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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 ap
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
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,
* 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.
“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 that means, not only contributing to increase the quantity of every actual discharge, but also frequently occasioning a discharge, where it might not otherwise have happened.
Experiments, Observations, and Facts, tending to sup
port the Opinion of the Utility of long, pointed Rods, for securing Buildings from Damage by Strokes of Lightning
READ AT THE COMMITTEE APPOINTED TO CONSIDER THE ERECTING OF
CONDUCTORS TO SECURE THE MAGAZINES AT PURFLEET, AUGUST 27th, 1772.
The prime conductor of an electric machine, A, B, (see Plate IV.) being supported about ten inches and a half above the table by a wax stand, and under it erected a pointed wire, seven inches and a half high, and one fifth of an inch thick, and tapering to a sharp point, and communicating with the table; when the point (being uppermost) is covered by the end of a finger, the conductor may be full charged, and the electrometer * will rise to the height indicating a full charge; but the moment the point is uncovered, the ball
“ If, therefore, we invite the lightning, while we are ignorant what the quantity or the effects of it may be, we may be promoting the very mischief we mean to prevent.
“Whereas if, instead of pointed, we make use of blunted conductors, those will as effectually answer the purpose of conveying away the lightning safely, without that tendency to increase or invite it.
“ My further reasons for disapproving of points, in all cases where conductors are judged necessary, are contained in a letter addressed to the Marquis of Rockingham, and published in the Philosophical Transactions, Vol. LIV. p. 247.
“ There are other reasons also, which I have to offer, for rejecting points on this particular occasion, and which were mentioned at the committce. Those I shall lay before the Royal Society at another opportunity, for the benefit of the public.”
* Mr. Henley's.