476 



SCIENCE. 



ing economical question, whether is 40 acres (the British 

 agricultural 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, obtained through steam from coals, maybe about 

 ten times the rental of forty acres, at £2 or ^3 per acre. 

 But the value of land is essentially much more than its 

 rental, and the rental of lan9 is apt to be much more 

 than £2 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 hun- 

 dred horse power, and in another forty acres. But, in- 

 deed, the question is hardly worth answering, consider- 

 ing the rarity of the cases, it they exist at all, where em- 

 bankments for the utilization of tidal energy are prac- 

 ticable. 



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 steam- 

 ers, 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 ascendancy, 

 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 if they are now 

 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 deca- 

 dence of wind-power. Is this decadence permanent, or 

 may we hope that it is only temporary? The sub- 

 terranean coal stores of the world are becoming exhaust- 

 ed surely, and not slowly, and the price of coal is up- 

 ward bound — upward bound on the whole, though no 

 doubt it will have 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 mines, 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 wind-mills 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 propor- 

 tion comparable to its present doing of work at sea. 



Evennow.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, 

 dow that we have dynamos and Faure's accumulator, the 

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

 A Faure cell containing 20 kilos, 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 sixty as a safe estimate). The charg- 

 ing may be done uninjuriously, and with good dynamical 

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

 The drawing off of the charge for use may be done safe- 

 ly, 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 en- 

 gine ready to set to work at any time after a four days' 

 calm, or the user of the light may have a few candles or c il 

 lamps in reserve and be satisfied with them when the wind 



fails for more then five days.) One of the 20-kilo. 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 soused 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 plan, if the windmill could be had for 

 something comparable with the prime cost of a steam 

 I engine capable of working at the same horse power as 

 the wind mill when in good action. But wind mills as 

 ; hitherto made are veiy costly machines ; and it does not 

 seem probable that without inventions not yet made, 

 wind can be economically used to give light in any con- 

 siderable class of cases, or to put energy into store for 

 other kinds of work. 



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 beenjnade in large 

 variety for utilizing rain-power in the various conditions, 

 in which it is presented, whether in rapdidly-flowing rivers 

 in natural waterfalls, or stored at heights in natural lakes 

 or artificial reservoirs. Improvements and fresh inven- 

 tions 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- 

 siderable degree with its assistance, been put in requisi- 

 tion 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 collec- 

 tion of rain-water in tanks placed artificially at sufficient 

 heights over flat country to supply motive power econo- 

 1 mically by driving water-wheels ? To answer it: Sup- 

 pose a height of 100 metres, which is very large for any 

 practicable building, or for columns erected to support 

 tanks ; and suppose the annual rainfall to be three-quar- 

 ters 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 therefore, dividing this by 75, we find 

 3153 sq. metres as the area of our supposed tank re- 

 quired for a continuous supply of one horse-power. The 

 prime cost of any such structure, not to speak of the 

 value of the land which it would cover, is utterly prohibi- 

 rory of any such plan for utilizing the motive power of 

 1 rain. We mav or may not look forward hopefully to the 

 time when windmills will again " lend revolving anima- 

 tion " to a dull flat country ; but we certainly need not be 

 afraid that the scene will be marred by forests of iron 

 columns taking the place of natural trees, and gigantic 

 tanks overshadowing the fields and blackening the horizon. 



To use rain-power economically on any considerable 

 scale we must look to the natural drainage of hill country, 

 and take the water where we find it either actually falling 

 or stored up and ready to fall when a short artificial 

 channel or pipe can be provided for it at moderate cost. 

 I The expense of acqueducts, or of underground water- 

 1 pipes, to carry water to any great distance — any distance 

 I of more than a few miles or a few hundred yards — is 

 I much too great for economy when the yield to be provided 

 I for is power ; and such works can only be undertaken 

 j when the water itself is what is wanted. Incidentally, in 

 connection with the water supply of towns, some part of 

 the energy due to the head at which it is supplied may 

 I be used for power. There are, however, but few cases (I 

 ! know of none except Greenock) in which the energy to 

 spare over and above that devoted to bringing the water 

 to where it is wanted, and causing it to flow fast enough for 

 I convenience at every opened tap in every house or fac- 

 [ tory, is enough to make it worth while to make arrange- 

 1 ments for letting the water-power be used without wast- 

 , ing the water-substance. The cases in which water-power 



