TBANSACTIONS OF SECTION A. 515 



the answer may be one hundred horse-power, and in another forttj 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 forms 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 Wind still supplies a large part of all the 

 energy used by man. But however much we 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 sixty 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 shut 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 coal is upward bound — upward bound on the whole, though no doubt it will 

 have its ups and downs in the future as it has had in the past, and as must be 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-m'ines 

 from which that little is to be excavated are so distant and deep and hot that its 

 price to the consumer is greatly higher than at present, it is most probable that 

 vdndmills or wind-motors in some form will again be in the ascendant, and that 

 wind will do man's mechanical work on land at least in proportion comparable to 

 its present doing of work at sea. 



Even now it is not utterly chimerical to think of wind superseding 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 want to let 

 the thing be done is cheap vsdndmills. A Faure cell containing 20 kilogrammes 

 of lead and minium charged and employed to excite incandescent vacuum-lamps 

 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 vsdth 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 somewhat 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 suffices 

 (there may be a little steam-engine ready to set to work at any time after a four- 

 days' calm, or theuser 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 

 hours at any 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 

 80 used would give as much light as four burners of London 16-candle gas. The 

 probable cost of dynamo and accumulator does not seem fatal to the pfan, if the 

 windmill could be had for something comparable with the prime cost of a steam- 

 engine capable of working at the same horse-power as the windmill when in good 

 action. But windmills as hitherto made 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 

 wanted for mills and factories of any kind, water-power is thoroughly appreciated. 

 From time immemorial, water-motors have been made in large variety for utilis- 



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