ABOUT THE 20tH OF DECEMBER, 1836. 165 



throughout the entire length of the United States, a distance of at least twelve 

 hundred miles, and probably much greater. In order, then, that the south- 

 west wind might drain off this vast torrent from the south-east, it would need 

 to blow continuously for at least thirty hours with the velocity observed. And 

 even on this supposition the south-west wind would only drain off the south- 

 eastj so as to prevent its advancing. But we have already seen that it must 

 recede with the same velocity as that with which the opposing wave advances. 

 I might call to ray aid here, if necessary, the observations of the thermometer, 

 but it would be superfluous. The case, to my mind, is perfectly made out. 

 This south-east current found its escape by ascending from the surface of the 

 earth. Having quit the surface, it might either flow on in its first direction 

 over the north-west current, or it might be entirely driven back over the south- 

 east current; or both of these motions might exist simultaneously. When we 

 come to consider the cause of the rain, we shall be able to judge of the proba- 

 bility of these several suppositions. 



If a south-east current should pass a north-west one by blowing over it, the 

 wind, to a stationary observer, would seem to change its direction by a calm. 

 But if one were S. S. E. and the other W. N. W., as was nearly the case in the 

 present instance, the wind would veer rapidly from one quarter to the other, 

 passing through each of the intermediate points. The two currents must ne- 

 cessarily influence each other to some extent. As the north-west wind ap- 

 proached, the south-east would feel its influence, and begin to veer to the south. 

 At a certain instant the two would exert equal power; and the direction of the 

 particles of air would be precisely intermediate between those of the two great 

 currents. Presently the second current w^ould predominate, and, in a short 

 time, the influence of the first would entirely disappear. 



The question naturally arises, What produced this southerly wind through- 

 out a territory so extended? The atmosphere is always warmer in the southern 

 states than in the northern; why, then, should not a current always set from 

 the north? There is no doubt, I think, that throughout this entire region the 

 general progress of the atmosphere near the surface of the earth is towards the 

 south. Not that the wind will necessarily blow a greater number of days in a 

 year from the north than from the south, for the northerly winds are ordinarily 

 far the most violent. One cause of southerly winds is found in the upper cur- 

 rent, which, in these latitudes, blows from a point a little south of west. This 



