Benjamin Franklin Papers

From Benjamin Franklin to Richard Dawson, 29 May 1772

To Richard Dawson2

ALS: British Museum; French translation by Barbeu-Dubourg: American Philosophical Society

This letter marked the start of an investigation that engaged Franklin’s attention intermittently for months to come, and embroiled him in one more dispute about the behavior of electricity. The problem was how best to protect the royal powder magazine at Purfleet against lightning. The magazine was new, and replaced the old one at Greenwich; transfer of powder from one to the other had begun in 1763 and been completed in 1768.3 A year later a disaster in Italy underlined the danger to all such arsenals: lightning struck the magazine at Brescia, and the resultant explosion destroyed much of the town and killed more than a thousand people.4 Parliament responded in 1771–72 by passing two gunpowder acts, which established strict supervision of the manufacture and storage of powder in private hands;5 the government, by implication, would supervise its own establishments.

It began to do so just when the second bill was becoming law. At some time before the beginning of May the Board of Ordnance requested Benjamin Wilson to examine the Purfleet magazine and recommend ways of protecting it. His report, dated May 4, suggested among other things the installation of conductors with blunt tops.6 The Board, apparently not satisfied, must have asked Franklin for his opinion, because he gave it in the letter below; his recommendation was conductors with pointed tops. This conflict of experts no doubt confused the Board, which on July 1 appealed to the Royal Society for a definitive opinion. By the 14th the Society had appointed a committee, including both Franklin and Wilson, to examine the question and bring in its report.7

Cravenstreet, London, May 29. 1772


Having yesterday at your Request view’d the Magazines of Gunpowder at Purfleet, in order to consider of their Preservation from Danger by Lightning, I am of Opinion

1. That all the Bars of Iron8 passing down through the Arches of the Roofs towards the Powder, be taken away, as at present they do, with the Copper Hoops on the Barrels, form an imperfect Conductor, more or less compleat as the Piles of Barrels happen to be higher or lower, but such an one as may tend to draw down among the Powder any Lightning that shall happen to strike the Roof, and are therefore dangerous.

2. That [the] Building which has a Coping of Lead all along the Ridge from End to End, may be secured by a Pointed Iron Rod erected near each End, communicating with that Coping, and extending down thro’ the Chalk Rock on which the Buildings are founded, till it comes to Water. The Rod may be of at least an Inch Diameter (that it may last the longer as well as afford a freer Passage to the Lightning thro’ its Substance) and painted to prevent its Rusting.9 Its upper End should extend ten feet above the Top of the Roof, tapering gradually to a sharp Point; the last six Inches (the better to preserve Sharpness) to be of Copper as less liable to be blunted by Rust.1 If the Rod cannot conveniently be made all of one Piece, the Lengths of which it is compos’d should be strongly scrued together or into each other by a close Joint, with a thin Plate of Lead between the Shoulders to exclude Moisture (which might occasion Rust) and to make the Joining or Continuation of Metal more perfect.

From all the Electrical Experiments I have made with this View, and all the Instances I have yet known of the Effect of Lightning on these Conductors, it appears to me equally safe (when they are good and compleat down to Water or very moist Earth) whether they are apply’d close to the Wall, and steadied by Staples driven into it, or supported by a Pole or Mast fix’d in the Ground at some Distance from it. The first Method is most convenient, as the Rod may so be bent for avoiding Doors and Windows that are directly under the Pitch of the Roof. Yet as it may be more satisfactory to some Apprehensions, if the Rods are supported in the latter Manner, I should have no Objection to it, provided they can be conveniently posited so as to obstruct no Passage, and are so firmly fix’d as that the Wind may not, by shaking them, break the Iron or Leaden Communication between the Side of the Rod and the Lead that Covers the Ridge.

3. The other four Buildings being, as I understand, to be new-roofed in the same manner with that just mentioned, the same Method will serve for them when so finished. But if it be required to secure them in the mean time, then, as their present Roofs are of a different Form, being hipped at the four Corners, and their Hip-joints as well as Ridges Cop’d with Lead extending down to the Eaves, I would advise the immediate boring or digging, near the Ends of each Building, the proposed Passages down to Water, and fixing in them that part of each Conductor which may reach upwards from the Water to the Height of the Eaves: From the Top of this I would extend two Iron Arms reaching to the Corners of the Eaves, where the Leaden Copings of the Hips must be connected with those Bars; and on the Joining of the Hips with the Ridge I would erect Rods nine or ten feet high, pointed as before directed, which, as any Roof is altered may be used as the upper Part of the more upright Conductor. I am, Sir, Your most obedient humble Servant

B Franklin

P S. Leaden Pipe may be used for the underground Part of the Conductors, as less liable to Rust.

Captain Dawson

Notation: 29 May 1772 Ordered that Capt. Dawson cause the same to be executed as above proposed H.S.C.2 [In another hand:] Letter wrote Copy sent.

[Note numbering follows the Franklin Papers source.]

2A captain at the time in the Corps of Engineers, and attached to the Ordnance; he had been commissioned as a lieutenant in the army in 1757, and was promoted to major in July, 1772, and to lieutenant colonel in 1777. A List of the General and Field-Officers as They Rank in the Army … (London, [1773]), p. 172; The Royal Kalendar … (London, [1770]), p. 168; Whitworth Porter, History of the Corps of Royal Engineers (3 vols., London, 1889–1915), I, 181, 188, 202–3.

3Purfleet, some miles down river from Greenwich and on the north shore, was where men-of-war customarily took on powder. For a brief contemporary description of the village and the magazine see Philip Morant, The History and Antiquities of the County of Essex … (2 vols., London, 1768), I, 93–4 of second pagination. For the move from Greenwich see Oliver F. G. Hogg, The Royal Arsenal: Its Background, Origin, and Subsequent History (2 vols., London, 1963), I, 109.

4For accounts of the explosion see the London Chron., Sept. 2–5, 23–26, 1769; The Annual Register … for 1769 (1770), p. 135 of first pagination; Gent. Mag., XXXIX (1769), 457; Benjamin Wilson, Observations upon Lightning … (London, 1773), pp. 58–65.

5II Geo. III, c. 35, and 12 Geo. III, c. 61.

6For Wilson, who combined painting with science, see above, IV, 391 n and subsequent vols. He is said to have been portrait-painter to the Board, and to have been commissioned by it as a direct result of the Brescia explosion: Benjamin Vaughan, ed., Political, Miscellaneous, and Philosophical Pieces … by Benj. Franklin... (London, 1779), p. 499 n. Wilson’s three-page MS report is in the British Museum.

7Vaughan (loc. cit.) says that the Board appealed to the Society because of BF’s and Wilson’s disagreement; Dubourg (Œuvres, I, 282) has a slightly different story. The request itself is in the Royal Soc. MSS; the formation of the committee is mentioned in Lord Barrington to Sir Charles Frederick, July 14, 1772, British Museum. For subsequent developments see below, the committee’s minutes under Aug. 12 and its report on Aug. 21.

8These were vertical bars in the roof of each building, to support a beam on which a crane traveled. Wilson agreed that they were the most dangerous structural defect; see his report in the British Museum.

9The buildings are more fully described in the committee’s report. For BF’s earlier recommendations about lightning rods see above, V, 76–8; X, 50–2; XIV, 262–3.

1BF’s experiments below, under Aug. 18, buttressed the argument for sharp points.

2Henry Seymour Conway, who became Lieutenant General of the Ordnance in 1767 and after 1770 was its de facto head: Namier and Brooke, House of Commons, II, 245–6. Whenever Conway’s order was given, BF’s advice does not seem to have been carried out in its entirety until after the committee reported: Dubourg, Œuvres, I, 282.

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