﻿408 SMITHSONIAN MISCELLANEOUS COLLECTIONS [VOL. 47 



difficulty of reducing" their readings to an absolute scale of units. 

 So that although the readings with two different instruments may 

 seem discordant, yet all the readings with each instrument may be 

 relatively quite comparable. While it is much to be desired that a 

 uniform and if possible an absolute system of actinometry should be 

 generally adopted, yet from the conclusions reached in this paper, 

 it appears that even now any good series of actinometric observations 

 on record may be possibly reduced to yield values proportional to 

 the bolometrically determined ones, and all these may later be 

 brought to a common system. 



Mr. Abbot hopes soon to have his continuously recording standard 

 actinometer in operation, and it was for use with this instrument 

 that this research was undertaken, and I wish to express my obliga- 

 tion to him here for his continuous help and suggestions in preparing 

 this paper. 



Conclusion 



By the application of a definite empirically determined correcting 

 factor, the use of Bouguer's formula with actinometer measurements 

 alone may serve to determine the extra-atmospheric value of the 

 solar radiation with nearly the same accuracy as by the bolometric 

 method. This correction for Washington is about 14 percent, addi- 

 tive, when the observations are between the air-masses 1.5 and 2.5. 



It seems probable that the same correction would be applicable at 

 other stations having, 1st, nearly the same altitude; 2d, similar air- 

 masses, and 3d, similar atmospheric transparency, — this latter condi- 

 tion being determinable by the slope of the actinometer curve. 



It is hoped that the correction may be later more accurately de- 

 • termined both as a function of the air-mass as also of the coefficient 

 of atmospheric absorption, and that it may even be extended to places 

 of much higher altitude. 



Smithsonian Astrophysical Observatory, 

 January 21, 1905. 



