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uneven, with hollows occasioned by the impressions of the hammer.
25. In those concave impressions the metal is thinner than it is around them, and probably thinnest near the centre of each impression.
26. The lightning, which in passing through the vane was not sufficient to melt its thicker parts, might be sufficient to melt the thinner (6, 7, 8, 9), and to soften those that were in the middle state.
27. The part of the tin (18), which covered the thinner parts, being more easily melted and exploded than copper (10), might possibly be exploded when the copper was but melted. The smoke appearing in several places (20) is a proof of explosion.
28. There might probably be more tin in the concave impressions of the hammer on one side of the plate, than on the convex part of those impressions on the other. Hence stronger explosions on the concave side.
29. The nature of those explosions is to act violently in all directions; and in this case, being near the plate, they would act against it on one side, while they acted against the air on the other.
30. These thin parts of the plate being at the same instant partly in fusion, and partly so softened as to be near it, the softened parts were pushed outwards, a hole made, and some of the melted parts blown away; hence there was not left metal enough to re-fill the vacancy by bending back the ragged parts to their places.
31. The concave impressions of the hammer, being indifferently made on both sides of the plate, it is natural, from 28, 29, 30, that the pushing outwards of the softened metal by explosions, should be on both sides of the plate nearly equal.
32. That the force of a simple electrical explosion
very great, appears from the Geneva experiment, wherein a spark between two wires, under oil in a drinking-glass, breaks the glass, body, stem, and foot, all to shivers.
33. The electric explosion of metal acts with still more force. A strip of leaf-gold no broader than a straw, exploded between two pieces of thick lookingglass, will break the glass to pieces, though confined by the screws of a strong press; and, between two pieces of marble pressed together by a weight of twenty pounds, will lift that weight. Much less force is necessary to move the melted and softened parts of a thin plate of copper.
34. This explication of the appearances on the vane is drawn from what we already know of electricity and the effects of lightning. The learned author of the account gives a different but very ingenious one, which he draws from the appearances themselves. The matter pushed out of the holes is found, that of some on one side of the plate, and of others on the other. Hence he supposes them to be occasioned (if I understand him right) by streams or threads of electric matter of different and contrary kinds, rushing violently towards each other, and meeting with the vane, so accidentally placed, as to be found precisely in the place of their meeting, where it was pierced by all of them, they all striking on both its sides at the same instant. This however is so extraordinary an accident, as to be in the author's own opinion almost miraculous; "Passeranno" (says he) "forse più secoli prima che ritorni tralle infinite combinazioni un caso simile a quello della banderuola che ora abbiamo per mano. Forza è che si esaurisca una non più udita miniera di fulmini sopra una grande città, pressoche seminata di campanili e di banderuole, il che è rarissimo; e può ancora [cento?] NN*
volte ciò succedere, senza che s'incontri giammai un altera banderuola tanto opportunamente situata tra i limiti della fulminea explosione."
35. But, though the author's explication of these appearances of the vane does not satisfy me, I am not so confident of my own as to propose its being accepted without confirmation by experiment. Those who have strong electric batteries may try it thus; form a little vane of paper, and spot it on both sides by attaching small pieces of leaf-gold or tinfoil, not exactly opposite to each other; then send the whole force of the battery through the vane, entering at one end of it and going out at the other. If the metal explodes, I imagine it will be found to make holes in the paper, forcing the torn parts out on the sides opposite to the metal. A more expensive but perhaps a more satisfactory experiment would be, to make a new vane as exactly as possible like that in question, in all the particulars of its description, and place it on a tall mast fixed on some hill subject to strokes of lightning, with a better conductor to the earth than the wood of the mast; if this should be struck in the course of a few years, and the same effects appear upon it, it would be still more miraculous to suppose it happened by accident to be exactly situated where those crossing threads of different electricities were afterwards to meet.
36. The perforation of glass bottles when overcharged is, I imagine, a different case, and not explicable by either of these hypotheses. I cannot well suppose the breach to be occasioned by the passage of electricity through it; since a single bottle, though so broken in the discharge, always is found to send round in its usual course the quantity with which it was charged. Then the breach never happens but at the instant of the circuitous discharge, either by the dis
charging rod, or in overleaping the borders of the glass. Thus, I have been present when a battery of twenty glasses was discharged by the discharging rod, and produced the same effect in its circuit as if none of the bottles had been pierced; and yet, on examining them, we found no less than twelve of them in that situation. Now, all the bottles of the battery being united by a communication of all the outsides together, and of all the insides together, if one of them had been pierced by a forced passage of the different kinds of electricity to meet each other, before the discharge by the discharging rod, it would not only have prevented the passage of the electricity by the common circuit, but it would have saved all the rest of its fellows, by conducting the whole through its own breach. And it is not easy to conceive that twelve bottles in twenty should be so equally strong as to support the whole strength of their charge, till the circuit of their discharge was opened, and then be so equally weak as to break all together when the weight of that charge was taken off from them by opening the circuits. At some other time I will give you my opinion of this effect, if you desire it.
I have taken the account of this stroke of lightning from an Italian piece, entitled Analisi d' un nuovo Fenomeno del Fulmine, the dedication of which is subscribed Carlo Barletti, delle Scuole Pie, who I suppose is the author. As I do not perfectly understand that language, I may possibly in some things have mistaken that philosopher's meaning. I therefore desire, my dear friend, that you would not permit this to be published, till you have compared and considered it with that original piece, and communicated to me your remarks and corrections. Nor would I in any case have it appear with my name, as perhaps it may occasion disputes, and I have no time to attend to them.
The Leyden Phial, and M. Volta's Experiment.* ́
I THANK YOU for the account you give me of M. Volta's experiment. You judge rightly in supposing, that I have not much time at present to consider philosophical matters; but, as far as I understand it from your description, it is only another form of the Leyden phial, and explicable by the same principles. I must, however, own myself puzzled by one part of your account, viz. "and thus the electric force once excited may be kept alive years together," which is perhaps only a mistake. I have known it indeed to be continued many months in a phial hermetically sealed, and suppose it may be so preserved for ages; but, though one may, by repeatedly touching the knob of a charged bottle with a small insulated plate, like the upper one of the electrophore, draw an incredible number of sparks successively, that is, one after every touch, and those for a while not apparently different in magnitude, yet at length they will become small, and the charge be finally exhausted. But I am in the wrong to give my opinion till I have seen the experiment.
I like much your pasteboard machine, and think it may, in some respects, be preferable to the very large glass ones constructed here. The Duc de Chaulnes has one, said, if I remember right, to be five feet in diameter. I saw it tried, but it happened not to be in order.
*It is not known to whom this letter was addressed. EDITOR.