Benjamin Franklin Papers

From Benjamin Franklin to David Hume, 21 January 1762

To David Hume

AL (incomplete): Royal Society of Edinburgh; printed in full in Essays and Observations, Physical and Literary. Read before the Philosophical Society in Edinburgh, and published by them, III (Edinburgh, 1771), 129–41.9

London, [Jany.]1 21. 1762

Dear Sir,

In Compliance with my Lord Marishall’s Request,2 communicated to me by you when I last had the Pleasure of seeing you,3 I now send you what at present appears to me to be the shortest and simplest Method of securing Buildings, &c. from the Mischiefs of Lightning.

Prepare a Steel Rod 5 or 6 Feet long, half an Inch thick at its biggest End, and tapering to a sharp Point, which Point should be gilt to prevent its rusting. Let the big End of the Rod have a strong Eye or Ring of half an Inch Diameter: Fix this Rod upright to the Chimney or highest Part of the Building, by means of Staples, so as it may be kept steady. Let the pointed End be upwards, and rise three or four Feet above the Chimney or Building that the Rod is fix’d to. Drive into the Ground an Iron Rod of about an Inch Diameter, and ten or twelve feet long, that has also an Eye or Ring, in its upper End. It is best that this Rod should be at some Distance from the Foundation of the Building, not nearer than ten feet if your Ground will allow so much. Then take as much Length of Iron Rod, of about half an Inch Diameter, as will reach from the Eye in the Rod above to that in the Rod below; and fasten it securely to those Rods, by passing its Ends thro’ the Rings, and bending those Ends round till they likewise form Rings. This Length of Rod may either be in one or several Pieces. If in several, let the Ends of the Pieces be also well hooked to each other. Then close and cover every Joint with Lead, which is easily done by making a small Bag of strong Paper round the Joint, tying it close below, and then pouring in the melted Lead. It being of Use in these Junctures, that there should be a considerable Quantity of metalline Contact between Piece and Piece: For if they were only hook’d together, and so touch’d each other but in Points, the Lightning in passing thro’ them might melt and break them where they join. The Lead will also prevent the Weakening of the Joints by Rust. To prevent the Shaking of this Rod by the Wind, you may secure it by a few Staples to the Building till it comes down within ten feet of the Ground, and thence carry it off to your Ground Rod; near to which should be planted a Post, to support the Iron Conductor above the Heads of People walking under it. If the Building be large and long, as 100 feet or upwards, it may not be amiss to erect a pointed Rod at each End, and form a Communication by an Iron Rod between them. If there be a Well near the House, so that you can by such a Rod form a Communication from your Top Rod to the Water, it is rather better to do so than to use the Ground Rod above-mentioned. It may also be proper to paint the Iron, to render it more durable, by preserving it better from Rust.

A Building thus guarded, will not be damaged by Lightning, nor any Person or Thing therein kill’d, hurt or set on fire. For either the Explosion will be prevented by the Operation of the Point, or, if not prevented, then the whole Quantity of Lightning exploding near the House, whether passing from the Cloud to the Earth or from the Earth to the Cloud, will be convey’d in the Rods. And though the Iron be crook’d round the Corners of the Building, or make ever so many Turns between the upper and lower Rod, the Lightning will follow it, and be guided by it without affecting the Building.

I omit the Philosophical Reasonings and Experiments on which this Practice is founded; for they are many, and would make a Book. They are besides already known to most of the Learned throughout Europe. In the North-American British Colonies, many Houses have been, since the Year 1752, guarded on these Principles. Three Facts only have come to my Knowledge of the Effects of Lightning4 on such houses,5 which I shall here give you.

The first was some years since, on the house of Mr. Raven, at John’s Island, near Charlestown, South Carolina.6 As the size of a conductor sufficient for conveying the lightning could only be discovered by experience, this gentleman has used small brass wire to connect the upper and lower rods. It went down on the outside of his chimney, which stood at the end of the house. Within the chimney stood, leaning against the back, a musket or gun. The lightning passed in the wire, small as it was, except where furnished with a better conductor by the gun-barrel, between the top of which and the wire it made a very small hole through the wall, and in that place, and all above it, melted and separated the wire. If I mistake not, (for I write this from memory only), some hurt to the lower part of the gun-stock, and a few bricks of the hearth broken up, was all the damage done at this house: And this being ascribed to the smallness of the wire, which occasioned the lightning to deviate into the gun-barrel, larger conductors were afterwards more generally used.

The second was the house of Mr. William Mayne of the same Province.7 He hath provided a conductor from the top to the bottom of his chimney composed of iron rods of near half an inch diameter, linked together by hooks, turned at their ends, the lowest joint being in the ground about three feet deep, close to the foundation of his chimney; and the highest about six inches above the top, terminating in three brass wires, with silver points. The lightning passed in this canal, melted and dissipated the brass wires with their points, melted and burst the joints of the rods, so as to unhook and separate them; but did no damage to the chimney, except near the foundation, where some bricks were torn out, and some bricks of the hearth within forced up; the earth also for eight or nine inches round the rod was furrowed and torn up. This small damage appeared to be occasioned by the ground-rod’s not entering the earth deep enough, and being placed too near the foundation of the chimney: Four persons were in the house, and one within a few feet of the chimney. They were stunned with the noise, which was extremely great; but otherwise received no hurt.

The third instance, I lately received an account of from Mr. Kinnersley, an ingenious electrician of Philadelphia, which I shall give you in his own words, from his letter to me, dated March 21.1761.8 “We had four houses in this city, and a vessel at one of the wharfs, struck and damaged by lightning last summer; one of the houses was struck twice in the same storm. But I have the pleasure to inform you, that our method of preventing such terrible disasters has, by a fact which like to have escaped our knowledge, given a very convincing proof of its great utility, and is now in higher repute with us than ever.

“Hearing a few days ago, that Mr. William West merchant in this city, suspected that the lightning, in one of the thunderstorms last summer, had passed through the iron conductor which he had provided for the security of his house, I waited on him to inquire what ground he might have for such a suspicion. Mr. West informed me, that his family and neighbours were all stunned with a very terrible explosion, and that the flash and crack were seen and heard at the same instant; whence he concluded that the lightning must have been very near; and as no house in the neighbourhood had suffered by it, that it must have passed through his conductor. Mr. White, his clerk, told me, that he was sitting at the time by a window about two feet distant from the conductor, leaning against the brick-wall, with which it was in contact, and that he felt a smart sensation like an electric shock in that part of his body which touched the wall. Mr. West farther informed me, that a person of undoubted veracity assured him, that being in a door of an opposite house on the other side of the street, he saw the lightning diffused over the pavement, which was then very wet with rain, to the distance of two or three yards from the foot of the conductor: And that another person of very good credit told him, that he, being a very few doors off, on the other side of the street, saw the lightning above, darting in such direction, that it appeared to him to be directly over that pointed rod.

“Upon receiving this information, and being desirous of farther satisfaction, there being no traces of the lightning to be discovered in the conductor, as far as we could examine it below, I proposed to Mr. West our going to the top of the house to examine the painted [pointed] rod; assuring him, that if the lightning had passed through it, the point must have been melted: And, to our great satisfaction, found it so. This iron rod extended in height about 9 feet and an half above a stack of chimneys to which it was fixed; it was somewhat more than half an inch diameter in the thickest part, and tapering to the upper end. The conductor, from the lower end of it to the earth, consisted of square iron nail rods, not much above a quarter of an inch thick, connected together by interlinking joints. It extended down the cedar roof to the eaves, and from thence down the wall of the house, four stories and a half, to the pavement, in Water-street, being fastened to the wall in several places by small iron hooks. The lower end was fixed to a ring in the top of an iron stake that was down about 4 or 5 feet into the ground. The above mentioned iron rod had a hole in the top of it, about two inches deep, wherein was inserted a brass-wire about two lines thick, and, when first put there, about ten inches long, terminating in a very acute point; but now its whole length is no more than seven inches and a half, and the top very blunt. Some of the metal appears to be missing, the slenderest part of the wire being, as I suspect, consumed into smoke. But some of it, where the wire was a little thicker, being only melted by the lightning, sunk down while in a fluid state, and formed a rough irregular cap, lower on one side than the other, round the upper end of what remained, and became intimately united therewith.

“This was all the damage that Mr. West sustained by a terrible stroke of lightning. A most convincing proof of the great utility of this method of preventing its dreadful effects.

“Mr. West was so good as to make me a present of the melted wire, which I keep as a great curiosity, and long for the pleasure of shewing it to you.9 And now, Sir, I most heartily congratulate you on the pleasure you must have in finding your great and well grounded expectations so far fulfilled,” &c. Thus far Mr. Kinnersly.

You will observe, Sir, that the size or thickness of the conductor I direct, is much greater than this of Mr. West’s, which nevertheless proved effectual; but, the quantity of lightning discharged in some strokes being probably much greater, I prefer a larger conductor as the safer. If I have not been explicite enough in my directions, I shall, on the least intimation, endeavour to supply the defect. I am, &c.

D. Hume Esqr

MS endorsed: Dr. Franklin 21 Augt. 17621

[Added at the end of the printed letter:]

Report on Dr. Franklin’s Method of securing Buildings, &c. from the Mischiefs of Lightning, by Professor Russel.2

The Doctor’s description of his method is the neatest and most distinct imaginable. No body can be at a loss to put it in practice. Two remarks only have occurred to me, which I shall lay before the society. One is, That when a building, proposed to be guarded in this manner, is partly covered with lead, or has the water conveyed from the roof to the ground in leaden pipes; the lead, so far as it goes, may easily be made to supply the place of the iron conductor. This would be done by conducting the iron rods from the top to the nearest part of the lead, and there connecting the iron with the lead, by means of solder, in the same manner that the Doctor proposes that the iron-rods should be connected to each other; and then forming the connection between the groundrod and that part of the lead which comes nearest to it, in the same manner. The other remark which I proposed to make is this: The Doctor directs, that when the conductor comes down within ten feet of the ground, it should thence be conveyed off from the building towards the ground-rod, and supported by a post above the heads of people walking under it. Now, I apprehend, it would be more convenient, and less operose, to carry this last part of the conductor a foot or two under ground.

[Note numbering follows the Franklin Papers source.]

9Only the two pages of the first leaf of the MS survive; the remainder of the letter and the remarks that follow are supplied from the printed text.

1In the MS an ink blot obscures the (obviously abbreviated) month; “January” is spelled out in the printed version. Someone at a later time apparently tried to overwrite what is left of the month in the MS to make it seem that BF had originally written “Augt” but this emendation was certainly in error. See the endorsement below and the footnote thereto. The MS clearly gives the day of the month as “21,” but the printed letter gives it as “24.” Sparks (Works, VI, 214), Bigelow (Works, III, 156), and Smyth (Writings, IV, 127), all follow the dating of the 1771 printed text.

2George Keith, 10th Earl Marischal (1693?–1778), served as a young officer with Marlborough but joined the Old Pretender in 1715. Attainted and his estates forfeited, he lived in Spain for some years and then settled in Prussia, where his brother was a field marshal in Frederick II’s army. The earl served as Prussian ambassador to France, 1751–52, and to Spain, 1759–60, and as governor of the principality of Neuchâtel, 1754–65. He was given a British pardon, 1759, and was allowed to return to Great Britain in 1760–61, receiving compensation for his forfeited estates. He met David Hume and they became warm friends. He returned to his post in Neuchâtel in 1762 and later settled in Potsdam. DNB; Edith E. Cuthell, The Scottish Friend of Frederic the Great: The Last Earl Marischall (2 vols., London, 1915).

3Hume had been in London in 1761 arranging with Strahan for the publication of the last volumes of his History of England.

4The MS original breaks off here.

5In printing this letter Sparks, Bigelow, and Smyth omit everything from this point to the final sentence. Sparks announced his omission by a footnote citing the pages in another volume of his edition in which the incidents described here were recounted; Smyth similarly cited the appropriate pages of Exper. and Obser., 1769 edit.; Bigelow jumped directly from “on such houses” to the final sentence of the letter with no indication that he had skipped more than 1200 words of the text.

6For BF’s source of information on this incident, See below, pp. 50, 53–5.

7For Maine’s extended account of his experience and BF’s remarks on it, See below, pp. 54–9.

8What follows is an extended extract from Kinnersley’s letter; see above, IX, 282–93.

9Here BF omitted two sentences of Kinnersley’s letter in which he mentioned and explained the “Representation” of the damaged lightning rod which he was sending to BF; see the illustration, above, IX, facing p. 286.

1This endorsement, in the margin of the second page of the MS, was almost certainly added much later, probably after the remaining pages of the letter had been lost. BF was still in England on August 21, 1762—at Portsmouth impatiently awaiting a favorable wind to set sail—but the date cannot be correct. Hume’s letter of May 10 acknowledging receipt of this long communication and BF’s reply of May 19 (See below, pp. 80, 82) make certain that he had written it substantially before May, and there seems no reason to doubt the January dating ascribed to it.

2James Russell, an Edinburgh surgeon-apothecary, was appointed professor of natural philosophy at the University of Edinburgh in 1764. The title here is explained by the fact that this “Report” was not printed until 1771. Hume sent BF a summary of these remarks in his letter of May 10, 1762, cited above, and BF commented on them in his reply of May 19.

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