THE AVAILABLE ENERGY OF NATURE. 9 i 



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 ac- 

 cumulator, the little want to let the thing be done is cheap windmills. 

 A Faure cell containing twenty kilogrammes of lead and minium, 

 charged and employed to excite incandescent vacuum-lamps, has a 

 light-giving capacity of sixty-candle hours (I have found considerably 

 more in experiments made by myself, but I take sixty as a safe esti- 

 mate). The charging may be done uninjuriously, and with good dy- 

 namical 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 

 wastefully, 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 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 kilo- 

 gramme-cells, charged when the windmill works for five or six hours at 

 any time, and left with its sixty candle-hours' capacity to be used six 

 hours a day for five days, gives a two-candle light. Thus, thirty-two 

 such accumulator-cells so used would give as much light as four burn- 

 ers of London sixteen-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 horse-power as the windmill when in 

 good action. But windmills as hitherto made are very costly ma- 

 chines, 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 or factories of any kind, water-power is 

 thoroughly appreciated. From time immemorial, water-motors have 

 been made in large variety for utilizing rain-power in the various con- 

 ditions in which it is presented, whether in rapidly-flowing rivers, in 

 natural waterfalls, or stored at heights in natural lakes or artificial res- 

 ervoirs. Improvements and fresh inventions of machines of this class 

 still go on, and some of the finest principles of mathematical hydro- 

 dynamics have, in the lifetime of the British Association, and, to a 

 considerable degree, with its assistance, been put in requisition for per- 

 fecting 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, 



