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CCCCXLI: TO MAJOR DAWSON, ENGINEER 1 - Benjamin Franklin, The Works of Benjamin Franklin, Vol. V Letters and Misc. Writings 1768-1772 [1904]

Edition used:

The Works of Benjamin Franklin, including the Private as well as the Official and Scientific Correspondence, together with the Unmutilated and Correct Version of the Autobiography, compiled and edited by John Bigelow (New York: G.P. Putnam’s Sons, 1904). The Federal Edition in 12 volumes. Vol. V (Letters and Misc. Writings 1768-1772).

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CCCCXLI

TO MAJOR DAWSON, ENGINEER1

  • Craven Street,

29 May, 1772.

Sir:—

Having visited yesterday, as you desired, the powder magazines at Purfleet, in order to see how they may be protected against danger from lightning, I think—

1. That all the iron bars, which pass down along the arches, from the top to the place where the powder is deposited, should be removed; as they now constitute, with the brass hoops with which the casks are bound, an imperfect conductor; imperfect in proportion to the greater or less height to which the casks are piled; but, in any case, such that they can only serve to attract towards the powder the first stroke that falls upon the arch; and that they are consequently very dangerous.

2. That the building, which has a leaden coping along the ridge from one end to the other, may be secured by means of a pointed iron rod, carried up near each end, communicating with this coping, and extending through the rock of chalk, which serves as the foundation of the building, till it meets with water. This rod should be at least an inch in diameter, that it may be more durable, and afford the lightning a more free course through its substance; and it should be painted, to preserve it from rust. Its upper extremity should be carried ten feet above the summit of the roof, and taper off gradually till it ends in a sharp point; and, the better to preserve this point, the last six inches should be of brass, because it is less liable to become blunted by rust. If the rod cannot well be made entirely of a single piece, the different pieces composing it should be strongly screwed together, or into one another very closely, with a thin plate of lead between the joints, in order to render the junction or continuation of the metal more perfect.

After all the electrical experiments that I have made in reference to this subject, and all the examples that have come to my knowledge of the effects of lightning on these conductors, it seems to me that (provided they are good and perfect, carried down till water or very moist ground is reached) they are equally safe, whether placed directly against the wall, and secured by staples driven into it, or whether supported by a pole or staff planted in the ground, at some distance from the wall. The former is the better rod, as the rod can be bent to avoid the windows or doors, which are situated directly below the summit of the roof. Yet, as certain apprehensions may be more effectually set at rest by supporting the rods in the other manner, I should make no objection to this, provided that they can be suitably placed, without interfering with any passage, and that they are so firmly fixed that the wind cannot, by causing them to vibrate, interrupt the communication of iron or lead between the side of the rod and the lead that covers the ridge.

3. As I am informed that the roofs of the other four buildings are to be reconstructed after the model of that of which I have just been speaking, the same method may be followed with regard to them, when they are finished in this manner. But if it be asked how they may be rendered secure in the meantime, I would advise that (as their roofs are now of a different form, being hip-roofs with four corners, and the joining at their corners, as well as their ridge-pieces, having a coping of lead, which extends to the gutters) the passages which it is proposed to carry down till water is reached, be bored or dug immediately, and that that part of each conductor which is to be carried up from the water as high as the gutters, be fixed in them. From the top of this conductor I would carry out two arms of iron to the corners of the gutters, where the leaden coping of the corners of the roof should be united to the ends of these bars; and at the junction of these corners with the ridge-piece, I would carry up rods to the height of ten feet, pointed as directed above; which, when a new roof is made, could be used for the upper part of a straight conductor. I am, Sir,

Your most obedient humble servant,

B. Franklin.

P. S.—For that part of the conductor which is to be carried under ground, leaden pipes should be used, as less liable to rust.1

[1 ]This letter is here printed in a translation from the French, as contained in M. Dubourg’s edition of the author’s writings (tom. i., p. 280).—Ed.

[1 ]In consequence of this letter the Ordnance Department directed that the advice of the writer should be followed in some respects; but that they might be still better authorized to proceed with regard to other points, these gentlemen were desirous to obtain the sanction of the Royal Society, and therefore requested their opinion. The Royal Society appointed Messrs. Cavendish, Watson, Franklin, Wilson, and Robertson, a committee to examine the subject, and report thereon—Dubourg.