﻿VOL. 47 ! 9°4 



Smithsonian 

 Miscellaneous Collections 



VOL. 2 QUARTERLY ISSUE PART 4 



THE DISCREPANCY BETWEEN SOLAR RADIATION 



MEASURES BY THE ACTINOMETER AND BY 



THE SPECTRO-BOLOMETER 



By F. E. FOWLE, Jr. 

 (Communicated by S. P. Langley, Secretary of the Smithsonian Institution.) 



The object of this paper is to show, that by means of actinometric 

 observations alone, of the intensity of the total solar radiation re- 

 ceived at the snrface of the earth, we may, by the application of a 

 certain correction, determine the radiation outside the atmosphere 

 with the same degree of accuracy which is reached in a holographic 

 determination made in Washington by the observation of homo- 

 geneous rays. 



Air. Langley demonstrated 1 many years ago that the use of 

 Bonguer's formula with ordinary actinometric observations neces- 

 sarily leads to too low a value of the solar radiation outside of our 

 atmosphere. This formula may be written E x =E n a m * where E x 

 and E are the actinometric readings in radiation units at the air- 

 masses m x and zero respectively, and " a " the coefficient of trans- 

 mission of our atmosphere for unit air-mass. The unit air-mass is 

 such that, with the sun in the zenith, the radiation passes through 

 one air-mass or atmosphere to the observer at the surface of the 

 earth. 



In employing the formula to compute E from actinometer meas- 

 ures, the error arises from the assumption that the coefficient of 

 transmission " a " is the same for all wavelengths, whereas at Wash- 

 ington the general transmission coefficients may vary from 0.39 to 

 0.97 (and in water vapor bands may even reach zero), between the 

 wavelengths 0^.4 to 2^.0; beyond these limiting wavelengths the 

 amount of solar radiation is negligible. 



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



399 



