ings, but more especially to powder magazines? Whether there is not some particular manner of buildings, invented of late, adapted to powder magazines, either to diminish the shock of the explosion, or to secure them against any accident, by the methods used at Philadelphia ?

M. Calandrini says further, that he himself has been eye-witness of the effects of lightning coming into a room, which had received much damage from it. That he looked for the place it went out at, and after long search perceived that it had followed the wire of the bell, which had conducted it through a very inconsiderable hole into the next room, whence it had opened itself a passage into a back yard. This accident was at that time thought very extraordinary, being anterior to Dr. Franklin's experiment.

To M. Calandrini's questions Dr. Watson sent the following an

swers

1. The apparatus used at Philadelphia consists either of a long iron rod, placed on the highest part of a house or other building, or of a shorter rod, inserted into a long wooden pole, placed in the same manner. The iron rod, mentioned by Mr. Kinnersley, in the "Philosophical Transactions Abridged," vol. xi, and which probably preserved the house in Philadelphia on which it was placed, extended in height about nine and a half feet above a stack of chimneys, to which it was fixed; but he supposes that three or four feet would have been sufficient. These rods are pointed at their upper extremity. It is indifferent which of these two is used, provided they are high enough above the chimneys, or any other part of the edifice. Connected to, or suspended from the metal of these, a metallic wire, generally of iron, is conducted in the most convenient manner to the nearest water, viz., to the well of the house, or any other water in the neighbourhood.

2. This method, wherever it has been employed, has hitherto perfectly answered the intention; no house in Philadelphia, or in any other place I have heard of, having suffered from the effects of lightning where this apparatus has been erected. The improvements I should recommend would be, first, that as iron wire soon becomes rusty, and when rusty to the centre is unfit for the present purpose, and as brass wire is, when long exposed to the weather, exceedingly brittle and liable to snap asunder, the wire should be of copper, and of a size not less than that of a large goose quill. Secondly, I prefer its being conducted from the rod at the top to the water below, on the outside of the building, and thus prevent the lightning from coming within the building. On houses where there are gutters and spouts of lead to carry off the rain, the wire need only be conducted to the lead of the gutters, and attention be had that the gutters and the spouts coming from them are in their whole length in contact, or very nearly so, one with the other. If the leaden spouts do not reach to the bottom of the building, a slip of lead, such as is employed for the gutters, and about an inch wide,

should be fastened to the bottom of one or two of the spouts, and conducted to the water. If such a slip of lead was to be conducted from the rod at the top to the gutters, it might with equal advantage be substituted for the copper wire: or further, a slip of lead of this kind may be connected with the rod at the top of the house; and where there are no leaden gutters nor spouts, may be conducted on the outside of the house down to the water, as before mentioned. I would recommend likewise an increase of their number, as the effects of one apparatus of this kind can extend only to a certain distance, and that to no great one; and the security, where mischiefs from lightning are frequent, must arise from their number. In countries and places so circumstanced, no house or other building should be without one at least; large edifices ought to have several. The number should be in proportion to the size of the building.

3. In powder magazines I should recommend the apparatus to be detached from the building itself, and to be only placed as near it as might be. Powder magazines should never be constructed so as to cover a large quantity of ground. If security from lightning was considered in their construction as a considerable object, I should recommend a circular building; in the periphery of which should be placed storehouses sufficient in their number and extent to contain the quantity of powder proposed. In the centre of this circle should be a well, very near to which should be erected a pole or mast, high enough to reach some feet above the buildings of the powder magazine, or the buildings in its neighbourhood. From this mast there should rise a brass rod, five or six feet in length, an inch in thickness, and ending in a point; and from this rod a wire of copper of a size not less than that of a large goose quill, should be conveyed down the mast, and terminate in the water of the well. If there is no well the wire should be laid into the nearest water; as the expence even of some hundred yards of a wire of this sort can hardly be considered as an object in an affair of this importance. For though there is reason to believe that the wire communicating with the ground would prevent the mischiefs of a thunder cloud, which came near an apparatus of this sort; yet as water is a more ready conductor than the ground, it should, if possible, be insisted on in this particular case, and employed. Mr. West's apparatus, described by the before mentioned Mr. Kinnersley, terminated in an iron stake, driven four or five feet into the ground; yet the earth did not conduct the lightning so fast but that, in a thunder storm, the lightning was seen to be diffused near the stake two or three yards over the pavement, though at that time very wet with rain. It is presumed, that had this iron stake been placed in water instead of earth, the lightning had not been visible, on account of the water's receiving the electric matter more readily than earth. Where this apparatus therefore is applied to powder magazines, it should certainly terminate in water. At Mr. Hamilton's, at Cobham, about

twenty miles from hence, where an apparatus of this sort was erected on a high and greatly exposed building, as there was no water but at a great distance, the bottom of the wire was placed deep in a hill of moist sand. If instead of one wire, two, three, or more, were adapted to the brass rod in this matter, and conducted to the water, or if the brass rod itself was continued to the water, I should consider it, in extraordinary cases, as an additional security. This will explain my sentiments on the 3rd, 4th, and 6th questions.

5. As the expectation of the utility of this apparatus is presumed to be the preventing of the accumulation of electricity in its neighbourhood, by affording a constant and easy passage to the electricity of the clouds surcharged with it, nothing in my opinion need be apprehended from the apparatus electrifying the air; as its principal operation is conceived to be the reverse of that, viz., divesting the air of its electricity. I am well apprised from experiments made here, that the earth is frequently electrified plus, and the clouds minus; and that this change of plus and minus between the clouds and earth are sometimes seen to vary several times in a quarter of an hour: but in that case it is presumed that the clouds, within the sphere of action of the apparatus, have by its operation their electricity brought to the same standard with that of the earth in its neighbourhood, and vice versa; and consequently that the mischiefs which might arise from the difference of the densities of the electricity in the earth and clouds are prevented, by the equilibrium between them being maintained. This subject, in regard to the electricity's being plus or minus, I many years ago considered, and laid my thoughts on it before the public, as may be seen in the "Philosophical Transactions Abridged," vol. ix.

That the atmosphere at times is very strongly electrified is evident, to say nothing of lightning, not only from our apparatus, but from the masts of ships being beset with St. Elmo's fires, which I believe would scarcely, if ever happen, were the masts provided with an apparatus of this sort: unless the cause might be so great, and come on so fast, that the metal employed between the tops of the masts and the water might not, on account of the vastness of the cause, be large enough for the purpose. If it should so happen, St. Elmo's fires might still appear at the tops of the masts, and thunder clouds might burst near them, and exert their dreadful effects. That even artificial electricity, when in too great a quantity, and hurried on too fast through a fine iron wire, has a remarkable effect on the wire, appears from a very curious experiment of Mr. Kinnersley, of Pennsylvania. This gentleman, in the presence of Dr. Franklin, by his case of bottles being electrified fully, and made to explode at once, after the manner of the experiment of Leyden, through a fine iron wire, the wire appeared at first red hot, and then fell into drops, which burned themselves into the surface of his table or floor. These drops cooled in a spherical figure, like very small shot, of which Dr. Franklin transmitted some hither to Mr.

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Canton, who had repeated this experiment. This proves the fusion to have been very complete, as nothing less than the most perfect fluidity could give this figure to melted iron. These effects from artificial lightning, are exactly similar to those of the natural; as we have several times known iron wires, nails, and other metallic substances to have been melted, and parts of them, while hot, bedding themselves in wood, by a thunder storm. Of this we had some instances here in a thunder storm, which happened in July, 1759, of which the effects were communicated to the public in the "Philosophical Transactions Abridged," vol. xi. As metal has been made red hot, and melted, by artificial lightning, how much greater must be presumed to be the effects of the natural; and how much larger ought to be the metallic part of the apparatus to avert its mischief? This requires particular attention.

7. I was of opinion, that iron bars to support weathercocks, if they were placed on the top of buildings made of brick or stone, and in contact with either of these materials, were not dangerous to ordinary buildings on the account you mention, except in very particular and extraordinary cases; as these substances when not much heated, conduct the electric matter in a very considerable degree. But what lately happened to St. Bride's steeple, as well as the mischief to South Weald church on the same day, evinces that the apparatus usually applied to weathercocks, should never be trusted in any building, without a metallic communication from them to some water, or at least to some very moist ground. St. Bride's steeple, one of the most beautiful in London, was on Monday, June 18, about ten minutes before three in the afternoon, very greatly injured, in one of the most severe thunder storms which ever happened here. From as attentive an examination as the steeple at present will admit of without scaffolding, it appears to me, that the weathercock and its apparatus had the principal share in occasioning the great mischief done to the upper part of the steeple. I am of opinion, that the lightning first took the weathercock, and was conducted, without injuring the metal or any thing else, as low as where the large iron bar or spindle, which is inserted into the top of the steeple, and comes down several feet of its length, ter.. minates. There the metallic communication ceasing, part of the lightning exploded, cracked and shattered the obelisk, which ter

The diameter of a piece of Mr. Kinnersley's wire, which I received from Dr. Franklin, was one part in 182 of an inch. Artificial lightning from a case of thirty-five bottles, I find will entirely destroy brass wire of one part in 330 of an inch. At the time of the stroke, a great number of sparks, like those from flint and steel, fly upwards and laterally, from the place where the wire was laid, and lose their light in the day-time at the distance of about two or three inches. After the explosion, a mark appears on the table the whole length of the wire; and some very small round particles of brass may be discovered, by a magnifier, near the mark; but no part of the wire itself can be found.-J. CANTON.

minates the spire of the steeple, in its whole diameter, and threw off at this place several large pieces of Portland stone, of which this steeple is built. Here it likewise removed a stone from its place, but not far enough to be thrown down. From hence the lightning seems to have rushed upon two horizontal iron bars, which are placed within the building, cross each other, to give additional strength to the obelisk almost at its base, and not much above the upper story: here, on the north east and east side, it exploded again at the end of the iron bar, and threw off a considerable quantity of stone. And here, for the sake of explanation, I must observe, that the spire of the steeple, where it rises above the bell tower, is composed of four stories, besides the obelisk placed over them. The lowest and second are of the Tuscan order; the third is Ionic; and the fourth or uppermost composite or Roman. The stone piers of these stories are connected together and strengthened by iron bars placed horizontally near the height of the capitals of the pilasters, and each story has only one set of these bars. From the cross bars near the base of the obelisk just mentioned, the lightning broke through the roof above the composite story, at the ends of another set of iron bars placed lower than the former, from which it tore out a large portion of the stone. It then struck the iron bars of this story, which are placed immediately under, and in contact with the stones, broke one of the iron bars directly across, and bent the larger part of it from its horizontal direction to near an angle of 45°. Its rapid progress being here in some measure prevented, at the end of one of the iron bars, it threw off the upper part of one of the composite pillars just above its capital, and a large portion of the cornice projecting over it, and that with such a force, that part of a stone which was placed here and formed a portion of the cornice, and weighed seventy-two pounds, was projected, not only the whole length of the body of the church, but beyond it, across St. Bride's-lane; where it fell on the top of a house, and broke through the roof, and lodged in the garret. The horizontal distance from the steeple to the place where it fell, was at least 150 feet; and the height from which it fell somewhat more that 200. This piece of stone was of a very irregular figure, and must have required an amazing force to rend it, detach it from the building, and throw it to such a distance. The shaft of the pillar, the next to the east of that whose upper part had suffered so much, was likewise violently struck; and a large portion of its diameter broken out and thrown down. The Ionic story has suffered considerably, more particularly the pilaster fronting the north-east, and placed directly under the composite column, whose top was thrown off. This pilaster is much injured, but the story in general has suffered less than the composite, and that chiefly where the irons are inserted; the upper Tuscan less than that, and the lower Tuscan but little, except in the north-east pier, which is considerably cracked and shaken; as if in its passage part of the force of the lightning was spent in these explosions, and part ab