PRODUCTION OF MECHANICAL EFFECT. 437 



" to the two classes mentioned above. Meteoric 

 " sources, including only the heat of newly-fallen 

 " meteoric bodies, and the combustion of meteoric 

 " iron, need not be reckoned among those avail- 

 " able to man for practical purposes." 



Thus we may summarise the natural sources of 

 energy as Tides, Food, Fuel, Wind and Rain. 



Among the practical sources of energy thus 

 exhaustively enumerated, there is only one not de- 

 rived from sun-heat that is the tides. Consider 

 it first. I have called it practical, because tide- 

 mills exist. But the places where they can work 

 usefully are very rare, and the whole amount of 

 work actually done by them is a drop to the 

 ocean of work done by other motors. A tide of 

 two meters' rise and fall, if we imagine it utilised 

 to the utmost by means of ideal water-wheels 

 doing with perfect economy the whole work of 

 filling and emptying a dock-basin in infinitely 

 short times at the moments of high and low water, 

 would give just one metre-ton per square metre 

 of area. This work done four times in the 

 twenty-four hours amounts to i-i62Oth of the 



