THE AMERICAN STORM OF DECEMBER 20, 1836. 9 



Storm tlie winds blew from the west, northwest, or north ; the average direction 

 being thirty degrees north of west ; and in the southwest part of the United States 

 the winds were somewhat more northerly than in the northwest part of the country. 

 In front of the storm the winds generally blew from some southerly point. TIio 

 average direction was ten degrees east of south ; and in the south part of the; 

 United States the winds were quite as much easterly as in the north part of tlic 

 country. 



We thus find that along a meridian line, for a distance of at least 1,200 miles, 

 v,'e have on the west side a violent current setting from a point thirty degrees north 

 of west ; and on the east side, in close proximity, we have also a strong current 

 setting from a point ten degrees east of south. These two currents blew with great 

 violence for at least forty-eight hours, in directions inclined to each other one hun- 

 dred and thirty degrees. It must be remembered that our observations are confined 

 to the southern half of this storm, and that in order to discover what law the winds 

 observed, it is important to have observations from every side of the centre of the 

 storm. The observations here presented indicate a decided crowding of the winds 

 towards the centre of the storm, and also some tendency to circulate around the 

 centre, in a direction contrary to the motion of the hands of a watch ; but this 

 storm was so much elongated that the direction of the winds cannot be even 

 approximately represented by circles, whose centre is at the point of greatest baro- 

 metric depression. 



Origin of the Storm. 



This storm probably commenced not for to the west of the Mississijipi river, 

 and its origin is to be ascribed to the high barometer prevailing along the eastern 

 part of the United States. This high barometer caused the wind on the eastern 

 side of the Mississippi river to fiow westward. This easterly current was opposed 

 by the normal west wind, which pi'evailcd at places further west. The west wind 

 flowed under the east wind, by which means the latter was lifted up above tlie 

 earth's surface, and its vapor was condensed by the cold of elevation. 1'his con- 

 densed vapor descended in the form of rain or snow, according to the temperature 

 of the place. The heat liberated in the condensation of this vapor expanded the 

 upper stratum of air, causing it to rise above its ordinary elevation, and thus to 

 flow off above, causing a diminished pressure at the earth's surface, so that through- 

 out tlie area of rain and snow the barometer fell below its mean height. This fall 

 of the barometer increased the strength of the east wind which set in from tlie 

 eastern part of the United States, and this being forced upward from the earth's 

 surface by the west wind which prevailed on the western side of the storm, caused 

 a still greater precipitation of moisture, which liberated a greater amount of heat, 

 and increased still further the fall of the barometer. A portion of tlie air which 

 flowed off above from the region of the storm spread eastward, un<l increased the 

 pressure over the eastern portion of the United States, thus adding to the strength 

 of the east wind which blew upon the eastern side of the storm. 



The air which thus spread eastward in a.lvance of tlie storm was heated by the 



