abbot: solar constant of radiation 107 



alter the results at both stations in the same direction on the 

 same day. Such a station was established at Bassour, Algeria, 

 in the years 1911 and 1912. Seventy-five days of simultaneous 

 measurement at Mount Wilson and at Bassour were obtained, 

 and of these days about fifty were so far free from the occurrence 

 of clouds, or other disturbing influences, at both stations, as to 

 be retained for purposes of comparison. The result of the com- 

 parison shows that when high values are obtained at Bassour, 

 high values are obtained also at Mount Wilson, and vice versa. 

 Thus the fluctuations which have been found, appear to be truly 

 existing in the solai- radiation outside the earth's atmosphere, for 

 the solar constant values obtained at two stations separated by 

 about one-third the circumference of the earth unite in showing 

 them. 



Value of the solar constant. During the whole solar constant 

 campaign from 1902 to 1913, about 700 measurements of the solar 

 constant of radiation have been obtained, all but three of the 

 values ranging between 1.80 calories and 2.10 calories. The 

 range of these numbers is mainly attributable to the actual fluc- 

 tuation of the sun itself, though part, especially in W^ashington 

 work, is due to accidental errors of measurement. The mean 

 value from 690 measurements is 1.933 calories per square cen- 

 timeter per minute. It is believed that this number represents 

 the average value of the solar constant of radiation for the epoch 

 1902 to 1913 within 1 per cent. There is still the possibility, 

 however, that an appreciable quantity of solar radiation be- 

 yond the wave length of 0.3^ in the ultra-violet has been absorbed 

 by ozone in the higher atmosphere of the earth, and has been 

 impossible of determination at the stations employed. However, 

 from the consideration of the form of energy curve of the sun's 

 spectrum it is improbable that this lost ultra-violet radiation 

 can exceed 1 or 2 per cent. 



Solar variability associated with sun-spots. Besides the short 

 irregular fluctuation of solar radiation above mentioned as having 

 been shown by the simultaneous measurements at Mount Wil- 

 son and Bassour, Algeria, it appears that a long period fluctua- 

 tion is associated with the sun-spot numbers. This connection 



