90 abbot: solar constant of radiation 



minute which would be produced by the complete absorption of 

 the solar radiation in free space at the earth's mean solar distance. 



Preparatory researches of great interest were made in the eight- 

 eenth century by Bouguer, Lambert, DeSaussure and Leslie. 

 Determinations of the solar constant of radiation, however, may 

 be said to have begun about eighty years ago with the investiga- 

 tions of Sir John Herschel, Principal Forbes, and Pouillet. The 

 problem comprises two parts : First, to measure the intensity of 

 the solar radiation at the earth's surface; second, to estimate the 

 loss it has suffered in passing through the atmosphere. It will 

 be convenient to consider the atmospheric influences briefly be- 

 fore taking up the methods of measuring the solar radiation, and 

 then to return to a more thorough discussion of the atmospheric 

 transmission. 



Atmospheric transmission. The determination of the trans- 

 mission of the atmosphere rests primarily upon the hypothesis 

 of Bouguer, first put forward in 1729 and elaborated in Bouguer's 

 posthumous work published in 1760. The late Dr. Langley has 

 placed this matter in so very clear a light in his paper on the 

 Amount of the Atmospheric Absorption- that I cannot do better 

 than to quote from his statement. 



If a beam of sunlight enters through a crevice in a dark room, the 

 light is partly interrupted by the particles of dust or mist in the air, the 

 apartment is visibly illuminated by the light laterally reflected or diffused 

 from them, and the direct beam, having lost something by this proc- 

 ess, is not so bright after it has crossed the room, as before. In com- 

 mon language, the direct light, to an observer in the path of the beam, 

 has been partly "absorbed," and the problem is, to determine in what 

 degree. If a certain portion of the light (suppose one-fifth) were thus 

 scattered, the beam after it crossed the room would be but four-fifths 

 as bright as when it entered it ; and, if we were to trace the now dimin- 

 ished beam through a second apartment altogether like the other, it 

 seems, at first, reasonable to suppose that the same proportion (i.e., 

 four-fifths of the remainder) would be transmitted there also, and that 

 the light would be the same kind of light as before, and only diminished 

 in amount (in the proportion i x f). The assumption originally made 

 by Bouguer and followed by Herschel and Pouillet, was that it was in 

 this mamier that our solar heat was absorbed by our atmosphere, and 

 that by assuming such a simple progression the original heat could 

 be calculated. 



2 Amer. Jour. Sci., third series, 28: September, 1884. 



