PRODUCTION OF MECHANICAL EFFECT. 447 



enough for convenience at every opened tap in 

 every house or factory, is enough to make it worth 

 while to make arrangements for letting the water- 

 power be used without wasting the water- substance. 

 The cases in which water-power is taken from a 

 town supply are generally very small, such as work- 

 ing the bellows of an organ, or " hair-brushing by 

 machinery," and involve simply throwing away the 

 used water. The cost of energy thus obtained 

 must be something enormous in proportion to the 

 actual quantity of the energy, and it is only the 

 smallness of the quantity that allows the conveni- 

 ence of having it when wanted at any moment, to 

 be so dearly bought. 



For anything of great work by rain-power, the 

 water-wheels must be in the place where the water 

 supply with natural fall is found. Such places are 

 generally far from great towns, and the time is not 

 yet come when great towns grow by natural selec- 

 tion beside waterfalls, for power; as they grow 

 beside navigable rivers, for shipping. Thus hitherto 

 the use of water-power has been confined chiefly to 

 isolated factories which can be conveniently placed 



