JOURNAL 



OF THE 



WASHINGTON ACADEMY OF SCIENCES 



Vol. Ill JUNE 4, 1913 No. 11 



ASTROPHYSICS.— r/ie variation of the sun. C. G. Abbot, F. 

 E. FowLE, and L. B. Aldrich.^ 



In the year 1902 preliminary experiments were begun at Wash 

 ington to detennine the solar constant of radiation. About 700 

 determinations of it have now been obtained, depending on obser- 

 vations at altitudes ranging from sea-level to 4420 meters. As 

 originally devised by Langley we determine spectral energy inten- 

 sities and atmospheric transmission coefficients for numerous 

 wave-lengths between about 0.30/x in the ultra-violet and 2.5fx 

 in the infra-red, by spectrobolometric observations at high and 

 low sun. The indications of the spectrobolometer are reduced to 

 the standard scale of calories per square centimeter per minute 

 by means of the readings of the pyrheliometer. 



At the time when the observations were begun in 1902 there 

 was no satisfactory establishment of the standard scale of pyr- 

 heliometry, nor indeed any pyrheliometer which was invariable 

 relatively to itself from year to year. We at first made use of 

 a modification of TyndalFs mercury pyrheliometer. This was 

 improved in 1906 as the copper disk pyrheliometer, which has 

 been in use on Mount Wilson ever since, and which is described 

 in volume 2 of the Annals of the Astrophysical Observatory. A 

 still later improvement took place in 1910 with the introduction 

 of the so-called "Silver-Disk Pyrheliometer" which has attained 

 considerable favor, and which is now in use in numerous countries. 

 Neither of these instruments is capable of yielding independently 



' Published by permission of the Secretary of the Smithsonian Institution. 



309 . 



