To Peter Franklin
MS not found; reprinted from extract in Benjamin Franklin, Experiments and Observations on Electricity (London, 1769), pp. 441–3.2
You may acquaint the gentleman that desired you to enquire my opinion of the best method of securing a powder magazine from lightning,4 that I think they cannot do better than to erect a mast not far from it, which may reach 15 or 20 feet above the top of it, with a thick iron rod in one piece fastened to it, pointed at the highest end, and reaching down through the earth till it comes to water. Iron is a cheap metal; but if it were dearer, as this is a publick thing, the expence is insignificant; therefore I would have the rod at least an inch thick, to allow for its gradually wasting by rust; it will last as long as the mast, and may be renewed with it. The sharp point for five or six inches should be gilt.
But there is another circumstance of importance to the strength, goodness, and usefulness of the powder, which does not seem to have been enough attended to: I mean the keeping it perfectly dry. For want of a method of doing this, much is spoilt in damp magazines, and much so damaged as to become of little value. If instead of barrels it were kept in cases of bottles well cork’d; or in large tin canisters, with small covers shutting close by means of oil’d paper between, or covering the joining on the canister; or if in barrels, then the barrels lined with thin sheet lead; no moisture in either of these methods could possibly enter the powder, since glass and metals are both impervious to water.
By the latter of these means you see tea is brought dry and crisp from China to Europe, and thence to America, tho’ it comes all the way by sea in the damp hold of a ship. And by this method, grain, meal, &c. if well dry’d before ’tis put up, may be kept for ages sound and good.
There is another thing very proper to line small barrels with; it is what they call tin-foil, or leaf-tin, being tin mill’d between rollers till it becomes as thin as paper, and more pliant, at the same time that its texture is extreamly close. It may be apply’d to the wood with common paste, made with boiling water thicken’d with flour; and, so laid on, will lie very close and stick well: But I should prefer a hard sticky varnish for that purpose, made of linseed oil much boil’d. The heads might be lined separately, the tin wrapping a little round their edges. The barrel while the lining is laid on, should have the end hoops slack, so that the staves standing at a little distance from each other, may admit the head into its groove. The tin-foil should be plyed into the groove. Then one head being put in, and that end hoop’d tight, the barrel would be fit to receive the powder, and when the other head is put in and the hoops drove up, the powder would be safe from moisture even if the barrel were kept under water. This tin-foil but about 18 pence sterling a pound, and is so extreamly thin, that I imagine a pound of it would line three or four powder barrels. I am &c.
2. Printed as Letter XLVII in Exper. and Obser., 1769 edit., and also in the edition of 1774, pp. 451–3.
3. The date of this letter is uncertain, but because it is headed in Exper. and Obser. as “To Mr. P. F. Newport,” it must have been written before Peter Franklin moved to Philadelphia. When BF and Sally returned from Boston in November 1763, Peter seems to have accompanied them, or followed them soon after (below, p. 392), perhaps in order to arrange for the assumption of the Philadelphia postmastership and to find a suitable house. By the summer of 1764 he and his wife appear to have been well settled into their new home, to which the Post Office was moved in October (BF to Jane Mecom, July 10, 1764, APS; Pa. Gaz., Oct. 17, 1764). Either in 1762 or early in 1763 BF apparently also wrote John Pringle on the preservation of gunpowder (below, p. 268). It seems probable, therefore, though there is no positive evidence, that BF wrote the present letter after his return from England; hence it is printed here.
4. The inquiry may refer to some proposal concerning the magazine at Fort George, on Goat Island, off Newport, R.I., though there is no reference to the matter in R.I. Col. Recs.