124 TABLES AND RESULTS OF THE PRECIPITATION, 



region of rather an exceptional character. According to the statements of Dr. 

 Gibbons and other meteorologists, 1 the most rainy point during the winter is in a 

 direct line with the trend of the southern coast, or about S. S. E. This is nearlv 

 the direction of the great basin formed by the Sacramento and San Joaquin River, 

 and (nearer to the coast) of the Bay of San Francisco and of the Valley of the Salinas 

 Eiver. These great hypsometrical features tend to keep up the direction of the 

 rainy south-easterly winds, as high up as latitude 40|°, beyond which limit a 

 different state of relation between rain and wind prevails. The warm S. E. wind 

 from the region of the Gulf of California and the western coast of Lower Cali- 

 fornia deposits a very small amount of rain in the latitude of San Diego (33°), but 

 parts with its moisture gradually and more freely on its way to the higher and 

 colder latitudes. The chart is here very imperfect for want of material, and con- 

 sidering the configuration of the country, and the variation in the rain-fall, as in- 

 fluenced by height, much labor remains to be expended before a complete analysis 

 and a perfect graphical representation of the distribution of rain can be made out. 

 The west and northwest winds are prevalent during the dry season, and it would 

 seem to be in accordance with facts that this season commences later and termi- 

 nates earlier in the higher than in the lower latitudes. 



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 dur- 

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

 to month, or to develop tho 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 we can hope to exhibit this inequality with 

 desirable clearness. 



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

 signed to show the regularity in the progression, from month to month, of the 

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

 ber 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 fifteen years 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. 



B = A + B 1 ein (0 + C x ) + B sin 2 (20 + C 2 ) + B 3 sin (30 -f Q -f 



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

 A = its mean monthly amount. 



1 See an article "Winds and Rains of California, by H. S. Warner," in the Proceedings of the 

 American Association for the Advancement of Science, Baltimore meeting, 1858. 



