seem to indicate, that the perforation is not made by the force of a projectile passing through, but rather by the explosion or the dilatation, in passing, of a subtile line of fluid?

14. Such an explosion or dilatation of a line of fluid, passing through a card, would raise burs round the hole, sometimes on one side, sometimes on the other, and sometimes on both, according to the disposition of the part of the paper near the surface, without any regard to the direction of the fluid.

15. Great thanks are due to the ingenious philosopher, who examined the vane at Cremona, and who took the pains to describe so exactly the effects of the lightning upon it, and to communicate that description. The fact is extremely curious. It is well worth considering. He invites to that consideration. He has fairly given his own opinion. He will with candor receive that of others, though it may happen to differ from his own. By calmly discussing rather than by warmly disputing, the truth is most easily obtained. I shall give my opinion freely, as it is asked, hoping it may prove the true one; and promising myself, if otherwise, the honor at least of acknowledging frankly my error, and of being thankful to him who kindly shows it to me.

16. By the account given of this stroke of lightning upon the steeple of Cremona, it appears that the rod of iron or spindle, on which the vane turned, was of about two inches circumference, terminating in a cross above the vane, and its lower end fixed in a marble pedestal.

17. That the plate of the vane was copper, eight or nine inches wide, and near twice as long. That it was about one line thick near the spindle, and growing thinner insensibly towards the other end, where its thickness did not exceed three quarters of a line, the weight twenty ounces and a half.

18. That the copper had been tinned over.

19. That the marble pedestal was split by the stroke into many pieces, and scattered over the roof, garden, and court of a neighbouring building. One piece was thrown to the distance of forty feet. The spindle was broken and displaced, and the vane thrown on the roof of the parsonage house, twenty feet from the steeple.

20. That the vane was perforated in eighteen places, the holes of irregular forms, and the metal which had filled them pushed outwards, in some of them on one side of the vane, in others on the other. The copper showed marks of having been partly melted, and in some places tin and copper melted and mixed together. There were marks of smoke in several places.

21. The ragged parts bent outwards round each hole, being brought back to their original flat position, were not, though evidently a little thinned and dilated, sufficient to fill the place.

22. From the effects described (19), it is clear that the quantity of lightning which fell on this steeple at Cremona was very great.

23. The vane being a thin plate of copper, its edges and corners may be considered as a series of points, and, being therefore sooner rendered negative by the repulsive force of an approaching positive cloud than the blunt and thick iron cross (12), was probably first struck, and thence became the conductor of that great quantity.

24. The plate of which the vane was formed, being thicker near the spindle, and diminishing in thickness gradually to the other end (17), was probably not of copper plated by passing between rollers, for they would have left it of equal thickness; but of metal plated by the hammer. The surface too of rolled copper is even and plain; that of hammered is generally

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

is 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?]

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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