THE SUN 7 



lower levels that is controlled by man and transformed into electric 

 power for his own purposes. 



It would take more than two billion earths placed side by side 

 to form a continuous spherical shell around our sun at distance 

 equal to the earth's distance, and thus to receive the total output 

 of solar heat. Therefore less than one two-billionth part of that 

 output falls upon the earth. The earth's share of solar energy, 

 expressed in horse-power or other familiar units, is too great to set 

 down in figures. If you should happen to own 250 acres of land in 

 one of the tropical deserts of the earth, you will be interested to 

 know that your quota of the solar energy, near the middle of a 

 summer day, is falling upon your tract of land at the rate of about 

 one million horse-power — more than enough heat and power to 

 supply all the needs of this great city — and this is but two-thirds of 

 the sun's good intentions toward you, for some 40 per cent, of the 

 energy is intercepted by the atmosphere overlying your farm, and 

 returned forthwith to outer space. 



Your neighbor's tract of 250 acres is also receiving solar energy at 

 the rate of one million horse-power. Figuring backward, if one 

 farm area receives a million horse-power and, there are more 

 than a hundred million such farm areas on the earth turned 

 toward the sun at one time, and the whole earth intercepts less 

 than one two-billionth of the sun's energy output, is it any wonder 

 that sun worship became one of the recognized religions? Ac- 

 curate knowledge saves us from that, but it is becoming in us to 

 give the sun our due respect. 



A great problem ahead of the scientific world is the storage of 

 the sun's beneficent heat rays for release as needed. Astronomers 

 are seeking intently for the sources of the sun's outpouring of 

 energy : how can the sun maintain the supply for tens of millions 

 of years, as it undoubtedly is doing? One important source has 

 been found — -the sun's own gravitation which tries constantly to 

 pull every particle of its material to the sun's center — but another 

 and greater source seems to await discovery. Does any one say 

 since the supply of solar energy will surely meet our needs for ten 

 or a hundred million years, why look further for the cause? Why 

 not let it go at that? This selfish spirit, if applied to all subjects, 

 would retrograde our civilization. Even the possession of the 

 truth is not so potent for good as the desire to know the truth, and 

 the struggle to discover it. Practically, a knowledge of the origin 

 of the sun's heat may be the key for locking up great quantities 

 of it on summer days and unlocking it when and wlicre needed. 



