Benjamin Franklin Papers

Franklin and Le Roy: Report on Lightning Rods at Strasbourg, 12 May 1780: résumé

Franklin and Le Roy: Report on Lightning Rods at Strasbourg

ALS (draft): Académie royale des sciences, Paris

Started between 1015 and 1028 in Romanesque style, the magnificent cathedral of Strasbourg underwent many transformations over the centuries. Around 1320, the intersection of nave and transept was marked by a Gothic construction resembling a bishop’s miter, hence called the Mitre. By 1365 the twin towers of the façade had been erected, and were joined to each other some thirty years later by a platform meant to support a belfry. The belfry, however, was never constructed. Twin spires were then planned, only one of which was built by 1439. A little guardhouse replaced what would have been the other one.4

In 1759, the cathedral’s lead-covered roof caught fire during a storm and the Mitre, destroyed, was rebuilt in the shape of a truncated pyramid. Twenty years after that catastrophe, on May 12, 1779, Barbier de Tinan, commissaire des guerres and member of the Academy of Dijon, presented to the Académie royale des sciences in Paris his translation of the abbé Toaldo’s work on lightning rods as well as his own memoir on the best way to preserve the cathedral from damage by lightning. Franklin and Le Roy were appointed commissioners to study Barbier’s memoir.5

Their twenty-page joint report, in Le Roy’s hand but signed by both men, was read at the Académie exactly one year later, May 12, 1780.6 The draft contains many corrections and several references to a gravure (now missing) which apparently was intended to accompany the report.

<May 12, 1780, in French: The Académie has asked us to report on a memoir by M. Barbier on the means of arming the Strasbourg cathedral with lightning rods.

Strasbourg is located in the middle of rather flat countryside dominated by the cathedral’s very high spire—so high indeed that no attempt at devising a protective system for so tall a building has ever been made. Its exterior, furthermore, is studded with a large number of iron bars and clamps which add to the danger posed by the frequent storms. Those storms originate southwest of the city, in the Vosges, and proceed in a northeasterly direction.

The two crucial issues we shall discuss are the shape given to the lightning rods’ ends and the means by which the lightning’s fire is brought down to earth.

On the first question, M. Barbier wisely prefers the rods with pointed ends7 and suggests that they be gilded so as to preserve them from the weather, and also to create an effect pleasant to the eye. As to the second problem, about which one cannot be too cautious, it should be noted that the spire is divided into three parts. Starting from the top, they are a tier made up of cross, lantern, and crown; then a pyramid in the shape of an octagon consisting of eight small winding stairs and turrets; and finally, resting on the platform, a square construction flanked by an openwork turret containing a staircase.

M. Barbier proposes, for the top part, a scaffolding of vertical iron rods off the northeast and southwest sides—those most often hit by storms—to join a “necklace” of horizontal rods just below the crown. From this necklace there would jut four rods descending between the little staircases of the pyramid and corresponding to the four turrets. They would finally follow at a distance the outer walls of the turrets belonging to the lowest part, which rests on the platform.

Protruding from the long drop between the cross and the turrets, a number of secondary conductors, five or six feet long and ending in copper-covered joints, would have to be installed for extra security. More of the same should be placed at each angle of the platform.

One third of the way down between the platform and the ground there runs a circular gallery, and more rods could be placed there, as well as a protective system extending to the copper roof of the nave. For better safety still, lead washers should be inserted between the various screws.

The final step is to connect this protective framework to the ground.

M. Barbier’s solution is to establish a tight metallic connection between the copper roof of the Mitre and the three adjacent copper roofs, two of which are well suited to attract lightning since they are garnished with very pointed pyramids.

Such a system would provide excellent protection against storms coming from the east. Since that part of the cathedral is so far removed from the tower, M. Barbier plans to give it extra safety by adding still sharper points to those of the pyramids. The next step is to take advantage of the two gutters situated right there, which can be made to communicate, through a leaden pipe, with an already established well. As usual, there will be a duplicate fallback system in case of accident, in the shape of metal rods constructed in the same way as those in the front of the cathedral. Both systems will be joined at the bottom by a thick iron bar pushed at least one foot into the ground at the bottom of the well.

Let us add, to complete this description, that M. Barbier proposes to secure the rods by flattening their extremities as well as those of the “necklace,” then drilling a hole through these flattened parts and attaching them to each other by means of a square-headed screw tightly shut by two keys.

It is our opinion that M. Barbier’s system would present still greater security if the Mitre were provided with more points and if the descending rods were made thicker. We agree with his observation that the work should proceed from bottom to top. We conclude by praising his plan and expressing the hope that, given the disastrous history of Strasbourg’s tower, it will become a reality. Thus protected, the tower will be a model for the rest of the kingdom and a symbol of the progress accomplished by physics in France when it is considered that only fifteen years ago lightning rods were still viewed as dangerous. >8

[Note numbering follows the Franklin Papers source.]

4See Georges Livet and Francis Rapp, eds., Histoire de Strasbourg (Toulouse, 1987), pp. 98, 107–8, 122. See also Bertrand Monnet, Métamorphoses de la Cathédrale de Strasbourg du XIe. siècle à nos jours (Paris, 1987).

5Procès verbaux, t. 98, fol. 158. In 1778, Giuseppe Toaldo (1719–1797), physicist and professor of astronomy at the University of Padua, republished in Venice a work that had originally appeared in 1772. Entitled Dei Conduttori per preservare gli edifizi da’ fulmini, it contained an Italian translation of BF’s Oct. 8, 1772, letter to Horace-Bénédict de Saussure (XIX, 324–7). Father Toaldo sent this volume to BF in homage. During the preparation of their joint report, presumably, Le Roy wrote BF a letter dated only “Tuesday morning” in which he asked for the loan of Toaldo’s book (APS). Barbier de Tinan’s translation of Toaldo’s 1778 version, augmented by some of his own comments, appeared in Paris in 1779. In 1780 Barbier published in Strasbourg his Mémoire sur la manière d’armer d’un conducteur la cathédrale de Strasbourg et sa tour: Quérard, France littéraire; Dizionario Enciclopedico Italiano (14 vols., Rome, 1955–74). A copy of the Mémoire, bearing BF’s notation on the title page, “De la part de l’Auteur à BF.”, is at the APS.

6Procès verbaux, t. 99. The report is included in a separate pochette. By June 30 it had already been printed; see BF’s letter to Cabanis of that date, below.

7For the controversy of pointed rods versus rounded ones, see XXIV, 163. The report specifies: “The most informed physicists argue today on the proper shape for lightning rods. The fears that M. Wilson tried to raise about pointed rods, through his equivocal experiments, have been laid to rest. The Committee of the London Royal Society declared that there was no reason to change the rods of the Purfleet magazines which, as one knows, are very pointed.”

8BF’s warm endorsement notwithstanding, nothing was done about protecting the cathedral’s spire until 1834–35, when a lightning rod was finally installed: Fonds de l’Oeuvre Notre-Dame, Budgets, pp. 137, 138, 590. Information kindly sent by M. Roger Lehni, of the Ministère de la Culture et de la Francophonie for Alsace. See also Georges Delahache, “Le premier projet de paratonnerre pour la cathédrale,” Bulletin de la Société des Amis de la cathédrale de Strasbourg, I (1925), 14, where the author mentions J.-B. Le Roy’s suggestions of 1770 and 1773 that the Academy put into practice BF’s brilliant discoveries.

But France was not yet ready to take that step. For ecclesiastical and other resistance to lightning rods see I. Bernard Cohen, Benjamin Franklin’s Science (Cambridge, Mass., and London, 1990), pp. 118–58. After a chapel of the basilica of St. Francis had been destroyed by lightning in 1791, Pope Pius VI, while seeing to its restoration, insisted that “electrical Franklin rods” be installed. Cohen, ibid., p. 140. We thank Professor Cohen for his helpful suggestions on this document.

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