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to the top, and from the top to the bottom seven other times, re. turned again into the room and was held in one hand of an ob. server near the machine. From the other hand of this observer another wire, of the same length with the former, was conducted in the same manner, and returned into the room, and was fastened to the iron rod with which the explosion was made. The whole length of the wires, allowing ten yards for their turns round the sticks, amounted to two miles and a quarter and six chains, or 12276 feet.

When all parts of the apparatus were properly disposed, several explosions of the charged phial were made; and it was invariably seen, that the observer holding in each hand fone of the extremities of these wires was convulsed in both his arms in the instant of making the explosions.

Instead of one, four men were then placed, holding each other by the hand near the machine, the first of which held in his right hand one extremity of the wire, and the last man the other in his left. These also were all seen convulsed in the instant of the ex. plosion. Every one who felt it complained of the severity of the shock.

On these considerations we were fully satisfied, that through the whole length of this wire, being 12276 feet, the velocity of the electricity was instantaneous.

[Id.

sectION III.

Atmospherical Electricity. Air is one of those bodies which have received the name of electric, because they are capable of being positively or negatively charged with electric matter. It not only contains that portion of electricity which seems necessary to the constitution of all terres. trial bodies, but it is liable also to be charged negatively or positively when electricity is abstracted or introduced by means of conducting bodies. These different states must occasion a variety of phæno. mena, and in all probability contribute very considerably to the various combinations and decompositions which are continually going on in air. The electrical state of the atmosphere, then, is a point of considerable importance, and has with great propriety oc. cupied the attention of philosophers ever since Dr. Franklin demon. strated that thunder is occasioned by the agency of electricity.

1. The most complete set of observations on the electricity of the atmosphere were made by Professor Beccaria of Turin. He found the air almost always positively electrical, especially in the day time and in dry weather. When dark or wet weather clears vp, the electricity is always negative. Low thick fogs rising into dry air carry up a great deal of electric matter.

2. In the morning, when the hygrometer indicates dryness equal to that of the preceding day, positive electricity obtains even before sunrise. As the sun gets up, this electricity increases more re. markably if the dryness increases. It diminishes in the evening.

3. The mid-day electricity of days equally dry is proportional to the heat.

4. Winds always lessen the electricity of a clear day, especially

if damp.

5. For the most part, when there is a clear sky and little wind, a considerable electricity arises after sunset at dew falling.

6. Considerable light has been thrown upon the sources of atmo. spherical electricity by the experiments of Saussure and other phi. losophers. Air is not only electrified by friction like other electric bodies, but the state of its electricity is changed by various chemical operations which often go on in the atmosphere. Evaporation seems in all cases to convey electric matter into the atmosphere; and Saussure has ascertained, that the quantity of electricity is as much increased when water is decomposed, as when water is dropt on a red hot iron. On the other hand, when steam is condensed into vesicular vapour, or into water, the air becomes negatively electric. Hence it would seem that electricity enters as a component part into water ; that it separates when water is decomposed or ex. panded into steam, and is reunited wben the steam is condensed again into water.

Farther, Mr. Canton has ascertained that dry air, when heated, beconies negatively electric, and positive when cooled, even when it is not permitted to expand or contract : and the expansion and contraction of air also occasion changes in its electric state.

Thus there are four sources of atmospheric electricity known: 1. Friction ; 2. Evaporation ; 3. Heat and cold ; 4. Expansion and contraction : not to mention the electricity evolved by the melting, freezing, solution, &c. of various bodies in contact with air.

7. As air is an electric, the matter of electricity, when accumu. lated in any particular strata, will not immediately make its way to the neighbouring strata, but will induce in them changes similar to what is induced upon plates of glass or similar bodies piled upon each other. Therefore if a stratum of air be electrified positively, the stratum immediately above it will be negative, the stratum above that positive, and so on. Suppose now that an imperfect conductor were to come into contact with each of these strata, we know, from the principles of electricity, that the equilibrium would be restored, and that this would be attended with a loud noise, and with a flash of light. Clouds which consist of vesicular vapours mixed with particles of air are imperfect conductors ; if a cloud therefore come into contact with two such strata, a thunder-clap would follow. If a positive stratum be situated near the earth, the intervention of a cloud will, by serving as a stepping-stone, bring the stratum within the striking distance, and a thunder-clap will be heard while the electrical Auid is discharging itself into the earth. If the stratum be negative, the contrary effects will take place. It does not appear, that thunder is often occasioned by a disch

rge electric matter from the earth into the atmosphere. The accidents, most of them at least, which were formerly ascribed to this cause, are now much more satisfactorily accounted for by Lord Stanhope's Theory of the Returning Stroke. Neither does it appear that electricity is often discharged into the earth, as the effects of few thunder-storms are visible upon the earth; that it is so sometimes, however, is certain.

In examining and detailing this curious phænomena, there is one remark entitled to particular attention : -it is this, that, during every discharge of electricity, whether natural or arti. ficial, through air, some change similar to combustion undoubt. edly takes place. The light and the peculiar smell with which all electrical discharges are accompanied demonstrate this ; for no light is perceptible when electricity is discharged in a va. cuum. What the change is which electricity produces -in air, or bow it produces it, are questions which, in the present state of our knowledge, are altogether insoluble. But the very extraordi.

of

nary galvanic phænomena which at present occupy the attention of philosophers promise not only to throw light upon this important subject, but to demonstrate a much closer connection between chemistry and electricity than has hitherto been suspected.

[Thomson.

CHAP. XLIII.

ELECTRICITY OF THUNDER AND LIGHTNING.

SECTION

General history of this curious and interesting discovery. There is no subject in natural history that has more attracted or more deserved to attract the attention of philosophers, than the cause of Thunder and Lightning. The magnificence, power and splendour of these combined meteors—the tremendous sound, the brilliant and rapid corruscation, and the awful effects produced without any visible instrumentality, have all concurred in fixing the mind in all ages upon these stupendous phænomena.

Among the Greek philosophers the sources of thunder and lightning were separated from each other. The former was as. cribed to a variety of causes, of which Lucretius enumerates not less than ten, the principal of them being the shock of clouds against clouds, and of winds against winds, meeting together from adverse points. The latter was attributed to an accumulation of pure etherial particles of elementary fire, of exquisite minuteness, concentrated in the cloud or clouds, whence the thunder-storm issued, and there creating a gass or vapour of a peculiar and indi. vidual quality *, an idea strongly congruous with the discoveries of modern times.

It was long afterwards supposed, that both phænomena had one common origin, and proceeded from sulphureous, nitrous, bitu. minous or other inflammable vapours ascending from the bowels of

* See Good's Lucretius and Notes, book vi. v. 98, and following.

the earth, and fermenting together in the atmosphere. Thomson, that he might be sure of being right, enlists the whole of these into service; first describing the ascent of inflammable substances into the atmosphere; and then telling us that they there gradually

Ferment; till by the touch etherial rous'd,
The dash of clouds, or irritating war
Of fighting winds, while all is calm below,
They furious spring.

SUMMER, 1103. Within the last half century, however, philosophers have confined themselves to the etherial touch alone ; to the accumulation of particles of elementary fire of a peculiar quality, to which alone the school of Epicurus ascribed the origin of lightning, and they have abundantly succeeded in proving, that this peculiar quality is that of the electric fluid; or, in other words, that thunder and lightning are altogether electrical phænomena.

The philosophers of the middle of the last century had not pro. ceeded 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 identity. 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 flat 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; whence the doctor infers, that in a thunder-storm, it would be safer to have one's clothes wet than

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