194 TABLES AND RESULTS OF THE PRECIPITATION, 



THE ANNUAL FLUCTUATION IN THE RAIN-FALL. 



In the preceding charts of the distribution of the rains in summer and winter 

 we already have had striking examples of the changes in the amount of rain during 

 these extreme seasons. We now propose to follow these changes from month to 

 month, or to develop the annual fluctuation. The data for this investigation are 

 furnished by the tables, series B, from which we select those stations where the 

 rain-record extends over the longest series of years. It is only by the combination 

 of results for a longer period that wc can hope to exhibit this inequality with desira- 

 ble clearness. 



The following three diagrams for Marietta, San Francisco, and Boston, are 

 designed to show the rcrjulariiy in the progression, from month to month, of the 

 annual fluctuation ; the diversity of distribution during the year in regard to number 

 of maxima and minima as well as to their epochs of recurrence ; and the range of 

 the yearly variation. 



To facilitate comparison all diagrams in this article are drawn to the same scale, 

 excepting those plotted from ratios. The apparent irregularities in the curves can 

 only be lessened by a series of observations extending over many years, and in the 

 present discussion no station was admitted with less than tifteen j^ears of record. 

 To render the comparisons, as far as practicable, independent of these accidental 

 irregularities, the mean monthly rain-fall was expressed analytically by means of 

 Bessel's circular function. 



R = A + B^ sin {o + C\) + B, sin (2o -\- C\) + B, sin {^o + C,) + 



where B = rain-fall in inches for any one month. 

 A — its mean monthly amount, 



