THE AVAILABLE ENERGY OF NATURE. 89 



restrial sources, including mountain quarries and mines, the heat of 

 hot springs, and the combustion of native sulphur perhaps, also, the 

 combustion of inorganic native combustibles are actually used ; but 

 the mechanical effect obtained from them is very inconsiderable, com- 

 pared with that which is obtained from sources belonging 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 available to man for practical 

 purposes." 



Thus, we may summarize the natural sources of energy as tides, 

 food, fuel, wind, and rain. 



Among the practical sources of energy thus exhaustively enumer- 

 ated, there is only one not derived 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 metres' rise and fall, if 

 we imagine it utilized 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 i6 t 20 of 

 the work of a horse-power. Parenthetically, in explanation, I may 

 say that the French metrical equivalent (to which, in all scientific and 

 practical measurements, we are irresistibly drawn, notwithstanding a 

 dense barrier of insular prejudice most detrimental to the islanders) 

 the French metrical equivalent of James Watt's " horse-power " of 

 550 foot-pounds per second, or 33,000 foot-pounds per minute, or 

 nearly two million foot-pounds per hour, is seventy-five metre-kilo- 

 grammes per second, or four and a half metre-tons per minute, or two 

 hundred and seventy metre-tons per hour. The French ton of one 

 thousand kilogrammes, used in this reckoning, is 0'984 of the British 

 ton. 



Returning to the question of utilizing tidal energy, we find a dock 

 area of 162,000 square metres (which is a little more than 400 metres 

 square) required for 100 horse-power. This, considering the vast cost- 

 liness of dock-construction, is obviously prohibitory of every scheme 

 for economizing tidal energy by means of artificial dock-basins, how- 

 ever near to the ideal perfection might be the realized tide-mill, 

 and however convenient and non-wasteful the accumulator whether 

 Faure's electric accumulator or other accumulators of energy hitherto 

 invented or to be invented which might be used to store up the energy 

 yielded by the tide-mill during its short harvests about the times of 

 high and low water, and to give it out when wanted at other times of 

 the six hours. There may, however, be a dozen places possible in the 

 world where it could be advantageous to build a sea- wall across the 



