146 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1921. 



reservoirs, and, passing through water wheels at lower levels, be 

 made to generate electric power for lighting, for heating, and for 

 the running of motors, it is the sun's energy which is transformed to 

 meet the needs of men. The sun's rays evaporate the surface waters 

 of the oceans, lakes, streams, and lands ; the winds, generated by the 

 unequal solar heating of our atmosphere, transport some of the 

 water vapor to the high mountains, where it is deposited as rain or 

 snow. It is merely the descent of these waters to the 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 1,000,000 horse- 

 power — more than enough heat and power to supply all the needs 

 of this great city (Cleveland) — 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 1,000,000 horsepower. Figuring backward, if one 

 farm area receives 1,000,000 horsepower, 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 wor- 

 ship became one of the recognized religions? Accurate 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 en- 

 ergy : 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 



