138 



TABLES AND RESULTS OP THE PRECIPITATION, 









New Bedford, Mass. 54 years, 1813- 



58. 









Epoch. 



Jan. 



Feb. 



March. 



April. 



May. 



June. 



July- 



August. 



Sept. 



Oct. 



Nov. 



Dec. 



1813-19 



6.6 



10.8 



8.7 



10. 1 



9-7 



8.2- 



3-9 



9.2 



10.4 



6.0 



10.5 



6.0 



1820-29 



7.2 



8.9 



9.8 



7.2 



8.9 



6.1 



6.8 



11. 8 



7-i 



9.2 



8-9 



8.0 



1830-39 



9.0 



6.6 



7.8 



8.0 



9.6 



7-7 



7-5 



9.8 



7.2 



8.7 



8.3 



9.8 



1840-49 



7-9 



7-3 



8.8 



7-7 



7.8 



6.7 



6.0 



9.0 



7.2 



9.6 



11. 1 



11.0 



1850-59 



9.0 



7-i 



7.0 



11. 2 



8.4 



5-6 



9.8 



7.6 



9.4 



6-3 



8.6 



9-9 



1860-68 



7-9 



8.6 



9-5 



7-3 



8.9 



7-5 



7.6 



9.2 



7-3 



7-9 



9-4 



9.0 









Marietta, Ohio 



. 48 years, 1818-67. 











1818-23 



5-5 



7.6 



9-3 



7.6 



9-3 



9-3 



12.6 



9-5 



8.0 



7-5 



8.1 



5-7 



1826-29 



6-3 



11. 1 



7-9 



10.7 



7-3 



10.5 



9.8 



7-7 



5-7 



6.9 



6.2 



9.8 



1830-39 



7.0 



7.2 



6.2 



5-8 



10.5 



13.0 



12.1 



9-3 



7.0 



7-i 



8.1 



6.7 



1840-49 



6.8 



5-i 



7-7 



6.9 



8.9 



11.5 



10. 1 



9.0 



8.4 



8-5 



6.7 



10.4 



1850-59 



5-7 



7-9 



6.1 



10.0 



10.8 



9.2 



8-3 



9-9 



6.9 



7-4 



8.0 



9.8 



1860-67 



9.2 



6.6 



8.7 



9-9 



9.6 



7.2 



10.0 



9.1 



7-3 



8.0 



6.9 



7-5 



Sacramento, Cal. 18 years, 1849-67. 



1849-5S 



12.5 



12. 1 



24.6 



10.9 



3-i 



0.2 



0.0 



0.0 



0.9 



1.4 



10.7 



23.8 



1856-61 



J 3-9 



14.9 



14.2 



7-3 



5-9 



0.6 



0.6 



0.0 



0.0 



4-5 



11.4 



26.7 



1862-67 



29.2 



13-9 



8-3 



6-3 



4-9 



0.0 



0.0 



0.0 



0.0 



1.1 



13.6 



22.7 



It will be seen that in the above specimens of successive annual fluctuations, the 

 incidental irregularities even for decennial means are yet far too great to allow a 

 recognition of any secular change in the annual inequality, which, at any rate, 

 must be quite small. 



Causes of the Annual Fluctuation.— The subject of the annual fluctuation may 

 be closed with a few remarks respecting the cause of the same. Of the various 

 forms which we have seen the annual inequality to assume, there is for the eastern 

 part of the United States an evident tendency of a maximum about summer and 

 of a minimum about winter, while on the Pacific Slope the reverse of this obtains. 

 In the first named locality two intermediate maxima, and particularly one minimum 

 in autumn, combine in various degrees with the principal extremes to shape the 

 annual fluctuation. The capacity of the atmosphere for rain is a maximum in sum- 

 mer, when it holds the greatest absolute amount of vapor, and a minimum in winter, 

 when the vapor is least in amount ; but the relative humidity, which is more directly 

 related to rain, at these seasons is the reverse of the above distribution. The 

 temperature changes, as measured by daily or monthly extremes, are greater in 

 winter than in summer, and supposing the air saturated with moisture, any fall in 

 temperature will precipitate rain or snow ; these temperature changes are generally 

 brought about by changes in the direction of the wind, and this is one of the causes 

 of the annual fluctuation; its principal origin, however, must be sought in the rela- 

 tive frequency of those winds which are charged with the vapors derived directly 

 from the surrounding oceans. We thus require, for the study of the rain-fall, a 

 classification of the winds, as recorded at the time of rain or snow, together with 



