ATMOSPHERIC ELECTRICITY. 337 



lands in the temperate zone.* Sometimes, however, the 

 stratum of cloud from which the thunder proceeds 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 — zigzag, 

 and sharply defined at the edges ; in sheets of light, illumin- 

 ating a whole cloud, which seems to open and reveal the light 

 within it ; and in the form of fire-balls. t 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 phenom- 

 enon), isolated clouds, standing high above the horizon, con- 

 tinue uninterruptedly for some time to emit a luminous ra- 

 diance from their interior and from their margins, although 

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

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

 have been seen to fall in a luminous condition, when the phe- 

 nomenon was not preceded by thunder. In the geographical 

 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$ from the testimony 

 of navigators (Scoresby, Parry, Ross, and Frankhn), 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 na- 

 ture which we are now concluding shows that the processes 

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

 riations in the elastic and electric tension, and in the hygro- 

 metric condition of the vast aerial ocean, are all so intimate- 

 ly connected together, that each individual meteorological 

 process is modified by the action of all the others. The com- 



* Arago, in the Annuaire dn Bureau des Longitudes pour 1838, p. 246. 

 t Arago, op. cit., p. 249-266. (See, also, p. 268-279.) 

 X Arago, op, cit., p. 388-391. The learned academician Von Baer, 

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

 taken into consideration the extreme rarity of storms in Iceland and 

 Greenland ; he has only remarked {Bulletin de V Academie de St. Piters- 

 bourg, 1839, Mai) that in Nova Zembla tmd Spitzbergen it is sometimes 

 heard to thunder. 



Vol. I.— P 



