Benjamin Franklin Papers

From Benjamin Franklin to Ingenhousz, [21 June 1782]

To Ingenhousz9

AL (draft), press copy of L, and copy:1 Library of Congress

[June 21, 1782]

An Attempt to explain the Effects of Lightning on the Vane of the Steeple of a Church in Cremona, August 1777. Address’d to Dr John Ingenhauss, C. & Archiat. Cæs. &c &c

  1. When the subtil 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 introduc’d 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 & acquire a mutual Repulsion, whence they fly from each other, either gradually or suddenly with great Force, as the separating Power is introduc’d gradually or suddenly.
  4. Thus Ice becomes Water, and Water Vapour, which Vapour is said to expand to 14000 times the Space it occupied in the Form of Water, and with an explosive Force in certain Circumstances capable of producing great & violent Effects.
  5. Thus Metals expand, melt, & explode. The two first effected by the gradual Application of the separating Power, and all three in its sudden Application, by artificial Electricity, or by Lightning.
  6. That Fluid in passing thro’ a Metal Rod or Wire, is generally suppos’d 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. 7. Thus the Links of a Brass Chain, with a certain Quantity of Electricity passing thro’ them, have been melted in the small Parts that form their Contact, while the rest have not been affected.
  8. Thus a Piece of Tin Foil cut in this Form, inclos’d in a Pack of Cards,
    and having the Charge of a large Bottle sent thro’ it, has been found unchang’d in the broadest Part between a and b. melted only in Spots between b & c, totally melted between c and d, and the Part between d and e reduc’d 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 suppos’d, (perhaps not yet prov’d) 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 Directions given to those Stones by the Explosion, is to be considered as different from the Direction of the Lightning, which happen’d to occasion those Explosions of the Matter it met with in its Passage between the Clouds & the Earth.
  12. When Bodies positively electris’d approach Sharp Pointed Rods or thin Plates of Metal, these are more easily render’d negative by the repulsive Force of the Electric Fluid in those positively electris’d Bodies, which chases away the natural Quantity contain’d in those mince Rods or Plates, tho’ it would not have Force enough to chase the same out of larger Masses. Hence such Points, Rods, & Plates, being in a negative State, draw to themselves more strongly and in greater Quantities the Electric Fluid offer’d them, than such Masses can do, which remain more nearly in their natural State. And thus a pointed Rod receives not only at its Point, tho’ more visibly there, but at all Parts of its Length that are expos’d. Hence a Needle held between the Finger & Thumb, and presented to a Charg’d Prime Conductor, will draw off the Charge, more expeditiously if held near the Eye & the rest of its Length is expos’d to the Elecl. Atmosphere, than if all but ½ an Inch of the Point is conceal’d & cover’d.—
  13. Lightning so differs from solid Projectiles, and from common Fluids projected with Violence, that tho’ 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 thro’ the Air when a Battery is discharg’d has what we call a Momentum, wch would make it continue its Course in a right Line tho’ a Conductor offer’d 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 tho’ it sometimes pierces them. Does not this seem to indicate that the Perforation is not made by the Force of a Projectile passing thro’, but rather by the Explosion or the Dilatation in passing, of a subtil Line of Fluid.
  14. Such an Explosion or Dilatation of a Line of Fluid passing thro’ a Card, would raise Burrs round the Hole sometimes on one side, sometimes on the other, & sometimes on both, according to the Disposition of the Parts of the Paper near the Surfaces, 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 extreamly curious. It is well worth considering. He invites to that Consideration. He has fairly given his own Opinion. He will with Candour receive that of others, tho’ 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 my self if otherwise, the Honour 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 at 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 fix’d in a Marble Pedestal.
  17. That the Plate of the Vane was Copper 8 or 9 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 20½ Ounces.
  18. That the Copper had been tinned over.
  19. That the Marble Pedestal was split by the Stroke into many Pieces, & scatter’d over the Roof, Garden & Court of a neighbouring Building, One Piece was thrown to the Distance of 40 feet. The Spindle was broken and displac’d, and the Vane thrown on the Roof of the Parsonage House 20 feet from the Steeple.
  20. That the Vane was perforated in 18 Places, the Holes of irregular Forms and the Metal which had filled them push’d outwards, in some of them on one Side of the Vane in others on the other. The Copper show’d Marks of having been partly melted, and in some Places Tin and Copper melted & mingled 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 Positition, were not tho’ evidently a little thinned & dilated, sufficient to fill the Place.
  22. From the Effects describ’d (19) it is clear that the Quantity of Lightning which fell on the Steeple at Cremona, was very great.
  23. The Vane being a thin Plate of Copper, its Edges & Corners may be considered as a Series of Points, and being therefore sooner render’d negative by the repulsive Force of an approaching positive Cloud, than the blunt & 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 & 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 center of each Impression.
  26. The Lightning which in passing thro’ 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 a middle State.
  27. That 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. Those thin Parts of the Plate being at the same Instant partly in fusion, and partly so softned as to be near it: the softned Parts were push’d outwards, a Hole made, and some of the melted Parts blown away; hence there was not left Metal enough to refill 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 softned Metal by the Explosions, should be on both Sides of the Plate in a proportion nearly equal.—
  32. That the Force of a simple electric 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 & 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 Looking Glass, will break the Glass to pieces, tho’ confin’d by the Screws of a strong Press. And between two Pieces of Marble press’d together by a Weight of 20 pounds, will lift that Weight.— Much less Force is necessary to move the melted & 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 & 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 push’d out of the Holes is found, that of some on one side of the Plate, & of others on the other. Hence he supposes them to be occasion’d (if I understand him rightly) 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 pierc’d 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 picès ecoli prima que ritorni tralle infinitè combinazioni un caso simile a quello della banderuola, che ora abbiamo per mano. Forza é que si esaurisca una non pisi udita miniera difulmini sopra una grande città, pressoque seminata di campanili, e di banderuole, il che è rarissimo; e può ancora picè volte ciò succedere, senza che s’incontri giammai un altra banderuola tanto opportunatamenta situata tra è limiti della fulmine a Explosione.”
  35. But tho’ the Author’s Explication of these Appearances on 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 thro’ the Vane, entring at one End of it & going out of 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 Side opposite to the Metal.— A more expensive, but perhaps 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 fix’d 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, & the same Effect appear upon it, it would be still more miraculous to suppose it happen’d by Accident to be exactly situated where those crossing threads of different Electricities were afterwards to meet.
  36. The Perforation of Glass Bottles when over-charg’d, is I imagine a different Case, & not explicable by either of these Hypotheses.— I cannot well suppose the Breach to be occasion’d by the Passage of Electricity thro’ it, since a single Bottle tho’ 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 discharging Rod, or in overleaping the Borders of the Glass. Thus I have been present when a Battery of twenty Glasses was discharg’d by the discharging Rod, and produc’d the same Effect in its Circuit as if the Bottles had none of them 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 pierc’d by a forc’d 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 sav’d all the rest of its Fellows, by conducting the whole thro’ its own Breach. And it is not easy to conceive that 12 Bottles in 20 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 Circuit.— At some other time I will give you my Opinion of this Effect if you desire it.—

I have taken the Acct. of this Stroke of Lightning from an Italian Piece, intitled, 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 compar’d 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.—

[Note numbering follows the Franklin Papers source.]

9Ever since May, 1780, BF had been promising to send Ingenhousz some thoughts on Carlo Barletti’s pamphlet analyzing a lightning strike on the weathervane atop a church in Cremona: XXXII, 343n, 348; XXXV, 548. He finally finished the paper on June 21, 1782. That same day he added a final section to an ongoing letter to Ingenhousz begun on Oct. 2, 1781 (XXXV, 544–51), and enclosed this paper with that letter.

1We publish from BF’s draft since the L (in L’Air de Lamotte’s hand) is flawed by numerous copying errors. The third version, a copy in Josiah Flagg’s hand, was made for BF in 1786.

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