ATMOSPHERIC ELECTRICITY. 845 



however, the stratum of cloud from which the thunder pro- 

 ceeds, sinks to a distance of 5000, or indeed only 3000 feet 

 above the plain. 



According to Arago's investigations the most comprehen- 

 sive that we possess on this difficult branch of meteorology 

 the evolution of light (lightning) is of three kinds: zig-zag, 

 and sharply denned at the edges ; in sheets of light, illuminat- 

 ing a whole cloud which seems to open and reveal the light 

 within it; and in the form of fire-balls.* The duration of 

 the two first kinds scarcely continues the thousandth part of 

 a second ; but the globular lightning moves much more slowly, 

 remaining visible for several seconds. Occasionally (as is 

 proved by the recent observations, which have confirmed the 

 description given by Nicholson and Beccaria of this pheno- 

 menon) isolated clouds standing high above the horizon, 

 continue uninterruptedly for some time to emit a luminous 

 radiance from their interior and from their margins, although 

 there is no thunder to be heard, and no indication of a storm ; 

 in some cases even hail-stones, drops cf rain, and flakes of 

 snow have been seen to fall in a luminous condition, when 

 the phenomenon was not preceded by thunder. In the geo- 

 graphical distribution of storms, the Peruvian coast, which is 

 not visited by thunder or lightning, presents the most striking 

 contrast to the rest of the tropical zone, in which, at certain 

 seasons of the year, thunder-storms occur almost daily, about 

 four or five hours after the sun has reached the meridian. 

 According to the abundant evidence collected by Arago f from 

 the testimony of navigators (Scoresby, Parry, Ross, and 

 Franklin) there can be no doubt that, in general, electric 

 explosions are extremely rare in high northern regions 

 (between 70 and 75 latitude.) 



The meteorological portion of the descriptive history of nature 

 which we are now concluding, shows, that the processes of 

 the absorption of light, the liberation of heat, and the varia-- 



* Arago, Op. cit., p^. '249-266. (See also pp. 268-279.) 

 t Arago, Op. cit., pp. 388-391. The learned Academician von Baoi, 

 who has done so much for the meteorology of Northern Asia, has not 

 taken into consideration the extreme rarity of storms in Iceland and 

 Greonland ; he has only remarked (Bulletin de I' A cademie de St. 

 Petersbourg, 1839, Mai), that in Nova Zembla and Spitsbergen it ia 

 aonie times heard to Uiundei. 



