280 PHILOSOPHY OF STORMS. 



the commencement of the south east wind was the fact that 

 the greatest depression of the "barometer was at some point 

 north of the United States. The chief cause of the depres- 

 sion of the barometer, he says, was this : " The south east 

 wind, which accompanied the rain, moved with an accel- 

 erated velocity. The particles, therefore, of air at one ex- 

 tremity of the current must have left those of the other ex- 

 tremity at an increased distance. Hence a mechanical rare- 

 faction, and of course diminished pressure, near the cen- 

 tral line of greatest depression : part of the depression, he 

 thinks, is due to the upward motion of the air." With 

 great respect for the opinion of the author of this very able 

 and most important paper, I dissent entirely from the views 

 here expressed as to the causes of the barometric depression. 

 There is but one cause of the great barometric fluctua- 

 tions the change of weight of the whole column of air 

 from the surface of the earth to the surface of the atmo- 

 sphere ; and this weight can be affected to any considerable 

 extent only by heat sensibly, however, by moisture. The 

 Professor thinks it cannot depend principally on heat, for 

 the barometer sometimes falls -as the temperature of the air 

 diminishes. But if the doctrine every where taught in this 

 book is correct, the upper regions of the air at this very 

 time are in temperature greatly above the mean, containing 

 the latent caloric of vapor recently condensed in a neigh- 

 boring storm of rain or snow. An upmoving current of air 

 is a proof that the barometer is depressed where it exists 

 below what it is in the borders of the upmoving column ; 

 but it is not the cause of that depression. On the contrary, 

 if the air were forced in on all sides by mechanical means, 

 by impulsion and not by aspiration, towards a central space, 

 it would ascend over that space, and at the same time the 

 barometer would rise ; and the laws of spouting fluids teach 

 us that the rise of the barometer would be in proportion to 

 the square of the velocity of the ascending air. Indeed, 



