LIGHTNING

, a large bright flame, shooting swiftly through the atmosphere, of momentary or very short duration, and commonly attended with thunder.

Some philosophers accounted for this awful natural phenomenon in this manner, viz, that an inflammable substance is formed of the particles of sulphur, nitre, and other combustible matter, which are exhaled from the earth, and carried into the higher regions of the atmosphere, and that by the collision of two clouds, or otherwise, this substance takes fire, and darts out into a train of Light, larger or smaller according to the strength and quantity of the materials. And others have explained the phenomenon of Lightning by the fermentation of sulphureous substances with nitrous acids. See Thunder.

But it is now universally allowed, that Lightning is really an electrical explosion or phenomenon. Philosophers had not proceeded far in their experiments and enquiries on this subject, before they perceived the obvious analogy between Lightning and electricity, and they produced many arguments to evince their similarity. But the method of proving this hypothesis beyond a doubt, was first proposed by Dr. Franklin, who, about the close of the year 1749, conceived the practicability of drawing Lightning down from the clouds. Various circumstances of resemblance between Lightning and electricity were remarked by this ingenious philosopher, and have been abundantly confirmed by later discoveries, such as the following: Flashes of Lightning are usually seen crooked and waving in the air; so the electric spark drawn from an irregular body at some distance, and when it is drawn by an irregular body, or through a space in which the best conductors are disposed in an irregular manner, always exhibits the same appearance: Lightning strikes the highest and most pointed objects in its course, in preference to others, as hills, trees, spires, masts of ships, &c; so all pointed conductors receive and throw off the electric fluid more readily than those that are terminated by slat surfaces: Lightning is observed to take and follow the readiest and best conductor; and the same is the case with electricity in the discharge of the Leyden| phial; from whence the doctor infers, that in a thunder-storm, it would be safer to have one's cloaths wet than dry: Lightning burns, dissolves metals, rends some bodies, sometimes strikes persons blind, destroys animal life, deprives magnets of their virtue, or reverses their poles; and all these are well-known properties of electricity.

But Lightning also gives polarity to the magnetic needle, as well as to all bodies that have any thing of iron in them, as bricks &c; and by observing afterwards which way the magnetic poles of these bodies lie, it may thence be known in what direction the stroke passed. Persons are sometimes killed by Lightning, without exhibiting any visible marks of injury; and in this case Sig. Beccaria supposes that the Lightning does not really touch them, but only produces a sudden vacuum near them, and the air rushing violently out of their lungs to supply it, they cannot recover their breath again: and in proof of this opinion he alleges, that the lungs of such persons are found flaccid; whereas these are found inflated when the persons are really killed by the electric shock. Though this hypothesis is controverted by Dr. Priestley.

To demonstrate however, by actual experiment, the identity of the electric fluid with the matter of Lightning, Dr. Franklin contrived to bring Lightning from the heavens, by means of a paper kite, properly fitted up for the purpose, with a long fine wire string, and called an electrical kite, which he raised when a thunder-storm was perceived to be coming on: and with the electricity thus obtained, he charged phials, kindled spirits, and performed all other such electrical experiments as are usually exhibited by an excited glass globe or cylinder. This happened in June 1752, a month after the electricians in France, in pursuance of the method which he had before proposed, had verisied the same theory, but without any knowledge of what they had done. The most active of these were Messrs. Dalibard and Delor, followed by M. Mazeas and M. Monnier.

In April and June 1753, Dr. Franklin discovered that the air is sometimes electrified negatively, as well as sometimes positively; and he even found that the clouds would change from positive to negative electricity several times in the course of one thunder-gust. This curious and important discovery he soon perceived was capable of being applied to practical use in life, and in consequence proposed a method, which he soon accomplished, of securing buildings from being damaged by Lightning, by means of Conductors. See the word.

Nor had the English philosophers been inattentive to this subject: but, for want of proper opportunities of trying the necessary experiments, and from some other unfavourable circumstances, they had failed of success. Mr. Canton, however, succeeded in July 1752; and in the following month Dr. Bevis and Mr. Wilson observed near the same appearances as Mr. Canton had done before. By a number of experiments Mr. Canton also soon after observed that some clouds were in a positive, while some were in a negative state of electricity; and that the electricity of his conductor would sometimes change, from one state to the other, five or six times in less than half an hour.

But Sig. Beccaria discovered this variable state of thunder clouds, before he knew that it had been observed by Dr. Franklin or any other person; and he has given a very exact and particular account of the external appearances of these clouds. From the observations of his apparatus within doors, and of the Lightning abroad, he inferred, that the quantity of electric matter in a common thunder storm, is inconceivably great, considering how many pointed bodies, as spires, trees, &c, are continually drawing it off, and what a prodigious quantity is repeatedly discharged to or from the earth. This matter is in such abundance, that he thinks it impossible for any cloud or number of clouds to contain it all, so as either to receive or discharge it. He observes also, that during the progress and increase of the storm, though the lightning frequently struck to the earth, the same clouds were the next moment ready to make a still greater discharge, and his apparatus continued to be as much affected as ever; so that the clouds must have received at one part, in the same moment when a discharge was made from them in another. And from the whole he concludes, that the clouds serve as conductors to convey the electric fluid from those parts of the earth that are overloaded with it, to those that are exhausted of it. The same cause by which a cloud is sirst raised, from vapours dispersed in the atmosphere, draws to it those that are already formed, and still continues to form new ones, till the whole collected mass extends so far as to reach a part of the earth where there is a deficiency of the electric sluid, and where the electric matter will discharge itself on the earth. A channel of communication being thus formed, a fresh supply of electric matter is raised from the overloaded part, which continues to be conveyed by the medium of the clouds, till the equilibrium of the fluid is restored between the two places of the earth. Sig. Beccaria observes, that a wind always blows from the place from which the thunder-cloud proceeds; and it is plain that the sudden accumulation of such a prodigious quantity of vapours must displace the air, and repel it on all sides. Indeed many observations of the descent of Lightning, consirm his theory of the manner of its ascent; for it often throws before it the parts of conducting bodies, and distributes them along the resisting medium, through which it must force its passage; and upon this principle the longest flashes of Lightning seem to be made, by forcing into its way part of the vapours in the air. One of the chief reasons why these flashes make so long a rumbling, is that they are occasioned by the vast length of a vacuum made by the passage of the electric matter: for although the air collapses the moment after it has passed, and that the vibration, on which the sound depends, commences at the same moment; yet when the slash is directed towards the person who hears the report, the vibrations excited at the nearer end of the track, will reach his ear much sooner than those from the more remote end; and the sound will, without any echo or repercussion, continue till all the vibrations have successively reached him.

How it happens that particular parts of the earth, or the clouds, come into the opposite states of positive and negative electricity, is a question not absolutely determined: though it is easy to conceive that when particular clouds, or different parts of the earth, possess op-| posite electricities, a discharge will take place within a certain distance; or the one will strike into the other, and in the discharge a flash of Lightning will be feen. Mr. Canton queries whether the clouds do not become possessed of electricity by the gradual heating and cooling of the air; and whether air suddenly rarefied, may not give electric fire to clouds and vapours passing through it, and air suddenly condensed receive electric fire from them.——Mr. Wilcke supposes, that the air contracts its electricity in the same manner that sulphur and other substances do, when they are heated and cooled in contact with various bodies. Thus, the air being heated or cooled near the earth, gives electricity to the earth, or receives it from it; and the electrified air, being conveyed upwards by various means, communicates its electricity to the clouds.—Others have queried, whether, since thunder commonly happens in a sultry state of the air, when it seems charged with sulphureous vapours, the electric matter then in the clouds may not be generated by the fermentation of sulphureous vapours with mineral or acid vapours in the air.

With regard to places of safety in times of thunder and Lightning, Dr. Franklin's advice is, to sit in the middle of a room, provided it be not under a metal lustre suspended by a chain, sitting on one chair, and laying the feet on another. It is still better, he says, to bring two or three mattresses or beds into the middle of the room, and folding them double, to place the chairs upon them; for as they are not so good conductors as the walls, the Lightning will not be so likely to pass through them: but the safest place of all, is in a hammock hung by silken cords, at an equal distance from all the sides of a room. Dr. Priestley observes, that the place of most perfect safety must be the cellar, and especially the middle of it; for when a person is lower than the surface of the earth, the Lightning must strike it before it can possibly reach him. In the fields, the place of safety is within a few yards of a tree, but not quite near it. Beccaria cautions persons not always to trust too much to the neighbourhood of a higher or better conductor than their own body; since he has repeatedly found that the Lightning by no means descends in one undivided track, but that bodies of various kinds conduct their share of it at the same time, in proportion to their quantity and conducting power. See Franklin's Letters, Beccaria's Lettre dell' Ellettricessimo, Priestley's Hist. of Electric., and Lord Mahon's Principles of Electricity.

Lord Mahon observes that damage may be done by Lightning, not only by the main stroke and lateral explosion, but also by what he calls the returning stroke; by which is meant the sudden violent return of that part of the natural share of electricity which had been gradually expelled from some body or bodies, by the superinduced elastic electrical pressure of the electrical atmosphere of a thunder cloud.

Artificial Lightning, an imitation of real or natural Lightning by gunpowder, aurum fulminans, phofphorus, &c, but especially the last, between which and Lightning there is much more resemblance than the others.

Phosphorus, when newly made, gives a sort of arti- <*>cial Lightning visible in the dark, which would sur- prise those not used to such a phenomenon. It is usual to keep this preparation under water; and if it is desired to see the corruscations to the greatest advantage, it should be kept in a deep cylindrical glass, not more than three quarters silled with water. At times the phosphorus will send up corruscations, which will pierce through the incumbent water, and expand themselves with great brightness in the upper or empty part of the glass, and much resembling Lightning. The season of the year, as well as the newness of the phosphorus, must concur to produce these flashes; for they are as common in winter as Lightning is, though both are very frequent in warm weather. The phoiphorus, while burning, acts the part of a corrosive, and when it goes out resolves into a menstruum, which dissolves gold, iron, and other metals; and Lightning, in like manner, melts the same substances.