164 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1912. 



seems most probable that we have found a real variation of the sun, 

 irregular in amount and period, but of an average period of 7 to 10 

 days, and an average magnitude of 3 to 5 per cent. This would be 

 "important if true." 



Note added January 13, 1913: 

 [In part from Annual Report of Secretary of the Smithsonian Institution for year 



ending June 30, 1912.] 



Congress having provided funds, an expedition under my charge 

 proceeded in July, 1911, to Bassour, Algeria, to make there a long 

 series of solar-constant observations simultaneously with similar 

 observations made by Assistant Aldrich on Mount Wilson. The 

 Algerian expedition included Mr. and Mrs. Abbot and Prof. F. P. 

 Brackett, of Pomona College, California. The apparatus carried 

 was the same which I had used on Mount Whitney in 1909 and 1910. 

 Station was reached on July 31, 1911, but owing to a most unfortu- 

 nate miscarriage of a box of apparatus, observations could not be 

 commenced until August 26, and several more days were required 

 to get the who]e outfit working satisfactorily. The weather of 

 August was excellent at both Mount Wilson and Bassour, but in the 

 subsequent months the good days at one station frequently coin- 

 cided with bad ones at the other. Hence, although 44 days of solar- 

 constant observations were secured at Bassour up to November 17, 

 when the camp was broken up, and a still greater number were 

 secured at Mount Wilson, only 29 of these coincided and 20 were good 

 at both stations. 



In spite of the loss of August and the unfavorable weather of sub- 

 sequent months, the results strongly confirm the supposed variability 

 of the sun. 



Expeditions of 1912. — While the simultaneous observations made 

 in 1911 at Bassour and Mount Wilson seemed justly mterpretable as 

 confii'ming the variability of the sun, yet it was felt that a result of 

 such uncommon interest ought to be put beyond the smallest war- 

 rantable doubt. Accordingly, m May, 1912, Mr. and IVIrs. Abbot 

 again returned to Bassour, where they were joined on May 20 by IVtr. 

 Anders Knutson Angstrom, as temporary assistant. Observations 

 were begun on June 2. Observations on J^Iount Wilson had already 

 been begun by IMr. Fowle in April. 



Between June 1 and September 9 nearly 50 days of observations 

 common to both stations Avere secured. This work is fully conclusive 

 in proving the variability of the sun. High solar -constant values at 

 Bassour correspond vntli high solar-constant values at Mount Wilson, 

 and vice versa. 



In the meantime it has been found that some measurements of the 

 kind illustrated in figure 6, which were made in 1908 at Washington 



