LIGHTNING. The general circumstances attendant on a thunder storm are familiar to moat persons. It will however be tieeful to state some of the most prominent, with a view to their explanation when regarded as electrical phenomena.
At first we see light clouds forming with jagged edges, the relative motions of which are frequently opposite and variable. The atmos phere at the surface of the earth enjoys a stillness and calm, accompanied with some elevation of temperature, as well as considerable barometric and hygrometric changes, which produce on the animal system the sensations of closeness, faintness, and oppreseion, and appear even to the brute creation indicative of some awful and impending changes. Some of these light clouds appear stationary, as if the force!, which produced contrary motions in the others made en equilibrium in those. A low murmuring and continued sound of distant thunder is soon board, after which the lower region of the air is refreshed with cooler but light breezes of uncertain direction. The calm is resumed, buts the thunder-clende are nearer, apparently larger, and much and their influence on the nervous eystem is felt by an indc.crihable sensation of uneasiness. Lightning flashes are now per ceived at short intervals ; their course is sometimes zigzag, when it is called forked lightning ; the aberrations in its course show that it is near terrestrial objects, and is therefore justly regarded as dangerous. In a few seconds after the discharge, heavy showers of rain or hail descend, and the atmosphere is again cooled. The blackness now comes universal ; the thunder, which ,before roared continuously and at a sensible interval after the discharge of the lightning, is now heard in a loud and sudden clap almost at the same instant that the lightning is seen descending towards the earth with immense velocity, and re sembling a globe of flame.
These phenomena are the most common concomitants of thunder storms, particularly in summer time. But storms are also produced by rapid changes in the atmospheric currents, for instance when the equinoctial gales usually set in ; or, as met with in violent and destruc tive gales, as in crossing the Atlantic with both a revolving and pro gressive movement, and becoming mixed with various strata of the air through the regions which they traverse, accompanied in several places by destructive thunder-storms.
The colour of the lightning is generally a variable yellow, but it may also be red, blue, or violet, depending much on the density and composition of the strata of air through which the discharge takes place.
In 1752, Franklin in America, and Dalibard in France, commenced, independently of each other, a series of experiments tending to identify lightning with the discharge of ordinary) electricity. [ELECTRICITY, ATMOSPHERIC.] Their identity might well be suspected from the number of analogies known to exist between them. For example, the zigzag path of the electric spark from the prime conductor of an electrical machine to a conductor held within striking distance of it resembles on a small scale the course of forked lightning ; both are drawn off by pointed bodies in preference to others, and lightning also prefers, ceteris paribus, the best electrical conductors : both can fuse metals and inflame combustibles, destroy sight and animal life, magne tise steel and reverse the poles of a magnet.
Franklin, in June, 1752, perceiving a thunder-cloud approaching, sent up a silk kite attached to a dry hempen cord. The loose threads of the cord stood erect, and upon pointing his finger to the cord, he drew sparks. When in consequence of a little rain the hempen cord became a better conductor, the supply of electricity from the cloud became more copious, and by the smartness of the shock ensuing, the danger of prolonging the experiment was sufficiently indicated.
A similar result had already been obtained in France a month earlier, by Messrs. Dalibard and Delor, acting however in obedience to the instructions sent to Europe by Franklin, in 1749. Atmospheric electricity became a favourite study now that the identity between lightning and ordinary electricity was established. it received a check however in consequence of an accident to Professor Richman, of St. Petersburg, who having attached a simple species of electrometer to his apparatus for measuring the electric intensity of a thunder-cloud, proceeded, immediately after a loud clap, to read off the indication of his instrument, when a globe of electric fire was discharged through his body, and lie fell instantly dead. This was on the 6th August, 1753.