434 



NATURE 



\_Sept. 8, 1 88 1 



obtained from tbem is very inconsiderable, compared with that 

 which is obtained from sources belonging to the two cla ses 

 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 summarise the natural sources of energy as 

 Tides, Food, Fuel, Wind, and Rain. 



Among the practical sources of enei-gy thus exhaustively enu 

 merated, 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 u efully 

 are very rare, and the while 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 utilised to the 

 utmost by means of ideal water-wheels doing with perfect eco- 

 nomy the whole work of filling and emptying a dock-bLisi'n 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 -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 practical measurements we are irre- 

 sistibly drawn, notwithsta iding a dense barrier of insular preju- 

 dice most detrimental to the islander,), — 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 tv\o 

 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 1000 kilogrammes used in this reckoning is 

 0"984 of the British ton. 



Returninj to the question of utilising 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, con- 

 sidering the vait costliness of dock construction, is obviously 

 prohibitory of every scheme for economising tidal energy by 

 means of artificial dock-ba-ins, however near to the ileal perfec- 

 tion might be the realised tide-mill, and however convenient and 

 uon-wasteful the accumulator — whether Faui'e's electric accumu- 

 lator, or other accumulators of energy hitherto invented or to 

 be invented — which mi^lit be used to store up the energy yielded 

 by the tide-mill during its short harvests about the times uf high 

 and low \^'ater, and to give it out \^hen wanted at other times of 

 the six hours. There may however be a dozen places po sible in 

 the world where il onld 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 d'jne, 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 agricultural measure for the area of 162,000 square 

 metres) or 100 horse-power more valuable. The annml 

 cost of 103 horse-power night and day, for 365 days of the year, 

 obtained through steam from coals, miy be about ten times the 

 rental of forty acres at 2/. or 3/. per acre. But the value of 

 land is e^sentiaky much more than its rental, and the rental of 

 1 rad is apt to be much more than 2I. 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 the answer may be one 

 Jiundred Jiorse-poiver, and in another fo7-ty acres. But, indeed, 

 the question is hardly worth answering, considering the rarity 

 of the cases, if they exist at all, where embankments for the 

 utilisation of tidal energy are practicable. 



Turning now to sources of energy derived from sun-heat, let us 

 take the wind first. When we look at the register of British 

 shipping and see 40,000 vessels, of which about 10,000 are 

 steamers and 30,000 sailing ships, and when we think how vast 

 an absolute amount of horse-power is developed by the engines 

 of those steamers, and how considerable a proportion it form; 

 of the whole horse-power taken from coal annually in the whole 

 world at the present time, and when we consider the sailing 

 ships of other nations, which must be reckoned in the account, 

 and throw in the little item of windmills, we find that, even in 

 the present days of steam ascendency, old-fashioned Wiad still 

 supplies a large part of all the energy used by man. But how- 

 ever much w-e may regret the time when Hood's young lady, 

 visiting the fens of Lincolnshire at Christmas, and writing; to her 

 dearest friend in London (both si.\ty years old now if they are 



alive), describes the delight of sitting in a bower and looking over 

 the wintry plain, not desolate, because " windmills lend revolving 

 animation to the scene," we cannot ihut our eyes to the fact of a 

 lamentable decadence of wind-power. Is this decadence permanent, 

 or may we hope that it is only temporary ? The subterranean coal- 

 stores of the world are becoming exhausted surely, and not slowly, 

 and the price of Ciial is upward bound — upward botmd on the 

 whole, tliough no doubt it will have its ups and downs in the 

 future as it has had in the past, and as mnst lie the case in 

 respect to every marketable commodity. When the coal is all 

 burned ; or, long before it is all burned, when there is so little of 

 it left and the coal-mines from which that little is to be exca- 

 vated are so distant and deep and hot that its price to the con- 

 sumer is greatly higher than at present, it is most probable thai 

 windmills or wind-motors in sone form will again be in the 

 ascendant, and that wind will do man's mechanical work on land 

 at least in proporti m comparable to its present doing of work 

 at sea. 



Even now it is not utterly chimerical to think of wind super- 

 seding coal in some places for a very important part of its 

 present duty — that of giving light. Indeed, now that we have 

 dynamos and Faure's accumulator, the little \^■ant to let the thing 

 be done is cheap windmills. A Faure cell containing 20 kilo- 

 grammes of lead and minium charged and employed to excite 

 incandescent vacuum-lanps has a light-giving capacity of 60- 

 candle hours (I have found considerably more in experiments 

 made by myself; but I take 60 as a safe estimate). The 

 charging may be done uninjuriously, and with good dynamical 

 economy in any time from six hours to twelve or more. The 

 drawing-off of the charge for use may be done safely, but some- 

 what wastefuUy, in two hours, and very economically in any 

 time of from five hours to a week or more. Calms do not last 

 often longer than three or four days at a time. Suppose then 

 that a five days storage-capacity suflices (there may be a little 

 steam-engine ready to be set to work at any time after a four- 

 days' calm, or the user of the light may have a few candles or 

 oil-lamps in reserve, and be satisfied with them when the wind 

 fails for more than five days). One of the twenty kilogramme 

 cells charged when the windmill works for five or six hiurs 

 at aity time, and left with its 60 candle-hours' capacity to be 

 used six hours a day for five days, gives a 2-candle light. Thus 

 thirty-two such accumulator cells so used would give as much 

 light as four burners of London i6-candle gas. The probable 

 cost of dynamo and accumulator does not seem fatal to the plan, 

 if the windmill could be had for something comparable with the 

 prime cost of a steam-engine capable of working at the same 

 horsepower as the windmill when in good action. But wind- 

 mills as hitherto ma'le are very costly machines, and it does not 

 seem probable that, without inventions not yet made, wind can 

 be economically used to give light in any considerable class of 

 cases, or to put energy into store for work of other kinds. 



Consider, lastly, rain-power. When it is to be had in places 

 where power is winted for mills or factories of any kind, water- 

 power is thorougUy appreciated. From tiue immemorial, 

 water-mo'ors have been made in large variety for utilising rain- 

 power in the various conditions in which it is presented, whether 

 in rapidly-flowing rivers, in natural waterfalls, or stored at 

 heights in natural lakes or artificial reservoirs. Improvements 

 and fresh inventions of machines of this class still go on, and 

 some of the finest principles of mathematical hydrodynamics 

 have, in the lifetime of the British Association, and, to a con- 

 sidera'ile degree, Avith its assistance, been put in requisition for 

 perfecting the theory of hydraulic mechanism and extending its 

 practical applications. 



A first question occurs : Are we necessarily 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 100 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 (30 inches). The annual yield of energy would be 

 75 metre-tons per square metre of the tank. Now one horse- 

 power for 365 times 24 hours is 236,500 foot-tons ; and there- 

 fore (dividing this by 75) we find 3153 square metres as the area 

 of our supposed tank required for a continuous supply of one 

 horse-pov\er. The prime cost of any such structure, not to 

 speak of the value of the land which it would cover, is utterly 

 prohiliitory of any such plan for utilising the motive power of 



