CONCERNING SMITHSONIAN 

 PYRHELIOMETRY 



By C. G. abbot, L. B. ALDRICH, and A. G. FROILAND 



Recently K. Wegener ^ has asserted that gross errors exist in 

 Smithsonian pyrheHometry. P. Courvoisier ^ showed that Wegener's 

 main contention is erroneous and also discussed at considerable length 

 the behavior of the Smithsonian silver-disk pyrheliometer. The im- 

 pression is left that certain not negligible errors, akin to that claimed 

 by Wegener, still existed. The largest of these he stated to be of 

 the order of i percent. But in a private communication, Dr. Mori- 

 kofer, in whose laboratory Courvoisier worked, informs us that this 

 estimate contained a numerical error and that the quantity considered 

 is very much smaller, only of the order of o.i percent. 



However, we wish to discuss the subject further. The late W. H. 

 Hoover and A. G. Froiland ^ have given pyrheHometry careful atten- 

 tion in volume 7 of the Annals of the Astrophysical Observatory. 

 We refer those interested to that source. We now present certain 

 facts, not always remembered by critics, and we add experimental re- 

 sults on the magnitude of the errors suggested by Wegener and 

 Courvoisier. 



The silver-disk pyrheliometer is not a standard instrument. It can 

 carry the standard scale of solar measurement only if it agrees, within 

 admissible limits of error, under all usual circumstances, with a 

 standard pyrheliometer. It is therefore of no consequence what errors 

 might belong to the silver-disk pyrheliometer, if, when used with its 

 accompanying Smithsonian papers, it will give, to the required degree 

 of accuracy, the same results as a standard pyrheliometer. 



1 Wegener, K., Die Messung der Sonnenstrahlung und der Solarkonstante. 

 Journ. Geophys. Res., vol. 54, p. 53, 1949; Die Sonnenstrahlung und ihre 

 Messung. Geof. Pura e Appl., vol. 22, p. 205, 1952. 



2 Courvoisier, P., Zur Definition der Sonnestrahlung. Arch, fur Meteorologie, 

 Geophysik und Bioklimatologie, ser. B, vol. 5, No. 2, p. 124, 1954. 



3 Hoover, W. H., and Froiland, A. G., Silver-disk pyrheHometry. Smith- 

 sonian Misc. Coll., vol. 122, No. 5, 1953. (Also, Ann. Astrophys. Obs., vol. 7, 

 ch. 4, pp. 99-137, 1954.) 



SMITHSONIAN MISCELLANEOUS COLLECTIONS, VOL. 123, NO. 5 



