84 



KEPORT OF THE SECRETARY. 



Table 2. — J^alitrs of the solar constant of radiation outside the earth's atmosphere. 



Date. 



July7 



Do 



August 24 



Do....... 



October 14 . . . 



Do 



October 29 . . . 

 December 7. . 

 December 23. 



Do 



January 27.. 



Do 



February 11 



Do 



April 4 



Do 



May 12 



Do 



May 25 



Do 



May 28 



Do 



Character of the 

 observation. 



Fair ... 



do. 



Good . . 



do. 



do. 



do. 



do. 



do. 



do. 



do. 



Fair 



do 



do 



do 



Indifferent . 



....do 



do 



do 



....do 



do 



Fair 



....do 



Air 

 mass. 



1.07 

 1.51 

 1.18 

 1.24 

 1.72 

 1.88 

 1.69 

 3.34 

 2.52 

 3.38 



2.20 

 2.98 

 1.81 

 2.27 

 1.21 

 1.72 

 1.08 

 1.59 

 1.53 

 1.94 

 1.09 

 1.81 



Solar radiation per 



.square centimeter 



per minute — 



Atearth's 

 surface 



Cat. 

 1.31 

 1.10 

 1.14 

 1.12 

 1.23 

 1.18 

 1.13 

 .92 

 1.16 

 1.02 



1.18 

 .99 

 1.18 

 1.02 

 1.39 

 1.25 

 1.29 

 1.19 

 1.08 

 .96 

 1.42 

 1.16 



Outside at- 

 mosphere 

 corrected 



to mean 

 solar 



distance. 



Oil. 



2.16 

 2.11 

 1.93 

 1.95 

 1.98 

 1.94 

 1.97 

 1.94 

 1.96 

 2.01 



2.05 

 1.98 

 2. 29 

 2. 24 

 2.08 

 2.15 

 1.90 

 2.07 

 2. 24 

 2.31 

 2.14 

 2.05 



Mean value of direct atmospheric transmission. — Since Table 2 was prepared, which 

 was done without any reference to the subject of the present paragraph, at your 

 request the numbers in the fifth column have been corrected to represent the solar 

 radiation at the earth's surface for zenith sun, and those in the sixth column to 

 actual rather than mean solar distance. The ratio of the amount of heat which 

 reaches the earth's surface for zenith sun to that outside the atmosphere is then 

 obtained by dividing the former values by the latter, and the mean result is 68 per 

 cent; whence the average absorption in our atmosphere is found to be 32 per cent, 

 which is the amount by which the sun's radiation is diminished in a vertical trans- 

 mission to the surface of the earth. In this connection attention may be invited to 

 a paper published by you in the American Journal of Science, as long since as Sep- 

 tember, 1884, in which you have stated that observations made up to that time by 

 the best authorities gave a value of al)sorption for the zenith sini of about 20 per cent, 

 and that owing chiefly to the neglect of selective absorption, this nearly unanimous 

 value is nevertheless far inferior to the truth. The value just deduced from selective 

 absorption methods may be consided as a confirmation of your then statement. 



You have elsewhere stated that the solar constant values obtained from high and 

 low sun measures at a low altitude station are likely to be below rather than above 

 the truth, and it may at least be admitted that there is a certain direct reflection of 

 radiation in passing from outer space into our atmosphere, differing as it does from 

 space in its optical density, and that a portion of radiation is here lost which can not 



