ORSERMXG THE ENERGY OE SUN-RAYS' 



By C. G. abbot, 



Secretary of flie Siiiitlisoiiiaii Institution, and Pirector of the 

 Smithsonian Astrol^Iiysieal (^I'ser-i'atory 



All terrestrial life depends on sun-rays. The chemical processes in 

 plants which are the fundamental hasis of all foo<ls ; the tem])erature 

 and rainfall which constitute essential life-conditions; the sup]:)lies of 

 coal, oil and wateri)ower from which manufacturin,^-, transportation, 

 and li^htin^- receive their ener«^y ; as well as the heauties of earth and 

 sky, are all the shifts of solar radiation. Such were the considera- 

 tions which led the third Secretary of the Smithsonian Institution. 

 Dr. Samuel I'. Langley, to estahlish the Astrophysical Ohservatory 

 for the study of sun-rays, their intensity, their quality, the transpar- 

 ency of the atmosphere to^ them, their transformation into heat, and 

 the important etit'ects in the atmosphere and on the earth's surface to 

 which they give rise. 



Ijeginning- in i8<)o. in temporary (juarters at Washington, the first 

 investigations were devoted to mapi)ing the lines and hands of solar 

 and terrestrial atmospheric ahsorption of the invisihle rays heyond 

 the red end of the colored spectrum. We then turned, in 1902, to the 

 measurement of what is called the " solar constant of radiation," which 

 is the measure of the intensity of solar energy availahle to warm 

 the earth. Some results of 1903 aroused the suspicion that this quan- 

 tity is not really constant. If so. its variations must afifect the climates 

 and weather of the whole world. As the smoke and dust of Washing- 

 ton raised difficulties for this research, in 1905 the observing was trans- 

 ferred to Mount Wilson in California, where the work of making daily 

 observations of the " solar constant " continued, hut only in summer 

 and autumn months, until 1920. Many associated studies relating to 

 the transparency of the atmosphere, the brightness and color of the 

 sky by day and by night, the distribution of brightness over the sun's 

 disk in all colors were also made on Mount Wilson. As evidences of 

 solar variation appeared, expeditions were made to Mount \Miitney 

 in California and Bassour in Algeria, to test whether the result 

 depended on atmospheric conditions. 



In 1918, in order to obtain daily measures of solar variation in 

 winter as well as summer, and where clouds and dust would be of 



^ All photographs in this article are reproduced by courtesy of the Xational 

 Geographic Society. 



