RECENT STUDIES OF THE SOLAR CONSTANT OF 

 RADIATION 



By C. G. abbot 



INTRODUCTION 



Within the last two years the observations of the Smithsonian 

 Astrophysical Observatory under the direction of the Secretary, 

 Mr. Langley, have been largely for the purpose of measuring the 

 total solar radiation, its distribution in the spectrum, and the 

 losses which it suffers by absorption in the solar and terrestrial 

 gaseous envelopes. In the experimental work and reduction of 

 observations Mr. Langley has been aided by the writer, but 

 chiefly by Mr. F. E. Fowle, Jr., whose able handling of the work 

 I wish particularly to acknowledge and commend. Preliminary 

 notices of this investigation have appeared in the Smithsonian 

 Report for 1902, and in an article by the Secretary in The Astro- 

 physical Journal for March, 1903, to which sources the reader is 

 referred for additional information in relation to the methods of 

 study. In the present paper will be found a summary of the results 

 thus far reached. 



ATMOSPHERIC ABSORPTION 



It is well known that the effectiveness of the solar and terrestrial 

 gaseous envelopes to intercept by reflection or absorption and thus 

 diminish the intensity of the solar radiations at the earth's surface, 

 varies greatly for rays of different wave-length. It is customary, 

 speaking of the matter in ready though not strictly accurate terms, 

 to combine these two effects of reflection and absorption under the 

 single head of absorption, but to distinguish two kinds of absorption, 

 namely, general and selective, of which the latter includes such 

 sudden alterations of transmission as are seep in the Fraunhofer 

 lines, while the former denotes merely a general weakening of the 

 radiation extending over wide ranges of wave-length. Using this 

 nomenclature, it appears to be the general absorption of the solar and 

 terrestrial envelopes which chiefly affects the amount of solar radia- 

 tion at the earth's surface, although the selective absorption of 



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