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TO JOHN INGENHOUSZ.
An Attempt to explain the Effects of Lightning on the
Vane of the Steeple of a Church in Cremona, August, 1777.
1. WHEN the subtile fluid, which we call fire or heat, enters a solid body, it separates the particles of which that body consists .farther from each other, and thus dilates the body, increasing its dimensions.
2. A greater proportion of fire introduced separates the parts so far from each other, that the solid body becomes a fluid, being melted.
3. A still greater quantity of heat separates the parts so far, that they lose their mutual attraction, and acquire a mutual repulsion, whence they fly from each other, either gradually or suddenly, with great force, as the separating power is introduced gradually or suddenly.
4. Thus ice becomes water, and water vapor, which vapor is said to expand fourteen thousand times the space it occupied in the form of water, and with an explosive force in certain cases capable of producing great and violent effects.
5. Thus metals expand, melt, and explode; the two first effected by the gradual application of the separating power, and all three, in its sudden application, by artificial electricity or lightning.
6. That fluid in passing through a metal rod or wire is generally supposed to occupy the whole dimension of the rod. If the rod is smaller in some places than in others, the quantity of fluid, which is not sufficient to make any change in the larger or thicker part, may be sufficient to expand, melt, or explode the smaller, the quantity of fluid passing being the same, and the quantity of matter less that is acted upon.
7. Thus the links of a brass chain, with a certain quantity of electricity passing through them, have been melted in the small parts that form their contact, while the rest have not been affected.
8. Thus a piece of tinfoil cut in this form, enclosed in a pack of cards, and having the charge of a large bottle sent through it, has been found unchanged in the broadest part, between a and b, melted only in spots between c and d, and the part between d and e reduced to smoke by explosion.
9. The tinfoil melted in spots between b and c, and that whole space not being melted, seems to indicate that the foil in the melted parts had been thinner than the rest, on which thin parts the passing fluid had therefore a greater effect.
10. Some metals melt more easily than others; tin more easily than copper, copper than iron. It is supposed (perhaps not yet proved), that those which melt with the least of the separating power, whether that be common fire or the electric fluid, do also explode with less of that power.
11. The explosions of metal, like those of gunpowder, act in all directions. Thus the explosion of gold leaf between plates of glass, breaking the glass to pieces, will throw those pieces into all parts of the room; and the explosion of iron, or even of water, between the joints of stone in a steeple, will scatter the stones in all directions round the neighbourhood. But the direction, given to those stones by the explosion, is to be considered as different from the direction of the lightning, 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 só 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 strụck, 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