MM. Franklin, Le Roy, Coulomb, Delaplace and l'Abbé Rochon made the following report.

M. the maréchal de Ségur sent two proposals to the Academy, to equip the powder magazines in Marseilles with lightning rods; in his letter accompanying the proposals, he explained that the King wished the Academy to have them examined, and to give him our advice on it. The Academy appointed us to report on the proposals; we are going to do this with all the care required for a subject of this importance, not only because of its subject, but also because of its beneficial results. For nothing is more apt to increase the number of lightning rods everywhere, than the example which the King will give by equipping his kingdom's powder magazines with them.

But before discussing the proposals which we have been entrusted to examine, it will be useful to briefly review the theory which must guide the installation of lightning rods or conductors.

To outfit a given building in order to protect it from lightning, using one or several conductors, there are different circumstances to which one must pay particular attention.

1. One must know or determine the area to be protected, in order to decide whether it is necessary to put up one or several rods, for this is an important point to resolve before undertaking anything. Unfortunately, electrical experiments have not yet taught us anything which might lead to a knowledge of the extent of the sphere of action of a conductor's tip. But since buildings have been equipped with them, several observations have taughtus that some parts of the building at a distance of more than forty-five feet from the conductor's tip were struck by lightning. It follows quite naturally that the rods must be placed so that their sphere of action need only protect parts situated at a lesser distance.

2. When there are several pointed rods or spires on a building, they must be well connected together, as well as all the parts of the roof of the building covered in lead or containing some other metal, such as the iron tips of weather cocks, or of any decoration, so that the whole produces a single body of metal elements, which, with the transmission bars, would suitably transmit the electrical energy of a lightning bolt from the top to the bottom of the building, no matter the direction it came from.

3. It is no less important that these bars be closely interconnected, so that the energy does not encounter any resistance in going from top to bottom. For a break of continuity in the bars always produces a larger or smaller resistance depending on the distance between them, because the electrical energy must jump from one bar to the other.

4. Finally, the bars must be in full contact with moist earth, or better yet, with water, so that there is an easy passage consistently open all along the transmission bars, to the earth's mass, the great common reservoir of electrical matter.

As for the height of these pointed rods, they must be at least 12 or 15 feet or even more, if the building is very large. It is certain that the higher they are, the larger the radius of their sphere of activity. They should be two inches square at the base or even more, in proportion to their height when this exceeds 15 feet. And as for the size of the transmission bars, it seems that a size of to inch square will be more than sufficient to transmit the electrical energy of the most violent lightning bolt. Certainly no example has been observed of iron bars of this width which have been melted or altered in any way by the passage of the bolt.

After these preliminary notions of how to install conductors and the dimensions of their elements, we must turn to the examination of the two proposals to equip the powder magazines of Marseilles with lightning rods, proposals sent by M. the maréchal de Ségur and signed by MM. Ravel, de Puy Comtal and Pierron, the first being an officer in the Royal Artillery Corps, and the other in the Royal Fortifications Corps.

These two proposals were created for the same magazine, which is 31 fathoms long by 8 wide or thereabouts. In the first proposal, three pointed rods or spires are mounted, on the ridgepole of the magazine and along its length. To these, four more are added, placed respectively at each corner of the building. The second proposal also recommends three pointed rods on the rooftop, but instead of placing them at the four corners of the building, they are arranged in a quincunx on the two slopes of the roof, with horizontal iron transmission bars all along it, connecting the pointed rods to each other.

As for how the rods are fastened or clamped to the rooftop, whose transmission bars are assembled together, and finally how they lead to the water, the system is the same in each proposal. In looking briefly at the drawings, the Academy will easily understand everything we have just said about the nature of the two proposals.

In order to decide which of the two should be preferred, we will note that the second, with its transmission bars laid horizontally along the length of the roof, would entail large expenses which seem unnecessary. However, the pointed rods from this proposal should be kept, only instead of placing them in a quincunx pattern as proposed in the plan, they should be put in such a way that each of them is exactly

between the pointed rod at the midpoint of the rooftop and the one at the edge. And instead of connecting the rods by the transmission bars laid along the length of the roof, they should be connected with those on the rooftop by bars which will join them in a manner perpendicular to the length of the roof. This way, all the pointed rods would be just as well connected, without the expense of laying the bars horizontally all around the roof. This point decided, we must proceed to the examination of the proposed way in which to connect or assemble the different elements of the lightning rods in these two proposals.

It seems that MM. Pierron and de Ravel, the authors of these two proposals, driven by the need to secure a very close connection between these elements, believed that no expense should be spared to achieve this aim, but we cannot help but think that these gentlemen pushed this scruple too far, and that the method they propose to connect the different elements of their conductor system would involve too much work and superfluous expense. In fact, the screws at the ends of the transmission bars, and the nuts in the hooks designed to accomodate the screws, would cost a fairly considerable sum if they are to be well done and achieve their purpose. We believe that one might proceed more simply: consequently, here is how we think that these magazines might be equipped, according to what has been presented to us, combining solidity and all else which might contribute to the economical construction of their lightning rods, in conformity with what M. the maréchal de Ségur appears to desire, according to the terms of his letter.

We will retain the struts for the spires in the powder magazines proposed for protection in Marseilles and in other ports in which they are exposed to heavy sea winds. However, other magazines found in the Kingdom's interior might be able to do without them, experience having shown that the pointed rods with the dimensions given in the drawings are sufficient to resist the efforts of the winds there; but we will make the spires higher, up to 15 feet from their base, and directly above the place where it will be fastened, in the freestone, we will fashion a circular flange on each side, about two inches in diameter and inch thick. We will make a hole in the center of it, inch in diameter; at the end of the first transmission bar to be attached to this spire, we will fashion another identical flange. We will do the same thing on the other end, so that all the transmission bars will have a latchet on each extremity. In order to connect these elements, we will put the latchet of one of these bars against that of another bar, putting a strip of lead between them. And by using a nut with a headed screw, the stem of which will pass through the eye of these flanges, they can be tightly fastened together. In this way the bars will be well attached to each other; they will be able to be separated easily, and this adjustment will be very easy to carry out. In order to reinforce the transmission bars along the roof, we will be able to establish and attach hooks in the freestone at intervals, which will be split at the top in order to receive them. As for the position and distribution of these pointed rods or spires, they will be constructed as we proposed, three on the ridgepole and two on each slope of the roof, located respectively at the midpoint of the interval between the spire at the middle of the building and the one at the edge. These spires on the slope will be connected to the transmission bars placed along the roof, as we stated above, and they will be tall enough to surpass the ridgepole by least six feet. By this arrangement of the different rods of the building, all the parts of the roof will be well protected by these spires, there being no part which is not well inside the sphere of activity discussed above. We can only applaud the way in which it has been proposed to link the transmission bars to water, by leading them to the sea. However, if at the other end of the building there were soil at the surface, and the ground was not entirely rock, the transmission bars could be made to descend from the pointed rod at this end: this could serve to reduce the distance that the electrical energy would have to travel to reach the other side. The way in which the copper tips are fitted to the spire is good, as they can be adjusted by screws; either method is fine, but in the latter [proposal], the tip can be easily dismantled, and this is an advantage, as it is sometimes necessary. Some of them have been observed to be melted by the passage of the electrical charge from a thunderbolt.

Such are the observations that we felt necessary to make regarding these two proposals which M. the maréchal de Ségur sent to the Academy, to equip the powder magazines of Marseille with lightning rods; and such are the suggestions which we believed necessary for connecting the different elements of the system together. But before concluding this report, we owe it to MM. Ravel de Puy Comtal and Pierron to say that we recognize by the manner in which their proposals were conceived, that they understand the subject matter very well, and that they are quite capable of equipping powder magazines with lightning rods, the task with which M. the maréchal de Ségur has entrusted them in order to protect these magazines against the ravages of lightning.”

Académie Royale des Sciences, minutes, tome CIII, fol. 90 v°-95 r° (session for Saturday April 24, 1784)