9 2 THE POPULAR SCIENCE MONTHLY. 



or may we look to the collection of rain-water in tanks placed arti- 

 ficially at sufficient heights over flat country to supply motive power 

 economically by driving water-wheels ? To answer it : Suppose a 

 height of one hundred metres, which is very large for any practical 

 building, or for columns erected to support tanks ; and suppose the 

 annual rainfall to be three quarters of a metre (thirty inches). The 

 annual yield of energy would be seventy-five metre-tons per square 

 metre of the tank. Now, one horse-power for 365 times twenty-four 

 hours is 236,500 foot-tons ; and, therefore (dividing this by 75), we 

 find 3,153 square metres as the area of our supposed tank, required for 

 a continuous supply of one horse-power. The prime cost of any such 

 structure, not to speak of the value of the land which it would cover, 

 is utterly prohibitory of any such plan for utilizing the motive power 

 of rain. We may or may not look forward hopefully to the time when 

 windmills will again " lend revolving animation " to a dull, flat coun- 

 try ; but we certainly need not be afraid that the scene will be marred 

 by forests of iron columns taking the place of natural trees, and gigan- 

 tic tanks overshadowing the fields and blackening the horizon. 



To use rain-power economically on any considerable scale we must 

 look to the natural drainage of hill-country and take the water where 

 we find it either actually falling or stored up and ready to fall when a 

 short artificial channel or pipe can be provided for it at moderate cost. 

 The expense of aqueducts, or of underground water-pipes, to carry wa- 

 ter to any great distance any distance of more than a few miles or a 

 few hundred yards is much too great for economy when the yield to 

 be provided for is power ; and such works can only be undertaken 

 when the irate)' itself is what is wanted. Incidentally, in connection 

 with the water-supply of towns, some part of the energy due to the 

 head at which it is supplied may be used for power. There are, how- 

 ever, but few cases (I know of none except Greenock) in which the en- 

 ergy to spare over and above that devoted to bringing the water to 

 where it is Avanted, and causing it to flow fast 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 working 

 the bellows of an organ, or " hair-brushing by machinery," and involve 

 simply throwing away the used water. The cost of energy thus ob- 

 tained must be something enormous in proportion to the actual quan- 

 tity of the energy, and it is only the smallness of the quantity that 

 allows the convenience 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 very 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 selection beside waterfalls, 



