I58 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 65 



SUMMARY 



Our pyrheliometric observations on the top of Mount Whitney, extending 

 from August 2 to August 12, 1913, have led to the following results : 



1. A variation in the solar constant amounting to more than 2 per cent during 

 this time is improbable. 



2. The solar constant computed from the measurements in a selected part 



of the spectrum, reduced to mean solar distance, came out 1.929 — „ ' . 



cm. mm. 



(Smithsonian scale), with a possible error of 1.5 per cent. This value is 



obtained on the assumption that the energy included between 0.484 u. and 



0.576 u is a constant known fraction of the total energy in the solar spectrum. 



3. The solar constant computed by the Angstrom-Kimball method was found 



to be 2.019 C f ' — (Smithsonian), 

 cm. mm. 



4. The solar constant computed according to Fowle's method comes out 



1.960 — 5-^ — (Smithsonian), 

 cm. min. 



The value of the solar constant given in (2) is in close agreement with 

 Abbot's mean value of 1.932 obtained from several series of observations 

 made during the years 1 902-1912 at much lower altitudes (e. g., at 1160 m. in 

 Algeria). The value given in (3) is also in close agreement with the solar 

 constant computed by Kimball according to the same method from measure- 

 ments at Washington. Consequently our observations give no support to a 



value of the solar constant greatly exceeding 2 — ~ — . 



cm." min. 



Because of their bearing upon the question of solar variability, it seems 



desirable that the observations in selected parts of the spectrum by means of 



absorbing screens should be extended to different localities, and that if possible 



simultaneous measurements at elevated stations should be undertaken. 



Cornell University, 

 December, 1913. 



Note. — After the publication of the paper treating the pyrheliometric 

 observations on Mt. Whitney by Dr. Kennard and myself, the spectrobolo- 

 metric observations at Mt. Wilson have been published by Dr. Abbot. From 

 both the simultaneous series, it is evident that our observations have been 

 carried out during a period of relatively high constancy of the solar activity. 

 No evidence in regard to the variability of the solar radiation can therefore 

 with safety be drawn from these few observations alone. If the doubtful 

 observations of August 8 and August 10 are excluded, the simultaneous 

 observations at the two places seem, however, to confirm one another very 

 well, as may be seen from figure 17. It seems, therefore, to be probable that 

 the variations in the computed solar constant values are due to a real solar 

 variability, the existence of which is very strongly indicated by the work of 

 several expeditions of the Smithsonian Institution. 1 



Anders Angstrom. 



1 Annals II and III of the Astrophysical Observatory of the Smithsonian 

 Institution. 



