298 M. De la Rive on the Formation of Hail 



But why, it may be inquired, dtoes this distribution of tem- 

 perature, which produces so great an accumulation of electricity 

 in the atmosphere, for the most part also produce the pheno- 

 menon of hail ? In answer to this question, we must recollect, 

 that the thicker the layer of clouds which intercepts the heat 

 from the earth, the colder is the upper po on of the atmo- 

 spheric column. Its temperature ought tu decidedly infe- 



rior to that of ice, for if the cloud completely interrupted all 

 the terrestrial heat, it would be reduced to the temperature of 

 the upper limit of the atmosphere, which is less than — 50° 

 Cent. ; but there is no necessity it should be so low as this. 

 It is enough that its temperature should be sufficiently low to 

 congeal the drops of water at the upper part of the cloud, and 

 so to freeze them as to render them capable, when impelled 

 by the wind, as noticed by M. Lecoq, to traverse the layers 

 of clouds, and descend towards the earth in consequence of 

 their weight, at the same time condensing and freezing upon 

 their surface the vapours through which they pass. Thus, the 

 same cause which favours the abundant accumulation of atmo- 

 spheric electricity in the cloud, will also be that which most as- 

 suredly effects the formation of hail. In particular, the im- 

 mense heat which is usually experienced before a thunderstorm 

 precisely indicates the existence of an invisible stratum, or of a 

 cloud which, placed in some part of the atmosphere above the 

 observer, intercepts the terrestrial heat, and sends it back 

 whence it comes, instead of allowing it to proceed into free 

 space ; consequently, the higher the temperature is raised on the 

 surface of the earth at any given time, the more it must be de- 

 pressed on the other extremity of the column, or on the other 

 side of the cloud, and consequently there must be the greater 

 tendency both to the development of electricity, and to the for- 

 mation of hail. 



Again, the hailstones once formed, enlarge more or less, ac- 

 cording to the length of the course which the wind causes them 

 to pursue through the atmosphere ; according to the quantity of 

 water which they meet with in this course, and, finally, according 

 to the temperature more or less low which they possess at the 

 moment of their formation. In winter, if they be small, it is 

 owing to the absolute quantity of water which the atmosphere 



