138 YALE AGRICULTURAL LECTURES. 



sel, the float rises. The stem is graduated so that a depth of 

 water of one one-hundredth of an inch is easily measured. 



The unequal distribution of rain over the surface of the earth 

 was touched upon, and the influence of mountain ranges was 

 pointed out in causing precipitation of rain. As a general rule 

 the amount of rain was in proportion to the average tempera 

 ture ; or, what is the same thing, to the amount of evaporation: 

 local causes, however, very greatly modify this general rule. 

 The number of rainy days bears no proportion (or an inverse 

 one) to the amount of rain which falls in particular latitudes. 

 Thus while the yearly fall of rain in the tropics is ninety-five 

 inches, there are not over seventy rainy days ; while here, with 

 an annual rain fall of about forty inches, we have one hun 

 dred and thirty or more rainy days. The following table 

 shows that the ordinary rains of the tropical regions are more 

 powerful than those of the temperate regions. 



M. LATITUDE. MEAN ANNUAL NUMBEE OF EAINY DAYS. 



From 12 to 43 78. 



" 43 " 46 103. 



" 46 " 50 134. 



" 50 " 60 161. 



In the northern part of the United States there are, on the 

 average, about 134 rainy days in the year ; in the southern 

 part, about 103. 



The greatest annual depth of rain occurs at San Luis, Maran- 

 ham, 280 inches ; the next in order are Vera Cruz, 278 ; Gre 

 nada, 126; Cape Fram;ois, 120; Calcutta, 81; Rome, 39; 

 London, 25 ; Uttenberg, 12*5. In our country the average 

 annual fall is 39-23 inches; at Hanover, N". H., 38; New York 

 state, 36 ; Ohio, 42 ; Missouri, 38'265. 



Prof. Silliman illustrated these general principles by an anal 

 ysis of the average results observed by Dr. S. P. Hildreth, at 

 Marietta, Ohio, Lat. 39 25' 1ST, and Long. 4 28' W of Wash 

 ington city, for 31 years, from 1828 to 1859. It appeared from 

 these tables that the rain fall at Marietta varied from 61 '84 



