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, compared 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 enumerated, 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 1620 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-kilogrammes 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 costliness of dock-construction, is obviously prohibitory of every scheme for economizing tidal energy by means of artificial dock-basins, however 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