Abbildungen der Seite
PDF
EPUB

about by a high wind at west or northwest, the bells rung for several hours (though with little intermissions) as briskly as ever I knew them, and I drew considerable sparks from the wire. This phenomenon I never observed but twice; viz. on the 31st of January, 1760, and the 3d of March, 1762.

I am, Sir, &c.

J. WINTHROp.

FROM ALEXANDER SMALL TO BENJAMIN FRANKLIN.

Flash of Lightning that struck St. Bride's Steeple.

I HAVE just recollected, that, in one of our great storms of lightning, I saw an appearance, which I never observed before, nor ever heard described. I am persuaded that I saw the flash which struck St. Bride's steeple. Sitting at my window, and looking to the north, I saw what appeared to me a solid straight rod of fire, moving at a very sharp angle with the horizon. It appeared to my eye as about two inches diameter, and had nothing of the zigzag lightning motion.. I instantly told a person sitting with me, that some place must be struck at that instant. I was so much surprised at the vivid, distinct appearance of the fire, that I did not hear the clap of thunder, which stunned every one besides. Considering how low it moved, I could not have thought it had gone so far, having St. Martin's, the New Church, and St. Clement's steeples in its way. It struck the steeple a good way from the top, and the first impression it made in the side is in the same direction I saw it move in. It was succeeded by two flashes, almost united, moving in a pointed

direction. There were two distinct houses struck in Essex Street. I should have thought the rod would have fallen in Covent Garden, it was so low. Perhaps the appearance is frequent, though never before seen by Yours,

ALEXANDER SMALL.

TO MR. PETER FRANKLIN, AT NEWPORT.

Best Method of securing a Powder Magazine from Lightning.

You may acquaint the gentleman that desired you to inquire my opinion of the best method of securing a powder magazine from lightning, that I think they cannot do better than to erect a mast not far from it, which may reach fifteen or twenty 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 public thing, the expense 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 spoiled 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 corked; or in large tin canisters, with small covers

shutting close by means of oiled 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, though it comes all the way by sea in the damp hold of a ship. And by this method, grain, meal, &c., if well dried before it is 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 tinfoil, or leaf-tin, being tin milled between rollers till it becomes as thin as paper, and more pliant, at the same time that its texture is extremely close. It may be applied to the wood with common paste, made with boiling water thickened 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 boiled. 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 tinfoil should be plied into the groove. Then, one head being put in, and that end hooped 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 tinfoil is but about eighteen pence sterling a pound, and is so extremely thin, that, I imagine, a pound of it would line three or four powder barrels. I am, &c.

B. FRANKLIN.

TO THOMAS RONAYNE, AT CORK.*

*

On the Electricity of the Fogs in Ireland.

SIR,

London, 20 April, 1766.

I have received your very obliging and very ingenious letter by Captain Kearney. Your observations upon the electricity of fogs and the air in Ireland, and upon different circumstances of storms, appear to me very curious, and I thank you for them. There is not, in my opinion, any part of the earth whatever which is, or can be, naturally in a state of negative electricity; and, though different circumstances may occasion an inequality in the distribution of the fluid, the equilibrium is immediately restored by means of its extreme subtilty, and of the excellent conductors with which the humid earth is amply provided. I am of opinion, however, that when a cloud, well charged positively, passes near the earth, it repels and forces down into the earth that natural portion of electricity, which exists near its surface, and in buildings, trees, &c., so as actually to reduce them to a negative state before it strikes them. I am of opinion, too, that the negative state in which you have frequently found the balls, which are suspended from your apparatus, is not always occasioned by clouds in a negative state; but more commonly by clouds positively electrified, which have passed over them, and which in their passage have repelled and driven off a part of the electrical matter, which naturally existed in the apparatus; so that, what remained after the passing of the clouds diffusing itself uniformly

Translated from M. Dubourg's French edition of the author's works; (Tome I. p. 265.) — EDITOR.

VOL. V.

52

II

through the apparatus, the whole became reduced to a negative state.

If you have read my experiments made in continuation of those of Mr. Canton, you will readily understand this; but you may easily make a few experiments, which will clearly demonstrate it. Let a common glass be warmed before the fire, that it may continue very dry for some time; set it upon a table, and place upon it the small box made use of by Mr. Canton, so that the balls may hang a little beyond the edge of the table. Rub another glass, which has previously been warmed in a similar manner, with a piece of black silk, or a silk handkerchief, in order to electrify it. Hold then the glass above the little box, at about the distance of three or four inches from that part, which is most distant from the balls; and you will see the balls separate from each other; being positively electrified by the natural portion of electricity, which was in the box, and which is driven to the further part of it by the repulsive power of the atmosphere in the excited glass. Touch the box near the little balls (the excited glass continuing in the same state) and the balls will again unite; the quantity of electricity which had been driven to this part being drawn off by your finger. Withdraw then both your finger and the glass, at the same instant, and the quantity of electricity which remained in the box, uniformly diffusing itself, the balls will again be separated, being now in a negative state. While things are in this situation, begin once more to excite your glass, and hold it above the box, but not too near, and you will find, that, when it is brought within a certain distance, the balls will at first approach each other, being then in a natural state. In proportion as the glass is brought nearer, they will again separate, being positive. When the glass is moved beyond them, and at some little farther

« ZurückWeiter »