EEPOET OF THE SECKETARY. 93 



of this instrument were prepared at the observatory shops by Mr. 

 Kramer and Mr. Abbot, and these were sent with the expedition to 

 California. In anticipation it may be said that the five instruments 

 were sent up on successive days, beginning July 30, 1913, and at the 

 time of writing this report two of them have been recovered. Each of 

 the two had a readable record of the ascension. A preliminary re- 

 duction of the results shows that, beginning at an altitude of about 

 6,000 meters and separated by altitude intervals of 2,000 or 3,000 

 meters for successive exposure, four determinations of the solar 

 radiation were obtained in each of the ascents. The rough computa- 

 tion mentioned results as follows: First ascent: 1.44, 1.60, l.TO, and 

 1.88 calories per square centimeter per minute. Second ascent : 1.62, 

 1.64, 1.76, and 1.89 calories per square centimeter per minute. 



These results are subject to later recomputation, but they indicate 

 at least that our solar-constant work of 1902-1912 by high and low 

 sun observations on homogeneous rays, according to Langley's 

 methods, gives results of the same order of magnitude as those 

 obtained by direct pyrheliometric observations at extremely high 

 altitudes. 



PERSONNEL. 



No change has occurred in the staff of the observatory, except that 

 Miss F. E. Frisby completed her temporary service as computer on 

 June 30, 1913, and Mr. A. K. Angstrom served as temporary bolo- 

 metric assistant in Algeria from July 1, 1912, to September 30, 1912. 



SUMMARY. 



The work of the observatory has been uncommonly successful. 



Volume III of its Annals has been published, including the work of 



the years 1907 to 1912. The observations at Bassour, Algeria, taken 



in connection with those made simultaneously at Mount Wilson, Cal., 



have established the variability of the sun. A variability connected 



with the sun-spot cycle has also been shown. The mean value of the 



solar constant of radiation has been fixed, it is thought, within 1 per 



cent. From about 700 observations, extending over the time interval 



from 1902 to 1912 and taken at different altitudes from sea level to 



4,420 meters, the mean value is 1.932 calories per square centimeter per 



minute. Pyrheliometers have been sent up by means of sounding 



balloons to very great altitudes, and preliminary results indicate that 



they give values of the solar radiation similar to those found by high 



and low sun observations on homogeneous rays. 



Eespectfully submitted. 



C. G. Abbot, 



Director Astrophysical Observatory^ 



Smithsonian Institution. 

 Dr. Charles D. Walcott, 



Secretary of the Smithsonian Institution. 



