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by the nature of the circle; therefore D H X BH = H K?; and consequently H K is a common tangent to the circles K E F, and DK BL, which passes through the points D, K, B.

B.

Philosophical Transactions, &c. Vol. LIII. Continued from

Page 205.

T

Papers in EXPERIMENTAL PHILOSOPHY.
Art. 22. New Experiments in Electricity. By Mr. Ebenezer

Kinnersley.
HESE experiments were made in Philadelphia, and

a are well imagined, and others puerile* enough. Those which Mr. Kinnersley effected by means of his newly-contrived electrical air-thermometer, are curious and interesting. The instrument itself also, is an ingenious contrivance for estimating the force of electrical explosions; but, as we cannot give the Reader a description of it without the plate, we shall content ourselves with the following specimens of this Writer's experiments and reasoning.

Dr. Franklin, it seems, before his last voyage to England, had suggested to Mr. Kinnersley, that it would be worth trying, whether, by hanging a weight to the end of a piece of bra's wire, and sending a great charge of electrical fire through it, the cohesion of the constituent particles of the wire might not so far relax, as that the weight would cause a separation. This experiment being tried, not only turned out as Dr. Franklin suggested, but the wire absolutely became red-hot; and on a second charge was fairly melted; a circumstance, of which, Mr. Kinnersey says, neither he nor the Doctor had entertained the least fufpicion.

That he might not be mistaken also, in the wire's being actually hot as well as red, he repeated the fame experiment on another piece of the same wire encompassed with a goose-quill filled with loose grains of gun-powder; which took fire as readily as

As for examplı, • I formed a cross of two pieces of wood of equal lengths, intersecting each other at right angles in the middle; hung it, horizontally, on a central pin, and set a light horse, with his rider, upon each extremity; whereupon, the whole being nicely balanced, and each courser urged on by an electrised point, initead of a pair of spurs, I was entertained with an electrical horse-race.'- A mighty pretty entertainment for a Philofopher truly !

if it had been touched with a red-hot poker. Tinder, tied to another piece of wire, also kindled by it: tho' from a wire twice as big, no fuch effects could be produced.

Hence this Writer concludes, that lightning does not melt *mictal by a cold fusion, as was formerly supposed * ; but that when it passes through the blade of a sword, if the quantity be not very great, it may heat the point so as to melt it, while the broadent and thickelt part may not be sensibly warmer than beforę. To this ob ervation Mr. Kinnerlley adds several pertinent reflections on the effects of thunder-storms; with the means of preservation against such awful and destructive phenomena. The recent inttances we have had of these effects, and particularly on St. Bride's church in Fleet-strect, may render the following relation acceptable to such perfons as are desirous of taking those means of prevention, which have now for some years been found, from experience, highly efficacious on the continent of North-America. · As the fact related serves to corroborate the above experiment of Mr. Kinnersey's, we shall give the whole in his own words, extracted from his letter to Dr. Franklin.

- We had four houses in this city, and a vessel at one of the wharfs, struck; and damaged, by lightning last summer. One of the houses was struck twice in the same storm. But I have the pleasure to inform you, that your method of preventing such terrible ditafters, has, by a fact, which had like to have escaped our knowlege, given a very convincing proof of its great utility, and is now in higher repute with us than ever,

Hearing a few days ago, that Mr. William West, Merchant in this city, suspected that the lightning, in one of the thunder storms laft fummer, had passed through the iron conductor, which he had provided for the fecurity of his house, I waited on bim, to enquire what ground he might have for such fufpicion. Mr. Welt informed me, that his family and neighbouis were all stunned with a very terrible explosion, and that the Aath and crack were seen and heard at the same instant: whence he concluded, that the lightning must have been very near; and -as no house in the ncighbourhood had suffered by it, that it must have palled through his conductor. Mr. White, his Clerk, told me, that he was fitting at the time by a window, about two tuet fiom the conductor, leaning against the brick wall with

Mr. Kinnersley, however, is not the first who hath drawn this conclufion of the erects of lightning. Dr. Knight haih made the fame reflection, in a paper inserted in the Philosophical Transactions, vol. LI, part I.--Se: Monthly Review, vol. XXIII. page 102.

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which it was in contact; and that he felt a smart fenfation, like an electrical shock, in that part of his body which touched the wall. Mr. Weft farther informed me, that a person of undoubted veracity assured him, that, being in the door of an opposite house on the other side of Water-street, (which you know is but narrow) he saw the lightning diffused over the pavement, which was then very wet with rain, to the distance of two or three yards from the foot of the conductor. And that another person, of very good credit, told him, that he, being a few doors off, on the other fide of the street, saw the lightning above, darting in such direction, that it appeared to him to be directly over that pointed rod.

• Upon receiving this information, and being desirous of farther satisfaction, there being no traces of the lightning to be discovered in the conductor, as far as we could examine it below, I proposed to Mr. Welt, our going to the top of the house, to examine the pointed rod; assuring him, that if the lightning had passed through it, the point must have been melted; and, to our great satisfaction, we found it so. This iron rod extend ed in height about nine feet and a half above a stack of chimnies to which it was fixed; (but I suppose, three or four feet would have been sufficient). It was somewhat more than half an inch diameter in the thickest part, and tapering to the upper end. The conductor, from the lower end of it to the earth, consisted of square iron nail-rods, not much above a quarter of an inch thick, connected together by interlinking joints. It extended down the cedar roof to the eaves, and from thence down the wall of the house, four story and a half, to the pavement in Water-street; being fastned to the wall, in several places, by small iron hooks. The lower end was fixed to a ring in the top of an iron stake, that was driven about four or five feet into the ground. The above-mentioned iron rod had a hole in the top of it, about two inches deep, wherein was inserted a brass wire, about two lines thick, and when first put there, about ten inches long, terminating in a very acute point; but now its whole length was no more than seven inches and a half, and the top very blunt. Some of the metal appears to be misling; the flenderest part of the wire being, as I suspect, consumed into smoke. But some of it, where the wire was a little thicker, being only melted by the lightning, funk down while in a fluid fate, and formed a rough irregular cap, lower on one side than the other, round the upper end of what remained, and became intimately united therewith.

This was all the damage that Mr. West sustained by a tersible stroke of lightning : A most convincing proof of the great utility of this method of preventing its dreadful effects. Surely

it will now be thought as expedient to provide conductors for the lightning as for the rain !! Art. 35. A Letter from Mr. George Edwards to Dr. Birch, con

cerning an Observation made by bim in Optics. This letter being short, and the observation contained in it somewhat singular, we shall give it the Reader verbatim.

I having lately accidentally discovered, that the shadows of things floating in water, a little below its surface, are reflected from the air above the water, more strongly (to my apprehenfion) than objects above the surface of the water are reflected from the water; and, consequently, that fishes playing beneath the surface of a still water, may see their images distinctly playing in the air, with this advantage over men, who view their faces in the water; for things in air, that are reflected from the water, must have, when placed over the water, their dark or shadowed sides reflected from it, which renders the images obfcure. On the contrary, the inhabitants of the waters have almost a hemisphere of light falling on their upper fides, which are the sides that are refcêted from the air, which consequently renders such images lighter, and more striking to the eye, than reflections of obscured things in air, when reflected from the water. As I have never heard of, or read, any account of this discovery, I imagine it may be new : but you, Sir, in far more extensive reading, may béacquainted with such a discovery. If so, I acknowlege my ignorance of it; and ask pardon for giving you this trouble, and desire it may be laid aside ; but, if it be thought worthy communicating to the Royal Society, I will be ready, in a very simple and easy manner, to demonstrate the truth of the above discovery. I do not see any use of this discovery at present, more than an amusing speculation ; tho', perhaps, when it is reconsidered by persons superior to me in penetrating into the secrets of Optics, some real use may be made of it.' Art. 51, Ratio confciendi Nitrum in Podelia. Auctore

Wolf, M. D. This paper contains an account of the method of making Salt petre in Poland; the process of which is particularly defcribed. Art. 54: A Letter from Mr. B. Wilson to Mr. Æpines of

Petersburg. We have been sometimes unjustly censured for speaking flightly of the abilities and labours of our modern Philosophers, as if we intended, in checking the presumption of some forward and ignorant Experimentalists, to depreciate the merit or utility

of

of experiments themselves. Nothing, however, can be farther from the intention of the Monthly Reviewers; who are not quite such Novices in Physics, as not to know the importance of the practical, to the theoretical, part of science. It must be owned, nevertheless, that it is with a mixture of contempt and indignation, they are so often obliged to attend to the futile reasonings, and inconclusive arguments, of mere Experimentmongers. It is, indeed, pleafant enough, to see the epithets learned, scientific, ingenious, &c. reciprocally bestowed on each other, by the several Academicians of Europe ; when it is notorious, that a considerable number of them can boast no better title to the appellation of Philosophers, than the merit of having made a few practical observations in Astronomy, Micography, Meteorology, or Electricity.

The case however is very different with the truly ingenious Author of the letter before us, whose penetration and fagacity in deducing rational conclufions from his experiments, is to be equalled only by his ingenuity and accuracy in making them. As to the subject of this his letter to Mr. Æpinus, it is extremely curious, relating to the very extraordinary phenomena observable in the Tourmalin ; but it will not admit of our making any abstract of it, fatisfactory to our Readers.

MECHANICS. Art. 25. The Properties of the Mechanic Powers demonstrated, with

fome Observations on the Methods that have been commonly used for that Purpose. By Dr. Hugh Hamilton, Fellow of Trinity College, Dublin.

In regard to propositions, whose truth is daily obvious to our senses, it is seldom that we give ourselves the trouble to investigate a strict demonstration of them. This appears to have been the case, with respect to the theory of Mechanic Powers, and particularly to that of an Equilibrium. In treating of the theory of sciences, however, which are strictly mathematical, the strictest demonstration is requisite. Dr. Hamilton, therefore, hath, with a good deal of propriety, taken a review, in this article, of the several methods in which former Writers have deduced the practical principles of mechanics from the laws of motion ; methods which, he observes, being very different, it may reasonably be supposed, that no one hath been looked upon as fatisfactory and unexceptiorable. The most general and remarkable theorem in mechanics, says the Doctor, certainly is this, That when two weights, by means of a machine, counterpoise each other, and are then made to move together, their quantities of motion will be equal.” Now an equilibrium always accompanying this equality of motions, bears such a re

semblance

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