6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 69 



On the afternoon of June 9, with a sky considerably clearer than 

 June 8, similar though not so frequent observations were made for 

 comparison. 



After the work of the expedition was concluded, the pyranomeu-r 

 and ammeter were taken by the writer to the Smithsonian station 

 at Mt. Wilson, Cal., and there comparisons were made with Secondary 

 Pyrheliometer A. P. O. No. IV on the sun alone, for the purpose of 

 determining the constant of the eclipse pyranometer. Five com- 

 parisons were made on June 26, and six on June 28, the mean of 

 these giving a value 23.8 as the constant of the eclipse pyranometer 

 when glass covered, and 22.6 with glass off. The computed value of 

 the constant (glass covered) is 25.9. The discrepancy is not sur- 

 prising when one considers the vicissitudes of the instrument since 

 the computation was made early in 1916. The value 23.8 was 

 adopted in reducing the readings to calories. 1 



Note by C. G. Abbot. — The value 22.6 stated by Mr. Aldrich as the constant 

 of pyranometer No. 5, as used for nocturnal work, without the glass cover is 

 determined by multiplying the day value, 23.8, by the fractions ^^ and $4, 

 The former fraction corrects for removing the hindrance to rays caused by 

 two reflecting surfaces of glass, the latter represents an attempt to take 

 account of the fact that the blackening of the pyranometer strip is less com- 

 pletely absorbing for long wave rays, such as are proper to a body at ordinary 

 terrestrial temperatures, than for sun rays. This latter assumption is quantita- 

 tively very uncertain. 



Dr. A. K. Angstrom has lately published a paper entitled " Determination 

 of the Constants of Pyrgeometers " (Arkiv for Matematik Astronomi och 

 Fysik, K. Svenska Vetenskaps akademien, Band 13, No. 8, 1918). In this 

 paper he explains clearly the methods and results of his recent investigation 

 to fix the scale of the Angstrom nocturnal radiation instruments, and gives 

 the constant of Pyrgeometer No. 22, now with the Smithsonian solar constant 

 expedition to Chile as 13.4. 



Messrs. Moore and L. H. Abbot of the Chilean expedition made careful 

 comparisons of that pyrgeometer with pyranometer S. I. No. 3, at Hump Mt., 

 N. C, on several nights of 1917 and 1918. They found that if the same 

 conventions adopted by Mr. Aldrich in computing the nocturnal radiation con- 

 stant of the pyranometer were employed, and if we assume that as so used 

 the pyranometer truly reads in calories, then the constant of Angstrom pyrgeo- 

 meter No. 22 is 9.8. Their value differs by 36 per cent of itself from Angstrom's, 

 so that Angstrom's results are therefore much higher than ours. 



I call attention to this glaring discrepancy, not intending to imply that the 

 Smithsonian pyranometer scale of nocturnal radiation is right or that 

 Angstrom's pyrgeometer scale is wrong, but rather to hinder readers from 

 accepting either scale as yet verified. Nocturnal radiation measurements can- 



1 See Smithsonian Misc. Coll., Vol. 66, No. 7, p. 7. 



