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SMITHSONIAN MISCELLANEOUS COLLECTIONS 



[VOL. 47 



i.o the value is what would have been obtained with the sun in the 

 zenith, but the points for air-masses between i.o and zero are purely 

 hypothetical and do not exist in nature. That is, the point at air- 

 mass 0.5 is not what the actinometer would have read had it been 

 carried above the earth vertically until half the atmosphere lay below 

 it, for such a value we have not the means of computing'. It is the 

 radiation that would be observed after passing vertically through a 

 column containing one-half the actual absorbing medium, but with 

 the same distribution of densities as actually exist in the zenith sec- 

 tion of our atmosphere. In other words instead of removing the 

 bottom layers in order to diminish the air-mass below unity a portion 

 of each layer is imagined removed. 



An inspection of this plot shows at once several points which Mr. 

 Langley pointed out. 1 



"(1) that the coefficient of transmission" (which is the anti- 

 logarithm of the tangent to the curve) " is not constant; (2) always 

 too large under any circumstances; (3) always larger and larger as 

 we approach the horizon, and (4) that the original energy of the sun 

 or star before absorption, as found by the thenuometric processes and 

 formulae in universal use, is always too small." Also that the rate 

 at which the tangents change becomes less and less the greater the 

 air-mass. 



Extrapolaticns, similar to those performed with the actual acti- 

 nometry, have been made from the computed curves of figure 63. 

 In the following table are given the corrections thus determined, to- 

 gether with " log a " and " a," as found from the slope of line used 

 for extrapolation. 



It thus appears that in extrapolation from ordinary air-masses 

 varying' from 1.5 to 2.5, a correction of 14 to 15 percent would be 

 expected. 



It is apparent from the table that the air-masses from which the 

 extrapolation is made is an important consideration; the smaller the 



1 American Journal of Science (3), xxviii, p. 163, 1884. 



