16 Dr. Hall on one of the Causes 



The rate of the expansibility of air in contact with water, is 

 also greater as the temperature rises ; for, from the experiments 

 of Saussuret, we learn that the power of air for retaining water 

 in a transparent state increases in a geometric progression, whilst 

 the temperature rises in an arithmetic progression only. 



From these considerations we are enabled to conceive how great 

 the contraction of a given volume of atmosphere must be by the 

 mere admixture of different portions of air charged with vapour 

 and of different temperatures, even without the precipitation of 

 water, but especially if, whilst its temperature is diminished, its 

 transparent vapour be condensed and withdrawn, in the form of 

 clouds and rain. 



To these considerations must be added, that of the vast extent 

 of country over which the changes which take place in the at- 

 mosphere are observed to diffuse themselves, — generally more 

 than one thousand miles, — that of the extent and height of that 

 region of the atmosphere occupied by the clouds — upwards of 

 three miles, — and that of the immense quantity of water which 

 must at some periods pass, in a very short space of time, from the 

 state of transparent vapour to that of clouds or rain. 



If we fully consider the extent and magnitude of these pheno- 

 mena, we must conceive that the degree of contraction in volume 

 in that part of the atmosphere which constitutes the region of the 

 clouds, must on some occasions be great indeed. Let us now 

 consider what must be the further effects of this contraction in 

 the immediate and adjacent region : — a portion of the atmosphere 

 being condensed and withdrawn, the elasticity of the remaining 

 portion occupying the same space must be diminished ; and, as 

 further consequences, the barometer must fall, and the air of the 

 adjacent regions must be attracted, and wind be produced ; — and 

 these effects and phenomena will continue as long as the atmo- 

 spheric process of the formation of clouds or rain continues. 

 When the deposition of atmospheric moisture ceases, the equili- 

 brium in the elasticity of the air is gradually restored, the 

 "barometer rises to its natural level, and the wind or influx of air 

 from the adjacent regions subsides. 



