264 UTILIZING THE SUN'S ENERGY. 



manufactures, will do much for us, and that will last as long as 

 humanity survives on this globe; hut it is doubtful whether it can be 

 considered as a possible complete substitute for steam power. Yet 

 the total available water power of the world will greatly ameliorate 

 the difficulties likely to arise from extinction of fuel supplies. The 

 mean annual rainfall of the world is 36 inches, and this means about 

 50,000,000 cubic feet per square mile per annum falling on the land 

 of both hemispheres. Taking the mean available height of fall as 

 10 feet, and assuming it possible to store the water effectively in 

 ample reservoirs, this would mean 500,000,000 X 60 = 30,000,000,000 

 foot-pounds of available energy, and, if expended in fhree thousand 

 'dug hours, it would give a total of 10,000,000 horsepower per 

 juarc mile for such countries as might be able to utilize such a fall. 

 This, however, is but a small fraction of the inhabited area of the 



7 7 ^ 



globe. As a fair estimate, the data for the Mississippi River, in the 

 United States, may be taken. This stream drains about 1,250,000 

 square miles, with a rainfall of 30 inches, an average, for each foot of 

 fall, of 11,000,000,000,000 foot-pounds per annum. The fall is 6 

 inches per mile, average, and the energy capable of use for that area 

 is about a quarter of a million horsepower per square mile. 



These figures are enormous, and give the impression that we need 

 not feel uneasy about our power supply, even though we entirely extin- 

 guish our fuel deposits. Thc}^ are, however, of little value; for they 

 give no idea of the practically available energy of rainfall, since it is 

 not possible to make use of more than a minute fraction of this total, 

 and it is not at all probable that we ever can. In the whole length of 

 the Mississippi River there are but three available water powers — one 

 with 78 feet fall, at Minneapolis, one with 24 feet, at Des Moines, and 

 one with 22 feet, at Rock Island. Taking the average flow as a half 

 million cubic feet per second utilized, the water powers at these points 

 would be a total of about 7,000,000 horsepower derived from an area 

 of a million and a quarter square miles, and directly from but a frac- 

 tion of that area, situated above the lowest fall. 



The deduction must evidently be that water power alone can not be 

 depended upon to provide the energy that will be needed by future 

 generations should fuel be unavailable, although it is equally obvious 

 that streams are likely to provide immense quantities of power, and 

 that manufactures in those coming days will group themselves about 

 the mill sites or within distances from them which can be spanned by 

 the electric high-tension wire. Of this process of displacement of 

 manufactures, Niagara and Buffalo are already giving impressive illus- 

 trations. As time goes on the part to be taken in power production 

 by waterfalls will become increasingly important. It is alread} T vastly 

 greater and more important economically than is generally supposed. 

 There are known water powers in the United States able to furnish, if 



