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applied to some beneficial uses. It has been thought, that water reduced to vapor by heat was rarefied only fourteen thousand times, and on this principle our engines for raising water by fire are said to be constructed; but, if the vapor so much rarefied from water is capable of being itself still farther rarefied to a boundless degree, by the application of heat to the vessels or parts of vessels containing the vapor (as at first it is applied to those containing the water), perhaps a much greater power may be obtained, with little additional expense. Possibly too, the power of easily moving water from one end to the other of a movable beam (suspended in the middle like a scale-beam) by a small degree of heat, may be applied advantageously to some other mechanical purposes.
The magic square and circle, I am told, have occasioned a good deal of puzzling among the mathematicians here; but no one has desired me to show him my method of disposing the numbers. It seems they wish rather to investigate it themselves. When I have the pleasure of seeing you, I will communicate it.
With singular esteem and respect, I am, dear Sir, Your most obedient humble servant,
On some Electrical Experiments made with Charcoal. Philadelphia, 13 October, 1770.
THE conducting quality of some sorts of charcoal is indeed very remarkable. I have found oak, beech, and maple to conduct very well; but tried several
pieces of pine coal without finding one that would conduct at all; perhaps they were made in a fire not hot enough, or not continued in it long enough. A strong line, drawn on paper with a black lead pencil, will conduct an electrical shock pretty readily; but this, perhaps, may not be new to you.
On the 12th of last July, three houses in this city, and a sloop at one of the wharfs, were, in less than an hour's time, all struck with lightning. The sloop, with two of the houses, was considerably damaged; the other was the dwellinghouse of Mr. Joseph Moulde, in Lombard Street, which was provided with a round iron conductor, half an inch thick, its several lengths screwed together, so as to make very good joints, and the lower end five or six feet under ground; the lightning, leaving every thing else, pursued its way through that, melted off six inches and a half of the slenderest part of a brass wire fixed on the top, and did no further damage, within doors or without. Captain Falconer, who brings you this, was in the house at the time of the stroke, and says it was an astonishing loud one.
TO MAJOR DAWSON, ENGINEER.*
On the Security of the Powder Magazines at Purfleet.
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,
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.) - EDITOR.
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,
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.
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.