16 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 60 
California, on the daily quantity of solar heat. The observations are 
arranged in such a manner as to indicate not only the quantity of 
solar heat reaching the earth, but also the quantity of heat which 
would reach a body with no atmosphere, like the moon. 
So far the observations have indicated that the sun is probably a 
variable star having a range of variation amounting to from five to 
ten per cent within an irregular interval of from five to ten days. 
Fic. 14.—Mr. Angstrom and the solar-constant apparatus at Bassour. 
Photograph by Abbot. 
In 1911 Mr. Abbot, assisted by Prof. F. P. Brackett, observed) in 
Algeria, while his colleague, Mr. Aldrich, observed on Mt. Wilson, 
in California. The object of thus duplicating the measurements was 
to avoid any errors due to local atmospheric conditions which might 
have affected Mt. Wilson observations. As nearly one-third of the 
circumference of the earth lies between Mt. Wilson and Bassour, it 
could not be expected that a similar local disturbance could affect 
both stations at the same time and in the same manner. The observa- 
tions of Ig11 strongly supported the belief that the sun is variable, 
but owing to cloudiness their number was not sufficient to fully 
