Virginia Advocate

SCIENTIFIC.

Phenomena of Thunder and Lightning.—Air, and all gases, are non-conductors: but vapour and clouds, which are composed of it, are conductors.—Clouds consist of small hollow vesicles of vapours, charged each with the same kind of electricity. It is this electric charge which prevents the vesicles from uniting together, and falling down in the form of rain. Even the vesicular form which the vapour assumes is probably owing to the particles being charged with electricity—The mutual repulsion of the electric particles may be considered as sufficient (since they are prevented from leaving the vesicle by the action of the surrounding air and of the surrounding vesicles) to give the vapour the vesicular form. In what way these clouds come to be charged with electricity, it is not easy to say. But, as electricity is evolved during the act of evaporation the probability is, that clouds are always charged with electricity, and that they owe their existence, or at least their form, to that fluid. It is very probable, that, when two currents of dry air are moving different ways, the friction of the two surfaces may evolve electricity. Should these currents be of different temperatures, a portion of vapour, which they always contain, will be deposited; the electricity evolved will be taken up by that vapour, and will cause it to assume the vesicular state constituting a cloud. Thus we can see in general how clouds come to be formed, and how they contain electricity. This electricity may be either vitreous or resinous, according to circumstances. And it is conceivable, that, by long continued opposite currents of air, the charge accumulated in a cloud may be considerable. Now, when two clouds charged, the one with positive and the other with negative electricity, happen to approach within a certain distance, the thickness of the coating of electricity increases on the two sides of the clouds which are nearest to each other. This accumulation of thickness soon becomes so great as to overcome the pressure of the atmosphere, and a discharge takes place, which occasions the flash of lightning. The noise accompanying the discharge constitutes the thunder clap, the long continuance of which partly depends upon the reverberations from neighboring objects. It is therefore longest and loudest, and most tremendous, in hilly countries.

A thunderstorm in this country commonly commences in the following manner. A low dense cloud begins to form in a part of the atmosphere that was previously clear. This cloud increases fast, but only from its upper part, and spreads into an arched form, appearing like a large heap of cotton wool. Its under surface is level, as if it rested on a smooth plane. The wind is hushed, and every thing appears preternaturally calm and still. Nevertheless, small ragged clouds, like teazled flakes of cotton, soon begin to make their appearance, moving about in various directions, and perpetually changing their irregular surface, appearing to increase by gradual accumulation. As they move about they approach each other, and appear to stretch out their ragged arms towards each other, before they coalesce with the upper cloud. But as frequently the upper cloud coalesces without them. Its lower surface, from being level and smooth, now becomes ragged, and its tatters stretch down towards the others, and long arms are extended towards the ground. The heavens now darken apace; the whole mass sinks down; wind rises and suddenly shifts in squalls; small clouds move swiftly in various directions; lightning darts from the cloud. A spark is sometimes seen co-existent through a vast horizontal extent, of a zigzag shape, and of different brilliancy in different parts. Lightning strikes between the clouds and the earth—frequently in two places at once. A very heavy rain falls—the cloud is dissipated, or it rises high and becomes light and thin.

These electrical discharges obviously dissipate the electricity: the cloud condenses into water, and occasions the sudden and heavy rain which always terminates a thunderstorm. The previous motion of the clouds, which act like electrometers, indicate the electrical state of different parts of the atmosphere.

Thunder, then, only takes place when the different strata of air are in different electrical states. The clouds interposed between these strata are also electrical, and owe their vesicular nature to that electricity. They are also conductors. Hence they interpose themselves between strata in different states, and arrange themselves in such a manner as to occasion the mutual discharge of the strata in opposite states. The equilibrium is restored, the clouds deprived of their electricity collapse into rain and the thunder terminates. * * * These electrical discharges sometimes take place without noise. In that case the flashes are very bright; but they are single flashes passing visibly from one cloud to another, and confined usually to a single quarter of the heavens. When they are accompanied with the noise which we call thunder, a number of simultaneous flashes, of different colors, & constituting an uninterrupted zigzag line, may generally be observed stretching to an extent of several miles.—These seem to be occasioned by a number of successive or almost simultaneous discharges from one cloud to another; these intermediate clouds serving as intermediate conductors or stepping stones for the electrical fluid. It is these simultaneous discharges which occasion the rattling noise which we call thunder. Though they are all made at the same time, yet, as their distances are different, they only reach our ear in succession, and thus occasion the lengthened rumbling noise so different from the sharp which accompanies the discharge of a Leyden jar. If the electricity were confined to the clouds, a single discharge (or a single flash of lightning) would restore the equilibrium. The cloud would collapse and discharge itself in rain, and the serenity of the heavens would be restored. But this is seldom the case. I have witnessed the most vivid discharges of lightning, from one cloud to another, which enlightened the whole horizon, continue for several hours, and amounting to a very considerable number, not fewer certainly than fifty, and terminating at last in a violent thunderstorm. We see that these discharges, though the quantity of electricity must have been immense, did not restore the equilibrium. It is obvious from this, that, not only the clouds, but the strata of air themselves, must have been strongly charged with electricity.—The clouds, being conductors, served the purpose of discharging the electricity with which they were loaded, when they came within the striking distance. But the electric stratum of air with which the cloud was in contact, being a non-conductor, would not lose its electricity by the discharge of the cloud. It would immediately supply the cloud with which it was in contact with a new charge.