1875.] bl«i [Chase. 



YEARLY RAINFALL IN THE UNITED STATES. 



By Pliny Earle Chase, 



Pkofessok of Mathematics in Haa^erford College. 



{Read before the American PJiilosophical Society, August 20, 1875.) 



At the Society's Meeting, on the 16th of April last, I submitted a com- 

 munication on the Lunar-Monthly rainfall in the United States, as de- 

 duced from an examination of the morning weather maps issued by the 

 Signal Service Bureau. The maps extended over a period of about three 

 years, and as the average number of reporting stations was about sixty, 

 the results represented an average of at least 2000 observations for each 

 of the thirty lunar-monthly days. For various reasons, enumerated in 

 the communication, the derived normals should be regarded as only pro- 

 visional ; still, the regularity of the curve, its magnitude, its resem- 

 blance to the Philadelphia curve for 43 years, and the indications of dis- 

 turbances originating beyond the Mississippi river, seem to justify my 

 estimate of the importance of such general comparisons as our National 

 Bureau has for the first time made possible. 



In oi'der to provide still further material for future use, I have tabula- 

 ted the same observations with reference to Earth's annual course around 

 the Sun. The rainfall for each year is divided into 30 periods of 12 or 

 13 days each, always dividing to the nearest day, the first division em- 

 bracing the last six days of one year, and the first six days of the follow- 

 ing year. The total fall for each period was divided by the total number 

 of reports for the same period, and the normals were deduced from the 

 resulting averages in the same manner as in my previous meteorological 

 papers. These normals, as given in the accompanying table, indicate an 

 average solar disturbance about 2.3 times as great as the lunar. This 

 suggests some kind of reciprocal tidal action, and it seems also to point 

 towards an important cosmical law, but more extended observations and 

 comparisons are needful in order to justify any conclusive decision. 

 There are some resemblances between the present curve and the corres- 

 ponding lunar-monthly curve which seem worthy of study, but it is 

 perhaps better to postpone their critical examination, until their signifi- 

 cance is either confirmed or changed by the observations for one or more 

 additional periods of like duration. For the convenience of those who 

 may desire to make comparisons without waiting for further data, I 

 copy the lunar normals alongside of the solar. 



