328 ox THE CONSERVATION OF FORCE. 



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 re- 

 stricted to a particular place, and only in mountainous 

 countries is it present in any quantity ; in level countries 

 extensive 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 trans- 

 fer 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, without being compelled to use a corre- 

 sponding exertion in raising the weight ? 



The answer to this is, that all these machines, in that 

 degree in which for the moment they facilitate the exer- 

 tion, also prolong it, so that by their help no motive power 

 is ultimately gained. Let us assume that four labourers 

 have to raise a load of four hundredweight, by means of 

 a rope passing over a single pulley. Every time the rope 

 is pulled down through four feet, the load is also raised 

 through four feet. But now, for the sake of comparison, 

 let us suppose the same load hung to a block of four 

 pulleys, as represented in Fig. 39. A single labourer 

 would now be able to raise the load by the same exertion 

 of force as each one of the four put forth. But when he 



