160 ON THE STORM EXPERIENCED THROUGHOUT THE UNITED STATES 



to this difference of latitude, for winter, in the United States, is about ten de- 

 grees. But the temperature of the air generally rose more than this number 

 of degrees above the mean. Moreover, there prevailed in the northern states a 

 vast body of snow and ice, which rapidly absorbed the caloric of this southern 

 current. The explanation, then, is insufficient. We shall find an additional 

 cause for this elevated temperature in the abundant precipitation of aqueous 

 vapour. When vapour becomes reduced to the liquid state, it gives up a large 

 amount of latent heat, which will, of course, be taken up by the surrounding 

 air. The temperature of the eastern states was found to be a little greater than 

 that of the western. At the former, the southerly wind came from the ocean, 

 which, in winter, has a higher temperature than the land. It is not clear, 

 however, that more rain fell in one section than the other. 



That the north-west wind was cold is not remarkable. It came from a high 

 latitude, where the prevalent temperature is, probably, far below zero. Its se- 

 verity, however, became somewhat tempered in its progress, the mean of the 

 minima at the eastern stations being fourteen degrees higher than at the 

 western. 



I come now to the phenomenon probably the most difficult of all to be ex- 

 plained, namely, the oscillation of the barometer. For the movements of this 

 instrument various causes have been assigned, some of them so unphilosophi- 

 cal that it is really astonishing that they could ever have been seriously pro- 

 posed by intelligent men. 



1. The oscillations of the barometer have been ascribed to the destruction of 

 large masses of air in the higher regions by electricity. The supposition is too 

 gratuitous to deserve serious consideration. 



2. They have been ascribed to the diminished pressure resulting from the 

 loss of rain. But the amount of rain which fell in the case under considera- 

 tion would be balanced by a column of mercury about one-fifteenth of an inch 

 in height. 



3. Heat, by expanding a column of air, causes it to ascend to a greater 

 height, and thus changes its centrifugal force arising from the earth's rotation. 

 This cause is too insignificant to produce the effect in question. 



4. They have been ascribed to the attractions of the sun and moon. La- 

 place estimates the greatest oscillation of the barometer due to this cause to be, 

 at the equator, 0.025 inch. 



