ning, which happened to occasion those explosions of the matter it met with in its passage between the clouds and the earth. 12. When bodies positively electrized approach sharp pointed rods or thin plates of metal, these are more easily rendered negative by the repulsive force of the electric fluid in those positively electrized bodies, which chases away the natural quantity contained in those mince rods or plates, though it would not have force enough to chase the same out of larger masses. Hence such points, rods, and plates, being in a negative state, draw to themselves more strongly and in greater quantities the electric fluid offered them, than such masses can do which remain nearly in their natural state. And thus a pointed rod receives not only at its point, though more visibly there, but at all parts of its length that are exposed. Hence a needle held between the finger and thumb, and presented to a charged prime conductor, will draw off the charge more expeditiously if held near the eye, and the rest of its length is exposed to the electrical atmosphere, than if all but half an inch of the point is concealed and covered. 13. Lightning so differs from solid projectiles, and from common fluids projected with violence, that, though its course is rapid, it is most easily turned to follow the direction of good conductors. And it is doubted whether any experiments in electricity have yet decisively proved; that the electric fluid in its violent passage through the air where a battery is discharged has what we call a momentum, which would make it continue its course in a right line, though a conductor offered near that course to give it a different or even contrary direction; or that it has a force capable of pushing forward or overthrowing the objects it strikes against, even though it sometimes pierces them. Does not this 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?] NN* VOL. V. 60 |