286 ON THE CONSERVATION OF FORCE. . 



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 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 domestic 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 in- 

 troduced among the Romans 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, spinning- frames and 

 looms, and so forth. It is the cheapest of all motive powers, it 

 flows spontaneously from the inexhaustible stores of Nature ; but 

 it is restricted to a particular place, and only in mountainous 

 countries is it present in any quantity ; in level countries exten- 

 sive reservoirs are necessary for damming the rivers to produce 

 any amount of water-power. 



Before passing to the discussion of other motive forces I 

 must answer an objection which may readily suggest itself. 

 "We all know that there are numerous machines, systems of 

 pulleys, levers and cranes, by the aid of which heavy burdens 

 may be lifted by a comparatively small expenditure of force. 

 We have all of us often seen one or two workmen hoist heavy 

 masses of stones to great heights, which they would be quite 

 unable to do directly ; in like manner, one or two men, by means 

 of a crane, can transfer the largest and heaviest chests from 



