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2. The same Subject continued.

By the same.

Mr. Watson laid before the Royal Society* an account of what had been done by some gentlemen, in order to ascertain the respec. tive velocities of electricity and sound ; from which it appeared, that through a space measuring 6732 feet, the electricity was perceptible in a quantity of time less than 766 of a second. But the gentlemen concerned were desirous, if possible, of ascertaining the absolute velocity of electricity at a certain distance; and a me. thod had been thought of, by which this might be determined with great exactness.

Accordingly, August 5, 1748, there met at Shooter's Hill for this purpose, the president of the R. S. the Rev. Mr. Birch, the Rev. Dr. Bradley, astronomer royal, James Burrow, Esq. Mr. Ellicot, Mr. George Graham, Richard Graham, Esq. the Rev. Mr. Lawrie, Charles Stanhope, Esq. and Mr. W. who were of the Royal Society; also Dr. Bevis, and Mr. Grischow, jun. a member of the Royal Academy of Sciences at Berlin.

It was agreed to make the electrical circuit of two miles; in the middle of which an observer was to take in each hand one of the extremities of a wire, which was a mile in length. These wires were to be so disposed, that this observer being placed on the floor of the room near the electrical machine, the other observers might be able in the same view to see the explosion of the charged phial, and the ob. server holding the wire ; and might take notice of the time lapsed between the discharging the phial and the convulsive motions of the arms of the observer in consequence of it; as this time would show the velocity of electricity, through a space equal to the length of the wire between the coated phial and the observer.

The electrifying machine was placed in the same house as it was last year. We then found ourselves, says Mr. W. greatly embar. rassed by the wire's being conducted by the side of the road, which we were compelled to, on account of the space necessary for the measuring of sound; but so great a distance from the machine was

• See the preceding article.

296 COMMUNICATION & VELOCITY OF ELECTRICITY.

not now wanted, though the circuit through the wire was intended to be at least two miles. We bad discovered, by our former experiments, that the only caution now necessary was, that the wires conducted on dry sticks should not touch the ground, nor each other, nor any non-electrical in a considerable degree, in any part of their length; if they did not touch each o her, the returns of the wire, be they ever so frequent, imported little, as the wire had been found to conduct electricity so much better than the sticks. It was therefore thought proper to place these sticks in a field fifty yards distant from the machine. The length of this field being eleven chains, or 726 feet, eight returns of the wire from the top to the bottom of the field, made somewhat more than a mile, and sixteen returns more than two miles, the quantity of wire intended for the electricity to pass through to make the experiment.

We had found last year, that on dischargiog the electrified phials, if two observers made their bodies part of the circuit, one of which grasped the leaden coating of the phial in one hand, and held in his other one extremity of the conducting wire; and if the other observer beld the other extremity of the conducting wire in one hand, and took in his other the short iron rod with which the explosion was made ; on this explosion, they were both shocked in the same instant, which was that of the explosion of the phial. If therefore an observer, making his body part of the circuit, was shocked in the instant of the explosion of the charged phial in the middle of the wire, no doubt would remain of the velocity of electricity being instantaneous through the length of that whole wire. But if, on the the contrary, the time between making the explosion, and seeing the convulsions in the arms of the observer holding the conducting wires, was great enough to be measured, we then should be able to ascertain its velocity to the distance equal to half the quantity of wire employed only, let the manner of the electricity's discharging itself be what it would.

To make the experiment, the same phial filled with filings of iron, and coated with sheet-lead, which was used last year, was placed in the window of the room near the machine, and was connected to the prime conductor by a piece of wire. To the coating of this phial a wire was fastened, which being conducted on dry sticks to the before-mentioned field, was carried in like manner to the bottom; and being conducted thus from the bottom of the field

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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 deinon. 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. Wben dark or wet weather clears up, 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, becomes 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 knowo: 1. Friction; 2. Evaporation ; 3. Heat and cold; 4. Expansion and contraction : not to mention the electricity evolved by tbo

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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 etfects will take place. It dues not appear, that thunder is often occasioned by a disch -rge of 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 Do light is perceptible when electricity is discharged in a va.

What the change is which electricity produces in air, or how it produces it, are questions which, in the present state of our knowledge, are altogether insoluble. But the very extraordi.

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