316 



DISCUSSION OF THE SECULAR VARIATION 



On the annexed diagram, the upper pair of curves refer to stations on the Atlantic 

 coast, the lower pair to stations in the Mississippi valley ; the heavy lines represent 

 the secular change in the temperature, the light ones that of the rain-fall. Though 

 the connection between the changes of temperature and rain-fall is not, in detail, 

 any way conclusive, yet in general following out the larger waves, there seems to 

 be some ground for concluding that years with a mean temperature above the 

 normal have a rain-fall above the normal or average amount, and years deficient in 

 the mean temperature present also a deficiency in the rain-fall. 



That this apparent law is not expressive in the minor undulations may be 

 explained by the small number of stations contributing information to both 

 temperature and rain-fall, and thus admitting the presence to some extent of local 

 peculiarities; yet it cannot be overlooked that there is some similarity in the general 

 character of the two phenomena; further comparisons, however, are desirable. 















































































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In explanation it may be remarked, that the greater the heat of the air, the 

 greater the amount of vapor it can hold, hence the greater the capacity for pre- 

 cipitation as well as for evaporation. 



Comparison of the secular variation in the temperatihre loith the average annual 

 direction of the ivind. — -The following numbers have been extracted from p. 42 of 

 my discussion of the Meteorological Observations' at Brunswick, Maine, made by 

 Prof. P. Cleaveland ; they give the deflections in degrees, -|- to the north (increasing 

 azimuth), — to the south (decreasing azimuth), from the mean assumed direction 

 of the wind x = 101°, counted like azimuths from the south around by west to 360°. 









I 



2 



3 



4 



S 



6 



7 



8 



9 



i8oo 























... 



... 





+ 3° 



i8io 



+ 5 



+ 6 







— 6 



— 7 



— 6 



— 8 



— 8 



+ 4 



+ 20 



1820 



-F28 



+ 25 



+ 13 



+ s 



+ 16 



+ 22 



+ 19 



+ 13 



+ 9 



+ 11 



1830 



+ S 



— 5 



— 2 



+ 3 



+ 4 



+ 6 



+ I 



— 6 



— 9 



—IS 



1840 



—18 



—14 



— 9 



— 12 



— 12 



■ — 12 



— 10 



— II 



— 7 



— 5 



1850 



— 7 



— 9 



—II 



-IS 



—IS 



— 10 



— II 



—13 



— 10 





' Smithsonian Contributions to Knowledge, No. 204 ; Washington, June, 1867. 



