THE GASEOUS PORTIONS OF THE ATMOSPHERE. 



26 



Now a rise of temperature in the warm column of 20°, 

 would reduce its weight and pressure below that of the 

 adjoining cold air, equal to about two inches of mercury. 

 A difference of 40°, which would be produced at a height 

 of only 4^000 yards, would reduce the pressure of the warm 

 column, at the surface, equal to 4 inches of mercury. We 

 see, then, that, — when condensation and congelation take 

 place successively in an ascending column, amply supplied 

 with fresh vapour from below, a force is brought into 

 action capable of reducing the pressure of that column, on 

 the surface of the earth, to a great extent, and that force 

 may be the cause of the greatest descents of the barometer 

 that have been known to occur. It is not here presumed 

 that the supposititious case that has been adduced includes 

 all the circumstances that should be examined, if we were 

 describing a real occurrence, but that case enables us to 

 perceive that there is a power which is sometimes brought 

 into action, that is capable of producing those alterations of 

 atmospheric pressure that have been hitherto considered in- 

 explicable. We see also, that the power is the same in is 

 nature as that which evidently produces other alterations that 

 frequently take place to a smaller extent; and that power 

 results, first, from the warming and lightening of the atmos- 

 pheric gases by the condensation of vapour that is intermixed 

 with and diffused through them ; and, secondly, it may be 

 from the freezing of the particles of water produced by the 

 condensation. 



