190 HELMHOLTZ 



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 introduced 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 extensive reservoirs are necessary for dam- 

 ming 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 bur- 

 dens 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 a ship to the quay. Now, it may 

 be asked, If a large, heavy weight had been used for driving 

 a machine, would it not be very easy, by means of a crane 

 or a system of pulleys, to raise it anew, so that it could 

 again be used as a motor, and thus acquire motive power, 



