•88 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1915. 



This value is 3.5 per cent above that of 1913 and 1 per cent above 

 the mean for former years. Indications are that the value for 1915 

 will also fall very high. 



The contrast of brightness between the center and edges of the 

 solar disk was greater in 1914 than in 1913, and, in fact, almost as 

 great as was found from Washington work of 1905 to 1907. 



These facts confirm the result derived from earlier observations, 

 namely, the solar emission of radiation varies along with the solar 

 activity as revealed by sun spots and other phenomena. Higher 

 values of solar radiation prevail at times of greater solar activity, as 

 expressed by sun spots. The connection does not, however, appear 

 to be a strictly numerical one between solar radiation and sun spot 

 numbers. In the return of solar activity presaged in 1914 the solar 

 radiation rose almost to its maximum value before the number of 

 sun spots had greatly increased. Associated with these changes, 

 greater contrast of brightness between the center and edges of the 

 solar disk prevails when the solar activity is greater. 



SHORT-PERIOD VARIATION OF THE SUN. 



In the year 1913, as in former years, considerable fluctuations of 

 the solar-constant values occurred from day to day. The values 

 found ranged over nearly 10 per cent between the extreme limits 1.81 

 and 1.99 calories, but seldom more than 3 per cent in any 10-day 

 interval. The periods of fluctuation were irregular, as heretofore. 

 Associated with these fluctuations, though perhaps not strictly con- 

 nected numerically, the contrast of brightness between center and 

 edges of the solar disk also varied. Curiously enough, however, the 

 correlation between solar-constant values and contrast values proves 

 to be of opposite sign for these short irregular fluctuations to that 

 which attends the long-period changes which are associated with the 

 general solar activity. In other words, in the progress of the sun 

 spot cycle high solar-constant values and increased contrast between 

 center and edges of the solar disk are associated together with numer- 

 ous sun spots, but for the short irregular period fluctuations of solar 

 radiation, higher solar-constant values are associated with diminished 

 contrast of brightness along the diameter of the solar disk. The year 

 1914 was singularly free from large fluctuations of solar radiation. 

 The extreme range of solar-constant values was only 4 per cent be- 

 tween limits 1.91 and 1.99 calories. Accordingly the year was not 

 very favorable for testing the relation just described. Nevertheless, 

 the results tend to confirm rather than disprove the conclusion 

 reached that for short, irregular fluctuations of the solar radiation 

 high values are associated with less contrast of brightness between the 

 center and edges of the sun. 



