LIGHTNING (Fr. eclair, Ger. Blitz), the name given to the sudden discharge of elm ' tricity between one group of clouds and another, or between the clouds and the ground. It is essentially the same, though on a much grander scale, as the spark obtained from . an electric machine. Clouds charged with electricity are called thunder-clouds, and are -easily known by their peculiarly dark and dense appearance. The height of thunder.
•clonds is very various; sometimes they have been seen as high as 25,700 feet, and a thuuder-cloud is recorded whose height was only 89 feet above the ground. According to Arago, there are three kinds of lightning, which he names lightning of the first, sec -ond, and third classes. Lightning of the first class is familiarly known as forked light ning (Fr. eclair en zig-zag). It appears as a broken line of light, dense, thin, and well ,detined at the edges. Occasionally, when darting between the clouds and the earth, it breaks up near the latter into one or two forks, and is then called bifurcate or trifurcate. The terminations of these branches are somethnes several thousand feet from each other.
• On several occasions the length of forked lightning has been tried to be got at trigono 'metrically, and the result gave a length of several nines. Lightning of tbe second class is what is commonly called sheet-lightning (Ger. Fliichenblitz). It has no definite form, but seems to be a great mass of light. It has not the intensity of lightning of the first class. Sometimes it is tinged decidedly red; at other times, blue or violet. When it • occurs behind a cloud, it lights up its outline only. Occasionally, it illumines the world • of clouds, and appears to come forth from the heart of them. Sheet-lightning is very much more frequent than forked-lightning. Lightning of the third kind is called ball lightning (Fr. globes de feu, Ger. Kugelblitz). This so-called lightning describes, perhaps, :more a meteor, which, on rare occasions, accompanies electric discharge, or lightning proper, than a phenomenon in itself electrical. It is said to occur in this way: After a violent explosion of lightning, a ball is seen to proceed from the region of the explo ,sion, and to make its way to the earth in a curved line like a bomb. When it reaches the ground it either splits up at once and disappears, or it rebounds like an elastic ball :several times before doing so. It is described as being very dangerous, readily setting fire to the building on which it alights; and a lightning-conductor is no protection :against it. Ball-lightning lasts for several seconds, and, in this respect, differs very widely from lightning of the first and second classes, which are, in the strictest sense, momentary.
The thunder (Fr. tonnerre, Ger. Donner) which accompanies lightning, as well as the snap attending the electric spark, has not yet been satisfactorily accounted for. Both,
no doubt, arise from a commotion of the air brought about by the passage of electricity; but it is difficult to understand how it takes place. Suppose this difficulty cleared, there still remains the prolonged rolling of the thunder, and its strange rising and fall ing to account for. The echoes sent between the clouds and the earth, or between •objects on the earth's surface, may explain this to some extent, but not fully. A person in the immediate neighborhood of a flash of lightning hears only one sharp report, which is peculiarly sharp when an object is struck by it. A. person at a distance hears the same report as a prolonged peal, and persons in different situations hear it each in a different way. This may be so far explained. The path of the lightning may he reckoned at one or two miles in length, and each point of the path is the origin of a separate sound. Suppose, for the sake of simplicity, that the path is a straight line, a person at the extremity of this line must hear a prolonged report; for though the sound -originating at each point of the path is produced at the same instant, it is some time before the sound coming from the more distant points of the line reaches the eEtr. A person near the middle of the line hears the whole less prolonged, bocause he is more equidistant from the different parts of it. Each listener in this way hears a different peal, according to the position he stands in with reference to the line. On this suppo ,sition, however, thunder ought to begin at its loudest, and gradually die away, because the sound comes first from the nearest points, and then from points more and more dis tant. Such, however, it is well known, is not the ease. Distant thunder at the beg,in ning is just audible, and no more; then it gradually swells into a crashing sound, and again grows fainter, till it ceases. The rise and fall are not continuous, for the whole peal appears to be made up of several successive peals, which rise and fall as the whole. :Some have attempted to account for this modulation front the forked form of the light ning, which makes so many different centers of sound, at different angles with each other, the waves coming from which interfere with each other, at one time moving in opposite directions and obliterating tlic sound, at another in the same way, and then streng,thening the sound produced by. each. Thunder has never been heard more titan 14 in. front the flash. The report of artillery.has been heard at much gteater distances. It is said that the cannonading at the battle of Waterloo was heard at the town of Creil, in the u. of France, about 115 m. from the field.