IN RAIN AND SNOW, IN THE UNITED STATES. 



125 



B 1 B 2 B 3 . . . the parameter of its fluctuations for the periods of one, one- 

 half, and one-third of a year. 

 6 = arc proportional to the time and inversely proportional to the length 

 of the period, which is represented by 360°. This angle consequently 

 counts at the rate of 30° a month. 

 C x C 2 C 3 . . . angular constants having reference to the epochs of their 

 respective periods. 

 Three terms (involving and its multiples) of this function are quite sufficient 

 when applied to the fluctuations in the rain-fall. 





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The following table contains the computed values of A B t B 2 B 3 G x C 2 C 3 for 

 all stations having fifteen or more years of record. The angle 6 counts from 

 January 1. 



The probable error of any computed monthly value, as given in the last column, 



is derived from the approximate formula 0.845 , in which 2A = sum of 



y n(n-l) 



differences of observed and computed monthly values, all taken as positive. The 



condition 2 A = is satisfied ; n = 12. Of the 12 values given by observation at 



any station, 7 are necessary for determining the constants. 



