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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 mode, 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 mean time, 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.*
REPORT ON LIGHTNING CONDUCTORS FOR THE POWDER MAGAZINES AT PURFLEET.
Drawn up by Benjamin Franklin, August 21st, 1772. To the PREsident AND Council of THE Roy Al Society.
The Society being consulted by the Board of Ordnance, on the propriety of fixing conductors for securing the powder magazines at Purfleet from lightning, and having thereupon done us the honor of appointing us a committee to consider the same and report our opinion, we have accordingly visited those buildings, and examined with care and attention their situation, construction, and circumstances, which we find as follows;
They are five in number, each about one hundred and fifty feet long, about fifty-two feet wide, built of brick, arched under the roof, which in one of them is slated, with a coping of lead twenty-two inches wide on the ridge, from end to end ; and the others, we were informed, are soon to be covered in the same manner. They stand parallel to each other, at about fifty-seven feet distance, and are founded on a chalk rock about one hundred feet from the river, which rises at high tides within a few inches of the level of the ground, its brackish water also soaking through to the wells that are dug near the buildings.
* 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. — Dc Bourg.
The barrels of powder, when the magazines are full, lie piled on each other up to the spring of the arches; and there are four copper hoops on each barrel, which, with a number of perpendicular iron bars (that come down through the arches to support a long, grooved piece of timber, wherein the crane was usually moved and guided to any part where it was wanted), formed broken conductors, within the building, the more dangerous from their being incomplete; as the explosion from hoop to hoop, in the passage of lightning drawn down through the bars among the barrels, might easily happen to fire the powder contained in them; but the workmen were removing all those iron bars (by the advice of some members of the Society who had been previously consulted), a measure we very much approve of. .
On an elevated ground, nearly equal in height with the tops of the magazines, and one hundred and fifty yards from them, is the house wherein the Board usually meet; it is a lofty building, with a pointed hiproof, the copings of lead down to the gutters; whence leaden pipes descend at each end of the building, into the water of two wells forty feet deep, for the purpose of conveying water, forced up by engines, to a cistern in the roof.
There is also a proof-house adjoining to the end of one of the magazines ; and a clock-house at the distance of feet from them, which has a weathercock on an iron spindle, and probably some incomplete conductors within, such as the wire usually extending up from a clock to its hammer, the clock, pendulum, rod, &c.
The blowing up of a magazine of gunpowder by lightning within a few years past, at Brescia in Italy, which demolished a considerable part of the town, with the loss of many lives, does, in our opinion, strongly urge the propriety of guarding such magazines from that kind of danger. And since it is now well known from many observations, that metals have the property of conducting, and a method has been discovered of using that property for the security of buildings, by so disposing and fixing iron rods, as to receive and convey safely away such lightning as might otherwise have damaged them, which method has been practised near twenty years in many places, and attended with success in all the instances that have come to our knowledge, we cannot therefore but think it advisable to provide conductors of that kind for the magazines in question.
In common cases it has been judged sufficient, if the lower part of the conductor were sunk three or four feet into the ground till it came to moist earth; but, this being a case of the greatest importance, we are of opinion, that greater precaution should be taken. Therefore we would advise, that at each end of each magazine a well should be dug in or through the chalk, so deep as to have in it at least four feet of standing water. From the bottom of this water should rise a piece of leaden pipe to or near the surface of the ground, where it should be strongly joined to the end