly-flowing, regularly-filled, brooks and 

 streams. We find water-power used for all purposes which 

 can possibly be effected by machines. It drives mills 

 which grind corn, saw-mills, hammers and oil-presses, 



