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though the ground about it be drier. In such case, this water may be exploded, that is, blown into vapor, whereby a force is generated, that may damage the foundation. Water reduced to vapor, is said to occupy fourteen thousand times its former space. I have sent a charge through a small glass tube, that has borne it well while empty, but, when filled first with water, was shattered to pieces, and driven all about the room. Finding no part of the water on the table, I suspected it to have been reduced to vapor; and was confirmed in that suspicion afterwards, when I had filled a like piece of tube with ink, and laid it on a sheet of clean paper, whereon, after the explosion, I could find neither any moisture nor any sully from the ink. This experiment of the explosion of water, which I believe was first made by that most ingenious electrician, Father Beccaria, may account for what we sometimes see in a tree struck by lightning, when part of it is reduced to fine splinters like a broom; the sap-vessels being so many tubes containing a watery fluid, which, when reduced to vapor, rends every tube lengthwise. And perhaps it is this rarefaction of the fluids in animal bodies killed by lightning or electricity, that, by separating its fibres, renders the flesh so tender, and apt so much sooner to putrefy. I think too, that much of' the damage done by lightning to stone and brick walls may sometimes be owing to the explosion of water, found during showers, running or lodging in the joints or small cavities or cracks that happen to be in the walls. '•
Here are some electricians, that recommend knobs instead of points on the upper end of the rods, from a supposition that the points invite the stroke. It is true that points draw electricity at greater distances in the gradual, silent way ;'but knobs will draw at the greatest
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distance a stroke. There is an experiment that will settle this. Take a crooked wire, of the thickness of a quill, and of such a length as that, one end of it being applied to the lower part of a charged bottle, the upper may be brought near the ball on the top of the wire that is in the bottle. Let one end of this wire be furnished with a knob, and the other may be gradually tapered to a fine point. When the point is presented to discharge the bottle, it must be brought much nearer before it will receive the stroke, than the knob requires to be. Points, besides, tend to repel the fragments of an electrized cloud, knobs draw them nearer. An experiment, which, I believe, I have shown you, of cotton fleece hanging from an electrized body, shows this clearly, when a point or a knob is presented under it.
You seem to think highly of the importance of this discovery, as do many others on our side of the water. Here it is very little regarded; so little, that, though it is now seven or eight years since it was made public, I have not heard of a single house as yet attempted to be secured by it. It is true the mischiefs done by lightning are not so frequent here as with us; and those who calculate chances may perhaps find, that not one death (or the destruction of one house) in a hundred thousand happens from that cause, and that therefore it is scarce worth while to be at any expense to guard against it. But in all countries there are particular situations of buildings more exposed than others to such accidents, and there are minds so strongly impressed with the apprehension of them, as to be very unhappy every time a little thunder is within their hearing; it may therefore be well to render this little piece of new knowledge as general and as well understood as possible, since to make us safe is not all its advantage; it is some to make us easy. And, as the stroke it secures us from might have chanced, perhaps, but once in our lives, while it may relieve us a hundred times from those painful apprehensions, the latter may possibly, on the whole, contribute more to the happiness of mankind than the former.
Your kind wishes and congratulations are very obliging. I return them cordially; being, with great regard and esteem, my dear Sir, your affectionate friend and most obedient humble servant,
Accounts from Carolina (mentioned in the foregoing Letter) of the Effects of Lightning on two of the Rods commonly affixed to Houses there, for securing them against Lightning.
Charleston, 1 November, 1760.
,....." It is some years since Mr. Raven's rod was struck by lightning. I hear an account of it was published at the time, but I cannot find it. According to the best information I can now get, he had fixed to the outside of his chimney a large iron rod, several feet in length, reaching above the chimney; and to the top of this rod the points were fixed. From the lower end of this rod, a small brass wire was continued down to the top of another iron rod driven into the earth. On the ground-floor in the chimney stood a gun, leaning against the back wall, nearly opposite to where the brass wire came down on the outside. The lightning fell upon the points, did no damage to the rod they were fixed to; but the brass wire, all down till it came opposite to the top of the gun-barrel, was destroyed.* There the lightning made a hole through the wall, or back of the chimney, to get to the gun-barrel,* down which it seems to have passed, as, although it did not hurt the barrel, it damaged the butt of the stock, and blew up some bricks of the hearth. The brass wire below the hole in the wall remained good. No other damage, as I can learn, was done to the house. I am told the same house had formerly been struck by lightning, and much damaged, before these rods were invented."
* A proof that it was not of sufficient substance to conduct with safety to itself (though with safety so far to the wall) so large a quantity of the electric fluid.
Mr- William. Maine's Jlccount of the Effects of the Lightning on his Rod, dated at Indian Land, in South Carolina, August 28lh, 1760.
...... u i nacj a set 0f electrical points, consisting of
three prongs, of large brass wire tipt with silver, and perfectly sharp, each about seven inches long; these were riveted at equal distances into an iron nut about three quarters of an inch square, and opened at top equally to the distance of six or seven inches from point to point, in a regular triangle. This nut was screwed very tight on the top of an iron rod x>f about half an inch diameter, or the thickness of a common curtainrod; composed of several joints, annexed by hooks turned at the ends of each joint, and the whole fixed to the chimney of my house by iron staples. The points were elevated (a) six or seven inches above the top of the chimney; and the lower joint' sunk three feet in the earth, in a perpendicular direction.
"Thus stood the points on Tuesday last, about five in the evening, when the lightning broke with a violent explosion on the chimney, cut the rod square off just under the nut, and, I am persuaded, melted the points, nut, and top of the rod, entirely up; as, after the most diligent search, nothing of either was found (/>), and the top of the remaining rod was cased over with a congealed solder. The lightning ran down the rod, starting almost all the staples (c), and unhooking the joints without affecting the rod (d), except on the inside of each hook where the joints were coupled, the surface of which was melted (e), and left as cased over with solder. No part of the chimney was damaged (/), only at the foundation (g), where it was shattered almost quite round, and several bricks were torn out (h). Considerable cavities were made in (he earth quite round the foundation, but most within eight or nine inches of the rod. It also shattered the bottom weather-board (t) at one corner of the house, and made a large hole in the earth by the corner post. On the other side of the chimney, it ploughed up several furrows in the earth, some yards in length. It ran down the inside of the chimney (k), carrying only soot with it, and filled the whole house with its flash (/), smoke, and dust. It tore up the hearth in several places (m), and broke some pieces of China in the buffet (n). A copper tea-kettle standing in the chimney was beat together, as if some great weight had fallen upon it (o); and three holes, each about half an inch diameter, melted through the bottom (/>). What seems to me the most surprising is, that the hearth under the kettle was not hurt, yet the bottom of the kettle was drove inward, as if the lightning proceeded from under it upwards (q), and the cover was thrown to the middle of the floor (r). The fire-dogs, an iron loggerhead, an Indian pot, an earthen cup, and a cat were all in the chimney at the time unhurt, though a great part of the
* A more substantial conductor.