PRODUCTION OF MECHANICAL EFFECT. 445 



limited to such natural sources of water-power as 

 are supplied by rain falling on hill-country, or may 

 we look to the collection of rain-water in tanks 

 placed artificially at sufficient heights over flat 

 country to supply motive power economically by 

 driving water-wheels ? To answer it : Suppose a 

 height of IOO metres, which is very large for any 

 practicable building, or for columns erected to sup- 

 port 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 24 hours is 2,365,000 metre- 

 tons ; and therefore (dividing this by 75) we find 

 31,530 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 

 utilising 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 country ; but we certainly need not 



