SOLAE RADIATION 



By C. G. Abbot 

 Secretary, Smithsonian Institution 



[With 3 plates] 



Directly or indirectly our most important interests depend on the 

 solar radiation. The sun rays keep the earth warm enough to 

 sustain life. Variations of their intensity associated with summer 

 and winter, and with night and day. produce climates. Slight 

 variations of the original output of rays from the sun itself seem 

 to be highly influential in altering the weather. All growth in plants 

 depends upon the application of solar energy. Our atmosphere is 

 the source of carbon, which is a principal plant constituent. The 

 trifling percentage of carbonic-acid gas contained in air is the es- 

 sential food of plants, but it can not nourish them without the help 

 of radiation. The health of animals, including man, requires radia- 

 tion. The prevention of rickets by the curious direct and indirect 

 influences of ultra-violet rays has formed a fascinating chapter in 

 the story of recent investigations. 



Power is principally derived indirectly from solar radiation. Solar 

 heat evaporates the oceans, drives the clouds inland, precipitates the 

 rain and snow, and thus maintains the world's hydroelectric power 

 sources. Enormous as these are, they are, nevertheless, trifling 

 compared to the power derived from oil and coal. Oil, the less 

 important of these two sources, comes mainly from animal life that 

 was sustained ages ago bj'' the vegetation fed by the ancient sun. 

 Coal, on the other hand, is the end product of decomposition of 

 vegetation. The enormous deposits of coal, which are now the 

 world's principal sources of power, represent but a trifling percentage 

 of the solar energy lavished on the earth in former geologic ages. 



These are highly indirect applications of solar radiation for power 

 supplies. It is possible, however, as numerous inventors have shown, 

 to produce heat for driving engines by the direct absorption of solar 

 rays. If the devices now available for this purpose should be but a 



^ Reprinted, by permission, from the Proceedings of the Ohio State Educational Con- 

 ference, Eievontli Annual Session, The Ohio State University Bulletin, vol. 36, No. 3, 

 Sept. 15, 1931. 



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