514 KEPORT — 1881. 



mechanical effect obtained by means of animals working, water-wheels worked by 

 livers, steam-engines, galvanic engines, windmills, and the sails of ships. 



'2. The motions of the earth, moon, and sun, and their mutual attractions, con- 

 stitute an important source of available mechanical effect. From them all, but 

 chiefly no doubt from the earth's motion of rotation, is derived the mechanical 

 effect of water-wheels driven by the tides. 



' 3. The other known sources of mechanical effect available to man are either 

 terrestrial — that is, belonging to the earth, and available without the influence of 

 any external body — or meteoric — that is, belonging to bodies deposited on the 

 earth from external space. Terrestrial 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 prabtical purposes.' 



Thus we may summarise the natural sources of energy as Tides, Food, Fuel^ 

 Wind, and Eain. 



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 prdctical, because tide-mills exist. But the places where they can work usefully 

 are very rare, and the whole amount of work actually done bj' 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 per- 

 fect 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 squai-e metre of area. This work done four times in the twenty-four hours 

 amounts to l-1620th of the work of a horse-power. Parenthetically, in explanation,, 

 I may say that the French metrical equivalent (to which in all scientific and prac- 

 tical measurements we are irresistibly drawn, notwithstanding a dense barrier of in- 

 sular 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 75 metre-kilogrammes 

 per second, or 4^ metre-tons per minute, or 270 metre-tons per hour. The French 

 ton of 1,000 kilogrammes used in this reckoning is 0-984 of the British ton. 



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

 162.000 square metres (which is 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 economising tidal energy bj^ means of 

 artificial" dock-basins, however near to the ideal perfection might be the realised 

 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 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 mouth of a natural basin or estuary, and to utilise the tidal energy of 

 filling it and emptying it by means of sluices and water-wheels. But if so much 

 could be done, it would in many cases take only a little more to keep the water 

 out altogether, and make fertile land of the whole basin. Thus we are led up to 

 the interesting economical question, whether is forty acres (the British a ffriculiural 

 measure for the area of 162,000 square metres) or 100 horse-power more valuable. 

 The annual cost of 100 horse-power night and day, for 365 days of the year, ob- 

 tained through steam from coals, may be about ten times the rental of forty acres 

 at 21. or 3/. per acre. But the value of land is essentially much more than its 

 rental, and the rental of land is apt to be much more than 21 or 3/. per acre in 

 places where 100 horse-power could be taken with advantage from coal through 

 steam. Thus the question remains unsolved, with the possibility that in one place 



