Members of the Purfleet Committee to the Royal Society
AL (incomplete draft): American Philosophical Society
On December 8 Benjamin Wilson, the vocal minority of one on the Purfleet committee, amplified his views in a long letter to the Surveyor General of the Ordnance, Sir Charles Frederick, in which he took sharp issue with the other members. On the 10th Sir Charles forwarded the letter to the Royal Society and asked for an opinion on it; the matter was urgent, he pointed out, because the Board of Ordnance needed to know at once what the best means were of safeguarding the magazine. The letter was consequently read at the Society’s meeting later that day, before the report that it criticized; Wilson hoped to persuade the meeting to reject the committee’s recommendation of pointed rather than blunt lightning rods, but he was disappointed. On the 17th his colleagues on the committee responded in a letter to the Society’s President, Sir John Pringle, that ignored his arguments and brusquely reaffirmed their stand.7
This chronology indicates that the draft below was composed during the week between the 10th and 17th. What happened, we believe, was that the committee members were so incensed at being attacked before the membership that they commissioned Franklin to write a reply to be read at the next meeting. He did so, elaborating the arguments for pointed rods; and this incomplete draft is what remains of his answer. As the days passed, we conjecture, the angry scientists realized that such a detailed rebuttal would lead only to an unseemly and unprofitable wrangle, and decided to content themselves with a curt letter to Pringle.
[After Dec. 10 and before Dec. 17, 1772]
The Committee appointed by you in July last on an Application from the Royal Ordnance to consider of the most proper Means of securing the Powder Magazines at Purfleet from Danger by Lightning, made their Report to the Council during your Recess, agreable to your Instructions, who immediately transmitted it to the said Board. That Report together with the Committee who sign’d it, having since been censured by Mr. Wilson in a Letter to Sir Charles Frederic, Surveyor of the Ordnance, which Letter was read at Sir Charles’s Request to the Society, and before the Report it self, with the Reasons on which it was founded could in the usual Course be laid before them, apparently tending to prejudice the Society against their Committee, especially as Mr. Wilson has stated his Objections with all the Strength they are capable of, and has passed over very slightly the Answers made to them in the Committee, and the Reasons on which it proceeded, only intimating his Ability and Intention of examining and invalidating those Reasons at some future time: We can not but think it a Piece of Justice due, not to ourselves merely, but to the Philosophical Part of the Question, that the Grounds of our Report in the Point controverted should be more fully explained than it could properly be in the Report itself. We therefore take the Liberty of acquainting you, that Mr. Wilson’s Objections to the Use of Pointed Conductors were heard and considered in the several Meetings of the Committee with Patience and with Candour; that they were all answered and obviated to the Satisfaction of every one of the Committee but himself; and the Substance of what was said on the Occasion being contained in a Paper of Experiments, Observations and Facts, drawn up by Mr. Franklin, and read to the Committee in the Presence of Mr. Wilson, several of the Committee having seen the Experiments themselves, we here present the Society with that Paper.
(Here insert the Paper.)8
The Committee had also before them an Experiment of Mr. Henley’s, related in a Letter from that Gentleman to Mr. Franklin, which as it seems to have been misapprehended by Mr. Wilson, is also here inserted, viz.
(Here insert it.)9
By this Experiment in which the Prime Conductor [is] suppos’d to represent a Cloud, it appeared, that the Point would operate at 12 times the striking Distance in drawing off and lessening the Quantity of Electric Fluid, before the Cloud could approach within that Distance; that a blunt Body drew off none of it till it came within that distance; that therefore if it struck on the blunt Body it would strike with its whole Force undiminish’d; on the Point it would either not strike at all or with a diminish’d Quantity for what is drawn off by a pointed Conductor, is at the same time convey’d away into the Earth. But these Consequences did not strike Mr. Wilson; to him, as he says in his Letter, “it appeared that the Difference in the Effects upon this Fluid, between pointed and blunted Metal, is as 12 to 1.” And thence he seems to conclude that in a Stroke upon a Point 12 times the Quantity must be discharged, that would have fallen on a Body which had no Point:1 For being persuaded from some Circumstances observed by the Verger of St. Pauls, in and about the Conductors upon that Building, the morning after the Thunder on March last; and examined by himself and Mr. Delaval about a Week after, that “a Bar of Iron near 4 Inches broad and about ½ an Inch thick,” had been “made considerably hot if not absolutely red; by a Stroke of Lightning;”2 he says, he “thinks it a happy Circumstance that there was no Point fixed upon the Top of the Church to solicit a greater Quantity;” for “the Difference in the Effects between blunted and pointed Ends, in causing a Discharge in our Electrical Experiments appearing to be as 12 to 1, it is easy to comprehend the very great Danger this noble Fabric has probably escaped, by having no pointed Apparatus upon it.” To your Committee this Deduction still appears quite unconnected with the Premises, which they conceive afford Conclusions directly contrary; and that, supposing the Fact tho’ a doubtful one, that so great a Quantity fell as to heat such an Iron Bar to the Degree imagined, yet if there had been a pointed Apparatus to have operated on the approaching Cloud while it was at 12 Times the Striking Distance, drawing off continually and conveying into the Earth Part of the Quantity it contained, all the Time of its Approach till within the Striking Distance, the Remainder must be less than the original Quantity, and strike therefore with less Force. And For another Reason, drawn from Exp. IV. it seems rather likely that for Want of Points on so great and lofty a Bulk of Metal as the leaded Roof of that Edifice presents to a Cloud, the electric Atmosphere of that Cloud might have been drawn from its more distant Parts, and accumulated over the Building before the Stroke, so as to afford an Explosion much larger than it could have done if such Accumulation had been prevented, or its Quantity previously drawn off and convey’d away by pointed Conductors.
Three of your Committee who signed the Report in question, were also on the Committee you appointed to consider of the securing St. Paul’s.3 They are now charg’d with Inconsistency, for advising Pointed Conductors in the present Case, and not in the former. “It is worthy of Note,” says Mr. Wilson, “that those Conductors did not terminate in a Point; nor was any Point put upon the Cross at the Top, and yet Dr. Franklin was of that Committee. If Points are so essential to our Safety, why was not the reason enforced at the Committee for having them on that capital Edifice?”
To explain this, it may be proper to observe, that the Advantages expected from Pointed Conductors have been three, viz. I. To prevent a Stroke as in Exp. V. by creating a greater Distance between the striking Parts of a Cloud and the Building. Or, 2dly. if the Cloud should come on so fast that a Stroke could not be prevented, then to lessen its Quantity and Force, by previously drawing off and conveying away into the Earth silently a Part of it. And 3dly. to direct the Lightning precisely upon the Conductor prepared to receive and convey it away without Damage, rather than suffer it to fall on any other Part of the Building, which it might damage in its Way from thence to a low unpointed Conductor. Those who have a Pleasure in Disputation are sometimes apt to make their Opponents say more or less, or something different from what they have really said, in order to obtain a little Advantage in Argument. No one of the Committee has ever asserted that Points were essential to our Safety. Dr. F. who first proposed them, and may therefore be supposed most fond of them, has, as quoted by Mr. Wilson, mentioned a Case in which a House might be safe without them, [that] is, to wit, “when the Roof is covered with Lead or other Metal, and Spouts of Metal are continued from the Roof into the Ground to carry off the Water.” Now this was precisely the Case of St. Pauls. The Dome and the whole Roof of the Church were covered with Lead, from whence a Number of large leaden Pipes descended into the common Sewers; but some Connections of Metal were wanting to unite the different Leaden Coverings with each other, and make the Communication with the Earth compleat. This being agreed to, none of the Committee thought it necessary to propose Points. Dr. F. would not propose them for a Dwelling House so covered, unless where some timid Inhabitant dreaded the Noise of an Explosion, which a Pointed Conductor might give a Chance of Preventing. For it being indifferent on what Part of such a Roof a Stroke should fall, the Conductors being compleat from all Parts, there would be no Reason for desiring to draw it to a particular Quarter. This too was in a great degree the Case of the Board House; another Instance given of our Inconsistency: for tho’ not intirely covered with Lead, yet every Angle of the Roof being well cop’d with broad Plates of that Metal, all communicating with the Leaden Gutters within the Parapet, and thence by large leaden Pipes quite down into the Water of two Wells one on each side the Building, it was thought sufficiently safe as a Dwelling House; and being no Powder Magazine itself, and plac’d at a considerable Distance from them, some of the Committee conceiv’d it not to be within our Charge, and therefore were [remainder missing]
To the Royal Society.
7. The committee’s report is above, Aug. 21, and its letter to Pringle below. Wilson’s dissent from the report and his letter to Frederick were published in Phil. Trans., LXIII (1773–74), 48–65. For Frederick’s communication to the Royal Society and Wilson’s reason for wanting his letter read before it see the latter’s Observations upon Lightning … (London, 1773), pp. [iii–v]. For Frederick (1709–85), an M.P. and F.R.S. and a man of considerable influence, see Thomas Thomson, History of the Royal Society … (London, 1812), app., p. xxxix; Namier and Brooke, House of Commons, II, 472–3.
8. We earnestly wish that the paper had been inserted, because it would have indicated which version of his experiments BF read to the committee; see the headnote on those experiments above, under Aug. 18. BF’s references to numbered experiments, later in the draft, are to this paper; most of the other matters he deals with are explained in the headnotes on the committee’s report above, Aug. 21, and on its letter to Pringle below, Dec. 17.
9. The experiment or experiments discussed in connection with the committee’s report above, p. 261 ns.
1. For Wilson’s misinterpretation of Henly see ibid.
2. During the storm on March 22, according to Wilson, St. Paul’s was struck: Observations, pp. 13, 22. Henly visited the Cathedral soon afterward, and could find no indication of the effects of lightning. He so informed the Royal Society: Phil. Trans., LXIV (1774), 148–51.
3. For the membership of the committee to examine St. Paul’s see above, XVI, 151. Four men were appointed to, and three served on, both committees: BF, Watson, and Wilson. Delaval took no part in the Purfleet committee; see above, p. 260 n. BF seems to have written originally “Two of your Committee”—meaning himself and Watson—and then for some unknown reason changed it to three.