Meteorological essays / by François Arago ... with an introduction, by Baron Alexander von Humboldt. Tr. under the superintendence of Colonel Sabine.

  • François Arago
Date:
1855
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Licence: Public Domain Mark

Credit: Meteorological essays / by François Arago ... with an introduction, by Baron Alexander von Humboldt. Tr. under the superintendence of Colonel Sabine. Source: Wellcome Collection.

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    exclusive]) entitled to tlie appellation of “ foudre,” or “ thun- derbolts.” * After tlie cases of bifurcation wbich I bave cited, I ouglit to mention tliat M. Gamot (a former pupil of the Ecole Poly- technique) bas written to me an account of having bimself seen, in October, 1838, ligbtnings proceeding from two very different points of a storm-cloud, unité, and descend to the earth united in one. He thougbt be could feel confident that it was not an ascending flash bifurcating before reacbing the cloud. § 2. Lightnings of the Second Class. We corne now to ligbtnings of the second class (Sheet Lightning). The ligbt of these, instead of being narrowed to brigbt sinuous Unes or traces, almost witbout apparent breadth, spreads on the contrary over immense surfaces. It is neitber so wbite nor so vivid as that of the fulminating lightning. It bas (not unfrequently) an intense red tinge, and sometimes blue or violet predominate. When a sheet of lightning of this class is furrowed by a zig-zag lightning of the first class, the différ- ence of colour becomes évident to the most unpractised eye. The hghtnings of the second class sometimes seem merely to illuminate the margins of the cloud from which'they emanate; sometimes their bright light embraces or pervades the whole surface of those clouds, and indeed seems to issue from their interior : the clouds seem to open and disclose their bright interior ; these are popular modes of expression, and I should seek in vain for any which should better depict the pheno- menon. Descriptions can be but very imperfectly successful in cha- racterising meteorological phenomena; but in regard to the liglitnings of which we now speak, readers who may find the above account insufficient, may be aided by being told that they are by far the most common of ail. Many persons hâve never seen, or at least hâve never remarked, any others. Dur- ing an ordinary thunderstorm, there may be as many thousands of them as there are single flashes of the narrow sinuous light- ning of the first class. * Seneca eut short the distinction his cotemporaries made between light- nings and thunderbolts ; saying, lightnings are thunderbolts which do not reach the ground, and thunderbolts are lightnings which do reach it. fQuæst. Nat. liv. ii. § 21.)
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