JOURNAL 



OF THE 



WASHINGTON ACADEMY OF SCIENCES 



Vol. IV MARCH 4, 1914 No. 5 



ASTROPHYSICS.— r/ie solar constant of radiation. C. G. Ab- 

 bot. ^ Smithsonian Institution.* 



We live in a world warmed by the sun. While it is not 

 to be expected that everybody will devote himself to the 

 measurement of solar radiation, yet it is not surprising that 

 many have concerned themselves with measuring the quantity 

 on which all lives depend. So far as I am aware, this subject 

 was not pursued by the ancients to such a point as to obtain 

 measurements worth much present consideration. This is a 

 great pity, for thus we lack proof whether the sun's radiation 

 has changed progressively. Beginning about a century ago in- 

 vestigations of solar radiation were pursued with great assiduity 

 by various observers. The need was almost immediately per- 

 ceived pf reducing the observations to represent conditions out- 

 side the earth's atmosphere, as for example, on the moon, so 

 as to be independent of the haze and water vapor, and even of 

 the gaseous constituents of the air. It is required to know the 

 measure of solar radiation in free space as an index of the con- 

 dition of the sun, quite apart from its influence on terrestrial 

 affairs, but secondly it is of great importance and interest to 

 apply this knowledge to promote meteorological inquiries. 



Sir John Herschel, who was a pioneer in solar radiation work, 

 proposed to express solar radiation in terme of a unit which 

 he called the actine, which is based on the melting of ice. But 

 by general consent, the gram calorie has been adopted as the 

 unit of measurement, and we say that the ''solar constant of 

 radiation" is the number of calories per square centimeter per 



1 Address of the retiring president delivered before the Philosophical Society 

 of Washington, January 3, 1914. 



89 



