ATMOSPHERIC ELECTRICITY. 369 



experimenter, Nairne,* in which the lightning, after having passed 

 through a tube for conducting rain water, followed a metallic wire, 

 by the aid of which a person could, without going out of his bed, 

 open and shut a gate ; after the passage of the lightning it was dis- 

 covered that this wire was shortened several inches. It is to be 

 observed that the common electric discharge produces the same elFecfe 

 as that mentioned by Nairne, which has been proved by M. Edmund 

 Becquerel.f The latter attributes this phenomenon to the expansive 

 force of the electric spark. 



Another effect of lightning, not less remarkable, is the magnetic 

 polarity which it communicates to iron and steel. The packet ship 

 New York, in May, 1827, exhibited a striking example of this 

 effect. When this ship reached Liverpool, after having been twice 

 struck with lightning, Mr. Scoresby| discovered that the nails of the 

 partitions and broken panels, the iron work of the masts, the knives 

 and forks, even the points of the mathematical instruments, had ac- 

 quired a very decided magnetic power. It has also been observed that 

 when the lightning passed near the needle of the compass it changes 

 its magnetism, sometimes entirely destroys it, or reverses the poles. 

 These eftects, which are common to lightning and to ordinary electri- 

 city, and of which the consequences may be very serious, are not rare ; 

 the annals of science furnish a great number of examples. 



Lightning, in condensing the particles of imperfect conductors of 

 electricity, sets free sufficient heat to kindle light substances, such as 

 straw, hay, cotton, &c. In forcing its way through sand to reach a 

 sheet of water at a certain distance below the surface of the ground, 

 it instantly fuses together the particles, and gives them the form of 

 vitrified cylinders, called fulminating tubes. These tubes, which 

 are sometimes 30 feet long, 2 inches in outer diameter and a few 

 lines in the inner diameter, are often divided into fragments by long 

 transverse fissures and terminated at their lower extremity by several 

 branches resembling roots. Their inside walls consist of glass uni- 

 formly brilliant and pure, while their covering is formed of a crust or 

 grains of agglutinated sand, the color of wjiich varies with the nature 

 of the strata through which the lightning passed. There can remain no 

 doubt as to the oiigin of these tubes, especially after the experiments ef 

 Messrs. Savart, Hachette, and Beudant § These experimenters obtained 

 tubes of 25 millimetres long with a millimetre of inside diameter, by 

 causing the discharge of a very strong battery to pass through glass 

 reduced to powder. Effects of fusion similar to tlie fulminating tubes 

 have been observed on the peaks of high mountains ; where the surfaces 

 of the rocks present balls and vitreous strata, which alike attest the 

 passage of lightning. || 



Lightning produces mechanical effects of an astonishing intensity. 

 Thus when it bursts into a room it almost always displaces or over- 

 throws the furniture and utensils ; tears pieces of metal from 



^Philosophical Transactions, Vol. LXXII, Part I, p. 357. 1782. 

 tBibliothtMiuo Universelle, noiivclle series, torn. XX, p. o50. 1839. 

 :{:Mdmoires de Savants strangers, torn. IV, p. 709. 1S33. 

 §A.nnales de Chimie et dc Physiqvie, toni. XXXVII, p. 319. 1826. 

 I' Voyages dans Ics Alpcs, torn. II, § 1153, p. COS. 



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