Imagens das páginas

posed to the escape of the superabundant electricity of the body, into that rare part of its ele&rical atmosphere into which the point projects.

Among other illustrations of this principle the Author produces the case of a pointed wire, placed between two round or prominent metallic bodies, with its point on a level with their furfaces. In this situation, when presented to an electrified body, it acts no longer as a point, or only in a very small degree: because the dense part of the electrical atmosphere of the two round bodies flows or is extended over it.

The Author's succeeding experiments shew that an insulated smooth body, a cylindrical conductor for instance, immerged within the electrical atmosphere, but beyond the striking distance, of another body, which we shall suppose to be charged politively *, is, at one and the same time, in different parts of it, in a state of three-fold electricity,' The end next to the charged body acquires negative electricity; and the farther end becomes positively electrified: while a certain part of the body, somewhere between its two extremities, is in a natural, uneliitrified, or neutral state: so that the two contrary electricities exactly counterbalance each other in that part. The Author on this occasion employs geometrical reasoning, as well as ex. periments, to determine the precise place of this un-electrified point, or rather line, in a cylinder or other given body; and to demonstrate that the density of electrical atmospheres is inversely as the square of the distance from the ele&rified body.

We scarce need to add, that if the body be not insulared, or have a communication with the earth, the whole of it will be in a negative state: a certain portion of its natural quantity of electricity being driven into the common mals, by the pressure, repulsion, or other action, of the electric matter belonging to the charged prime conductor. The extenfive and fruitful principle, on which this and the preceding effects depend, has frequently been explained or referred to in the course of our Journal; particularly, and very lately, in our Review for December last, page 408; where it is noticed for the purpose of explaining the phenomena of the electrophorus. We take the more particuJar notice of it at present; as one of the Author's most remark able observations on the subject of thunderstorms, and from which he draws some very itriking, indeed formidable, conclufions, is founded upon it.

We allude to what the Author calls the electrical returning firske ;' by means of which, he alleges that, in a thunderstorm,

* To avoid repetition, or circumlocution, we fall, throughout the remainder of this Article, conftantly luppose the electrified body to be charged with pofitive electricity.


the most fatal effe&ts may be produced, even at a vast distance from the place where the lightning falls. This observation appears to be of so much importance, that we shall endeavour to give as clear an idea of the experiments on which it is founded, as can be conveyed by us, without the use of plates : not confining ourselves to any particular experiment; but relating such material circumstances common to them all, as may best convey the Author's meaning in the fewest words.

We ought to premise that the Author used a very powerful machine, made by Mr. Nairne; the prime conductor of which (fix feet long, by one foot diameter) would generally, when the weather was favourable, ftrike into a brass ball connected with the earth, to the distance of eighteen inches, or more. In the following account this brass ball, which we fhall call A, is supposed to be constantly placed at the firiking distance; so that the prime conductor, the instant that it becomes fully charged, explodes into it.

Another large conductor, which we shall call the second conductor, is suspended, in a perfectly insulated state, farther from the prime conductor than the striking distance, but within its electrical atmosphere;—at the distance of six feet, for instance. A person standing on an insulating stool touches this second conductor very lightly with a finger of his right hand; while, with a finger of his left hand, he communicates with the earth, by touching very lightly a second brass ball fixed at the top of a metallic stand, on the floor, and which we shall call B.

While the prime conductor is receiving its electricity, sparks pafs (at least if the distance between the two conductors is not too great) from the second conductor to the insulated person's right hand; while fimilar and simultaneous sparks pass out from the finger of his left hand into the second metallic ball B, communicating with the earth. These sparks are part of the natural quantity of electric matter belonging to the second conductor, and to the insulated person ; driven from them, into the earth, through the ball B, and its stand, by the elastic pressure or action of the electrical atmosphere of the prime conductor. The second conductor and ihe insulated person are hereby reduced to a negative state.

At length however the prime conductor, having acquired its full charge, suddenly strikes into the ball, A, of the first metallic ftand, placed for that purpose at the striking distance of 17 or 18 inches. The explosion being made, and the prime conductor suddenly robbed of its electric atmosphere, its preflure or action on the lecond conductor, and on the insulated person, as suddenly ceases; and the latter instantly feels a smart returning froke, though he has no direct or visible communication (except by the floor) either with the striking or struck body; and is placed at the dis


Ff 3

tance of five oro fix feet from both of them. This returning Stroke is evidently occasioned by the sudden re-entrance of the electric fire naturally belonging to his body and to the second conductor, which had before been expelled from them by the action of the charged prime conductor upon them; and which returns to its former place, the instant that action or elastic pressure ceases. The Author Mews that there can be no reason to fuppofe that the electrical discharge from the prime conductor should, in this experiment, divide itself at the instant of the explofion, and go different ways ; so as to strike the second conductor and insulated person in this manner, and at such a distance from it.

When the second conductor and the insulated person are placed in the densest part of the electrical atmosphere of the prime conductor, or just beyond the striking distance; the effects are still more considerable: the returning froke being extremely severe and pungent, and appearing considerably sharper than even the main stroke itself, received directly from the prime conductor. This circumstance the Author alleges as an unanswerable proof that the effect which he calls the returning stroke was not produced by the main proke being any wise divided at the time of the explofion, fince no effect can ever be greater than the cause by which it is immediately produced.'-Having taken the returning froke eight or ten times one morning, he felt a considerable degree of pain across his chest during the whole evening, and a disagreeable sensation in his arms and wrists all the next day.

We come now to the application of this experiment, and of the doctrine deduced from it, to what paffes in natural elettricity, or during a thunderstorm; in which there is reason to expect similar effects, but on a larger scale:-a scale so large indeed, according to the Author's representation, that persons and animals may be destroyed, -and particular parts of buildings may be considerably damaged, by an electrical returning froke, occasioned even by some very diftant explosion from a thundercloud ;'- possibly at the distance of a mile or more.

It is certainly easy to conceive that a charged extensive thun. dercloud must be productive of effects similar to those produced by the Author's prime conductor. Like it, while it continues charged, it will, by the superinduced elastic electrical preffure' of its atmosphere-to use the Author's own expression drive into the earth a part of the electric Auid naturally belonging to the bodies which are within the reach of its widely extended atmosphere; and which will therefore become negatively electrical. This portion too of their electric fire, as in the artificial experiments, will, on the explosion of the cloud, at a diflance, and the cellation of its action upon them, suddenly return

to them; so as to produce an equilibrium, and restore them to their natural state.

We cannot however agree with the ingenious Author, with respect to the greatness of the effects, or of the danger, to be apprehended from the returning stroke in this case: as we think his estimate is grounded on an erroneous foundation.- Since, says he, the electrical density of the electrical atmosphere of a thundercloud, is so immense, when compared to the electrical density of the electrical atmosphere of any prime conductor, charged by means of any electrical apparatus whatsoever; and fince a returning stroke, when produced by the sudden removal of even the weak elastic electrical pressure of the electrical atmosphere of a charged prime conductor, may be extremely strong, as we have seen above: it is mathematically evident, that, when a returning stroke comes to be produced by the sudden removal of the very strong elastic electrical pressure of the electrical at. mosphere of a thundercloud powerfully charged; the strength of such a returning firoke must be enormous.'

If indeed the quantity of electric Auid naturally contained in the body of a man, for instance, were immense, or indefinite, the Author's estimate between the effects producible by a cloud, and those caused by a prime conductor, might be admitted. But surely an electrified cloud, -how great soever may be its extent, and the height of its charge, when compared with the extent and charge of a prime conductor-cannot expel from a man's body (or any other body) more than the natural quantity of electricity which it contains. On the sudden removal therefore of the pressure by which this natural quantity had been expelled, in consequence of the explofion of the cloud into the earth; no more (at the utmost) than his whole natural flock of electricity can re-enter his body f. But we have no reason to suppose that this quantity is so great, as that its sudden reentrance into his body should destroy or even injure him.

In the experiment above described, the insulated person re. ceives into his body, at the instant of the returning /troke, not only all that portion of his own natural electric fire which had been expelled from it; but likewise transmits through it, at the same instant, in consequence of his peculiar situation, all the electric fire of which the large second conductor had been robbed ; and which must necessarily re-pass through his body, to arrive at that conductor. To render the case somewhat parallel, in natural electricity, the man's body must be lo peculiarly circumstanced, fuppofing him to be in a house, that the electric matter which

I We suppose the person not to be so situated, that the returning fire of other bodies must necessarily pass through his body.



has been expelled from the house into the earth, by the pressure of an extensive thundercloud, could not return back into the building, on the explosion of the cloud at a distance, without passing through his body: a case not likely to happen, un. less the house were insulated (like the second conductor in the preceding experiment), and his body became the channel through which alone the house could have its electric matter restored to it: it appears much more probable that the electric matter returns to the house through the same channels by which it before insensibly passed out, and with equal filence, though more suddenly.

In the case of a man who is abroad, and in an open field, during the time of an explofion ;-as he is unconnected with other masses of matter above him, no more than the precise quantity of electric fire, which had been before expelled from his body, will suddenly return into it at the instant of a distant explofion : and that this quantity is not usually very large may be inferred from many considerations.

When a person standing on the ground holds a pair of Mr. Canton's balls in his hand, while a highly charged thundercloud is suspended over his head; the angle made by the balls indicates the electrical state of that person, or the quantity of natural electric city of which his body is at that time deprived, by the action of the (positively) charged cloud hanging over him. But we have never seen the repulsion of the balls to considerable, as to furnith any just apprehensions that the return of his natural electric matter, however sudden, could be attended with injury to him: nor would he be sensible of any commotion on the bails suddenly coming together; though a spark might undoubtedly be per. ceived, at that instant, were he insulated, and placed in the same manner with the Author, when he tried the above related experiment.

The Author nevertheless obferves, that there have been inItances of persons, who have been killed by natural ele&tricity, having been found with their shoes torn, and with their feit damaged by the electrical fire; but who have not had, over their whole body, any other apparent marks of having been struck with lightning. He adds, if a man walking out of doors were to be killed by a returning stroke; the electrical fire would rush into that man's body through his feet, and his feet only; which would not be the case, were he to be killed by any main froke of explosion, either positive or negative.'

It would be no difficult task, we think, to account for these appearances in a different manner; were all the circumstances attending the case minutely ascertained : but without interrogating the dead on this subject, we may more satisfactorily appeal


« AnteriorContinuar »