76 REPORT OP THE ACTING SECRETARY. 



OBSERVATIONS ON MOUNT WILSON. 



The departure of an expedition in charge of Mr. Abbot to Mount Wilson, in 

 California, in May, 1905, and the circumstances which led to its location tliere 

 in response to the liind invitation of the director of the Carnegie Solar Ob- 

 servatory and the president of the Carnegie Institution were mentioned in last 

 year's report. Observations were continued by the expedition on Mount Wilson 

 until October 27, 1905, when the approach of cold weather and the frequent 

 rise of fog over the mountain made the observing so difficult, especially in the 

 temporary shelter in which the bolometric apparatus was situated, that Mr. 

 Abbot was recalled. By request and invitation of the officials of the Carnegie 

 Institution most of the apparatus was left in place on Mount Wilson, in order 

 to provide in some way for the continuation of solar constant work there during 

 1906. Later it was determined to renew the Smithsonian expedition, and Mr. 

 Abbot returned about May 5, 190G, and after June 15, 1906, was again aided, 

 as in 1905, by Mr. L. R. Ingersoll, temporary bolometric assistant . 



During 1905 the solar constant was observed on Mount Wilson on fifty-six days, 

 depending on about 600 holographs and about 1,000 pyrheliometer readings. 

 Observations were made on a number of days on the transmission of the solar 

 envelope and in svui spots. Pyrheliometer readings were made on Mount San 

 Antonio (over 10,000 feet high) sinuiltaneous with solar constant determina- 

 tions on Mount Wilson (0,000 feet high). A few days of observation were de- 

 voted to the determination of the reflective power of clouds and the scattering 

 of light by the sky. 



In 1906 additional instrumental equipment was provided, and a tower has 

 been erected at the point of a ridge overlooking deep canyons on three sides for 

 use in conducting from its summit careful measurements by the bolometer of 

 the comparative brightness of equal angular areas of cloud and sun at all 

 solar altitudes, and thus to determine what effect clouds have on the total 

 amount of solar radiation available to the earth. Almost ideal conditions for 

 this experiment are often had on Mount Wilson, for a level-topped sea of fog 

 often rises and remains stationary for hours within a few hundred feet of 

 the base of the tower above mentioned. It is also intended to observe the 

 brightness of the sky with the bolometer, making supplementary spectro- 

 photometric eye observations. Solar constant determinations were begun by 

 Mr. Abbot about May 15, 1906, and are being continued almost every day, ex- 

 cept when clouds prevent. As regards transparency of sky and freedom from 

 clouds, the conditions in 1906 are much less favorable than they were in 1905 

 on Mount Wilson. It may be possible that the prolonged eruption of Mount 

 Vesuvius has had a similar effect to that of the great eruption of Krakatoa 

 in 1883 in perturbing the air. It will be interesting to see if the observa- 

 tions of others indicate a general decrease of the transparency of the earth's 

 atmosphere. 



GENERAL STATEMENT OF RESULTS. 



Reserving for the forthcoming Volume II of the Annals of the Astrophysical 

 Observatory a detailed statement of the results obtained and a discussion of 

 them, the principal results of the year may be summarized as follows : 



A. Agreement between Washington and Mount Wilso7i solar constant 



ohservations. 



So far as the reductions have enabled the results to be compared, there 

 seems to be no difference of a systematic kind between Washington and Mount 

 Wilson spectro-bolometric determinations of the solar radiation outside the 

 earth's atmosphere. The probable error of Mount Wilson determinations is 



