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dry: lightning burns, dissolves metals, rends some bodies, some. times 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 exhi. biting any visible marks of injury; and in this case Signior Beccaria supposed 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; his hypothesis, however, was controverted by Dr. Priestley and a better theory, that of the returning stroke, has since been introduced by Lord Stanhope to explain the phænomenon.
To demonstrate, 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, hence 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 verified 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 capa ble of being applied to practical use in life, and in consequence he
proposed a method, which he soon accomplished, of securing build. ings from being damaged by lightning, by means of conductors.
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 unfavorable circumstances, they had hitherto failed of success. Mr. Canton, however, succeeded in July 1752; and in the following month, Dr. Bavis and Mr. Wilson observed nearly 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 fre. quently struck to the earth, the same clouds were the next moment ready to make a still greater discharge, and his apparatus conti. nued 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 first 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 fluid, and where the electric matter will discharge itself on the earth. A
channel of communication being thus produced, 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. Beccaria observes further, that a wind always blows from the place from which the thunder-cloud proceeds; and 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, confirm 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 their production 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 the vibration, on which the sound depends, commences at the same moment; yet when the flash is directed toward 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 er 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 opposite electricities, a discharge will take place within a certain distance; or the one will strike into the other, and that in the discharge a flash of lightning will be seen. 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 upward by various means, communicates its electricity to the clouds.
It has since been observed by Lord Mahon, now Earl Stanhope, that damage may be done by lightning, not only by the main stroke and lateral explosion, but also by what his lordship calls THE RETURNING STROKE; by which is meant, as we shall notice more fully in a subsequent article, the sudden and violent return of that part of the natural share of electricity gradually expelled from some body or bodies within the range of the main stroke, by the additional passage of the electrical atmosphere discharged from the thunder cloud.
[Phil. Trans. Rozier. Payne. Editor.
Invention and curious properties of the Electrical Kite*.
1. Letter from Benjamin Franklin, Esq. to Mr. Peter Collinson, F.R.S. dated Philadelphia, Oct. 1, 1752.
As frequent mention is made in the public papers from Europe, of the success of the Philadelphia experiment, for drawing the electric fire from clouds, by means of pointed rods of iron erected on high buildings, &c. it may be agreeable to the curious to be informed, that the same experiment has succeeded in Philadelphia, though made in a different and more easy manner, which any one may try, as follows:
Make a small cross, of two light strips of cedar, the arms so long as to reach to the four corners of a large thin silk handker. chief, when extended; tie the corners of the handkerchief to the extremities of the cross, so you have the body of a kite, which being properly accommodated with a tail, loop, and string, will rise in the air like those made of paper; but this, being of silk, is fitter to bear the wet and wind of a thunder-gust without tear. ing. To the top of the upright stick of the cross is to be fixed a very sharp-pointed wire, rising a foot or more above the wood. To the end of the twine, next the hand, is to be tied a silk rib band; and where the twine and silk join, a key may be fastened. * See further on this subject, section v. 1.
The kite is to be raised when a thunder gust appears to be com. ing on, and the person who holds the string must stand within a door, or window, or under some cover, so that the silk ribband may not be wet; and care must be taken that the twine does not touch the frame of the door or window. As soon as any of the thunder-clouds come over the kite, the pointed wire will draw the electric fire from them; and the kite, with all the twine, will be electrified; and the loose filaments of the twine will stand out every way, and be attracted by an approaching finger.
When the rain has wet the kite and twine, so that it can con duct the electric fire freely, you will find it stream out plentifully from the key on the approach of your knuckle. At this key the phial may be charged; and from electric fire thus obtained spirits may be kindled, and all the other electrical experiments be performed, which are usually done by the help of a rubbed glass globe or tube, and thus the sameness of the electric matter with that of lightning completely demonstrated.
Phil. Trans. 1752.
2. The same subject continued.
By W. Watson, Esq. F.R.S.
AFTER the communications received from several correspondents in different parts of the continent, acquainting us with the success of their experiments last summer, in endeavouring to extract the electricity from the atmosphere during a thunder-storm, in consequence of Mr. Franklin's hypothesis, it may be thought extraor. dinary that no accounts have been yet laid before the Society of our success here from the same experiments. That no want of attention therefore may be attributed to those here, who have been hitherto conversant in these inquiries, he states, that though several members of the Royal Society, as well as himself, did, on the first advices from France, prepare and set up the necessary apparatus for this purpose, they were defeated in their expectations, by the uncommon coolness and dampness of the air here, during the whole summer. They had at London only one thunder-storm; viz. on July 20; and then the thunder was accompanied with rain, so that by wetting the apparatus the electricity was dissipated too soon to be perceived on touching those parts of the apparatus