298 



ANNUAL, EEPOET SMITHSONIAN INSTITUTION, 19 3 



and determined the mean of the " solar constant " to be 1.938 calo- 

 ries per minute per square centimeter. Sohir radiation is subject 

 to variations which, although slight, are important. The value of 

 the " solar constant " at a given time may differ from the mean. 

 The highest monthly mean for the years 1918 to 1929 was 1.969 for 

 September, 1921 ; the lowest, 1.912 for July, 1922, giving a range of 

 0.057, or 2.9 per cent. The highest yearly mean for the same period 



was 1.952 for 1921, and 

 the lowest 1.927 for 1922, 

 or a range of 1.3 per cent, 

 according to solar con- 

 stant values supplied by 

 Doctor Abbot in a per- 

 sonal communication of 

 November, 1929. 



The "solar constant" 

 variations for the period 

 1905 to 1929, inclusive, 

 have been arranged in the 

 form of a graph. Figure 

 1, by entering the yearly 

 mean fluctuations on the 

 ordinates and the years 

 on the abscissa. This is 

 the same method and grid 

 used in Figures 2 to 13 

 for the varved clays. The 

 value, 1.900, is equivalent 

 to the base line. The 

 original drawing has been 

 reduced one-half during 

 publication. 



According to Wolfer's 

 sun-spot numbers, there 

 was a maximum of spots 

 in February, 1907, a min- 

 imum in May-June, 1913 ; 

 a maximum in August, 1917, a minimum in August, 1923, and a maxi- 

 mum in July, 1928. These figures, when compared with the solar 

 radiation values, as noted in Figure 1, show a relation between them 

 in the 11-year cycle. Abbot (1929) states that the solar radiation 

 rises to a maximum with medium sun-spot numbers and declines 

 thereafter as sun-spot numbers increase. Furthermore, superposed 

 on the sun-spot influence variation there appears to be three pulses 

 of regular periods of about 25, 15, and 11 months respectively, and 



910 1915 1920 mo 



30LAR. C0N5TAMT VAR1AT10N5 



Figure 1. — Graphs of solar constant and sun-spot 

 variations 1905 to 1929 



Curves smoothed by taking the mean of successivo 

 four (solid line) and eight (dotted line) years, 

 appear above each graph. In the solar-constant 

 graph the vertical linos have heen dotted where 

 the data were incomplete for the year. (Data 

 from Dr. C. G. Abbot, November, 1929.) 



