[ 6o ] 



vapor as 12 to 10, therefore the gain here Is 0,49 of an inch, 

 which deduded from 0,59 the lofs, leaves the lofs ■— of an inch. 

 This, therefore, is the variation the barometer fhould undergo by 

 the paflage of a column of air from abfolute drynefs to complete 

 faturation, a circumftance which perhaps never takes place, as 

 the atmofphere is never abfolutely dry ; and yet previous to 

 heavy rains we often obferve the barometer to fall 3, 4 or 5-tenths 

 of an inch, a fall which we fee cannot originate from the fatura- 

 tion of the atmofphere with vapor. Nor is there any proportion 

 between the afcent of ? after heavy rains and the weight of 

 vapor condenfed, for in fuch cafes the $ frequently rifes 3 or 

 4-lenths of an inch ; and yet the heavieft rain feldom produces- 

 one cubic inch of water, and the weight of a cubic inch of water 

 is not equal to that of even i^ of a cubic inch of ?. 



Of the unequal Diffujion of the higher Atmofphere. 



Having thus fliewn the infufficiency of thofe caufes to which 

 the variable weight of the atmofphere and height of the baro- 

 meter have been ufually referred, I now proceed to explain that 

 ■which alone feems to me adequate to the efFe£ls produced j 

 namely, the accumulation of air over thofe parts of the globe in 

 which the mercury exceeds its mean height, that is the height 

 fuited to its fituation, and the diminution or fubtradion of the 

 natural quantity of air over thofe regions in which the ? falls 

 beneath its mean height^ To trace the origin of this accumulation 

 and diminution we muft confider what may be called the natural 

 ftate of the atmofphere, and how this flate is difturbed. 



I call 



