8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 65 



ditions which lead to our underestimating by nearly 50 per cent the 

 intensity of solar radiation outside the atmosphere, these would 

 probably be variable from' day to day; so that such minute real 

 changes of the total intensity of the sun's radiation as we have found 

 would have been swallowed up in the irregular local fluctuations of 

 the transparency of the atmosphere. 



CRITICISMS OF THE WORK 



We turn from this summary of the work and the circumstances 

 which heretofore indicated its validity, to a discussion of the criti- 

 cisms which have been made of it by several authors, and the new 

 experiments we have made to refute them. We take the following 

 summaries of objections from several recent articles : 1 



1. Mr. F. W. Very remarks that there are several reliable acti- 

 nometers, capable, when properly handled, of giving results correct 

 to 1 or 2 per cent, but that unfortunately some of them may give 

 results 20 per cent in error when inefficiently used or imperfectly 

 corrected. Although Mr. Very says in another place that our deter- 

 minations rest upon perfected instruments and admirable care, yet 

 he has seemed to indicate by his praise of values of the solar radiation 

 obtained from observations on the summit of Mt. Whitney, which 

 reached 2.0 calories per sq. cm. per minute, that he perhaps considers 

 our results to be 15 per cent too low, because in three different years 

 we have never observed on Mt. Whitney values exceeding 1 .7 calories 

 per sq. cm. per minute. 



2. It is pointed out that we employ the equation 2 



log R = m log a + log A 



as the equation of a straight line. In this equation R is the intensity 

 of one wave length of radiation at the station ; A, the corresponding 



1 F. W. Very, Astrophysical Journal, 34, 371, 1911; 37, 25 and 31, 1913; 

 American Journal of Science, 4th Series, 36, 609, 1913 ; 39, 201, 1915 ; Bulletin 

 Astronomique, xxx, 5, 1913. 



F. H. Bigelow. Boletin de la Oficina Meteorologica Argentina, 3, 69-87, 

 1912 ; American Journal of Science, 4th Series, 38, 277, 1914. 



E. Kron, Vierteljahrsschrift der Astronomischen Gesellschaft, 49, 53, 1914. 



2 As pointed out by Radau, Langley, and others, this equation is applicable 

 only to homogeneous radiation, that is, radiation of approximately a single 

 wave length. It is always -with this limitation that we employ it in our defini- 

 tive solar constant determinations. We have, however, pointed out that for a 

 limited range of two or three air masses good observations of total solar 

 radiation, when plotted thus logarithmically, deviate so slightly from the 

 straight line that the smallness of the deviations is a useful guide to the 



