9 o THE POPULAR SCIENCE MONTHLY. 



mouth of a natural basiu or estuary, and to utilize 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 ques- 

 tion, whether is forty 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, may be 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 land 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 hundred horsepower, and in another forty acres. 

 But, indeed, the question is hardly worth answering, considering the 

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

 utilization 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 day 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 can not 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 be- 

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

 ward 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-mines from which that little is to be exca- 

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

 is greatly higher than at present, it is most probable that windmills or 

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



