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 ROYAL SOCIETY. GENTLEMEN, 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 * 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. 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. 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 of an upright bar, an inch and a half in diameter, fastened to the wall by leaden straps, and extending ten feet above the ridge of the building, tapering from the ridge upwards to a sharp point; the upper twelve inches to be copper; the iron to be painted. We mention lead for the underground part of the conductor, as less liable to rust in water and moist places, in the form of a pipe, as giving greater stiffness for the substance; and iron for the part above ground, as stronger and less likely to be cut away. The pieces of which the bar may be composed should be screwed strongly into each other by a close joint, with a thin plate of lead between the shoulders, to make the joining or continuation of metal more perfect. Each rod, in passing above the ridge, should be strongly and closely connected by iron or lead, or both, with the leaden coping of the roof, whereby a communication of metal will be made between the two bars of each building, for a more free and easy conducting of the lightning into the earth. We also advise, in consideration of the great length of the buildings, that two wells, of the same depth with the others, should be dug within twelve feet of the doors of the two outside magazines; that is to say, one of them on the north side of the north building, the other on the south side of the south building; from the bottoms of which wells, similar conductors should be carried up to the eaves, there joining well with a plate of lead extending on the roof up to the leaden coping of the ridge, the said plate of lead being of equal substance with that of the coping. We are further of opinion, that it will be right to form a communication of lead from the top of the chimney of the proof-house to the lead on its ridge, and thence to the lead on the ridge of the corridor, and thence to the iron conductor of the adjacent end of the magazine; and also to fix a conductor from the bottom of the weathercock spindle of the clock-house, down on the outside of that building into the moist earth. As to the board-house, we think it already well furnished with conductors by the several leaden communications above mentioned, from the point of the roof down into the water; and that, by its height and proximity, it may be some security to the buildings below it; we therefore propose no other conductor for that building, and only advise erecting a pointed rod on the summit, similar to those before described, and communicating with those conductors. To these directions we would add a caution, that, in all future alterations or repairs of the buildings, special care be taken that the metalline communications are not cut off or removed. It remains that we express our acknowledgments to Sir Charles Frederick, Surveyor-general of the Ordnance, for the obliging attention with which he entertained and accommodated us on the day of our inquiry. With very great respect we are, Gentlemen, Your most obedient humble servants, H. CAVENDISH, J. ROBERTSON.* * Mr. Benjamin Wilson, one of the committee appointed by the Royal Society, dissented from the part of the above report, which relates to pointed conductors. EDITOR. "I dissent from the report," said he, "in that part only which recommends, that each conductor should terminate in a point. 66 "My reason for dissenting is, that such conductors are, in my opinion, less safe than those which are not pointed. "Every point, as such, I consider as soliciting the lightning, and, by |
