314 ABBOT, FOWLE, ALDRICH : VARIATION OF THE SUN 



station will be 0.9 per cent. Had the condition of the sky in 

 1912 been free from the haze which prevailed owing to the vol- 

 canic eruption of Mount Katmai, we believe the probable error of 

 the separate determinations of 1912 would have scarcely reached 

 0.5 per cent. 



It will be seen that the measurements of 1912 are on the aver- 

 age above those of 1911, at both stations. The difference 1912- 

 1911 is 0.03 calories per square centimeter per minute. This in 

 itself may be regarded as an indication of the variation of the 

 sun depending upon nearly twenty days of observation in 1911 

 and about thirty days of observation of 1912. 



In further study of the variation of the sun we have compared 

 the mean solar-constant values obtained on Mount Wilson for the 

 different months of the years 1905 to 1912 with the monthly 

 values of the sun-spot numbers as published by Wolfer. We find 

 a fluctuation of solar radiation in the sense that when the sun- 

 spot numbers are high the solar radiation is high and vice versa. 



It is also indicated that when the solar radiation is increased 

 the intensity of the violet and ultra-violet rays of the solar spec- 

 trum (as it would be found outside the atmosphere) is increased 

 with respect to the intensity of the red and infra-red. 



Again it seems to be indicated that when the solar radiation 

 is high the contrast between the brightness of the centers and 

 edge of the solar disk is greater than normal. 



These and other results of this long investigation are published 

 with details in volume 3 of the Annals of the Astrophysical Obser- 

 vatory of the Smithsonian Institution, now in press and expected 

 to appear about July 1, 1913. The most important conclusions 

 are as follows: 



1. The mean value of the solar constant of radiation for the 

 epoch 1905-1912 is 1.932 calories per square centimeter per min- 

 ute. 



2. An increase of 0.07 calories per square centimeter per minute 

 in the ''solar constant" accompanies an increase of 100 sun-spot 

 numbers. 



3. An irregular variation frequently ranging over 0.07 calories 

 per square centimeter per minute within an interval of ten days 



