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by the nature of the circle; therefore D H X BH = HK2; and confequently H K is a common tangent to the circles K E F, and D K B L, which paffes through the points D, K, B.

Philofophical Tranfactions, &c. Vol. LIII. Continued from Page 205.

Papers in EXPERIMENTAL PHILOSOPHY.

Art. 22. New Experiments in Electricity, By Mr. Ebenezer Kinnerfcy.

THE

HESE experiments were made in Philadelphia, and communicated in a letter to Dr. Franklin. Some of them are well imagined, and others puerile enough. Those which Mr. Kinnerfley effected by means of his newly-contrived electrical air-thermometer, are curious and interefting. The inArument itself alfo, is an ingenious contrivance for estimating the force of electrical explofions; but, as we cannot give the Reader a defcription of it without the plate, we shall content ourselves with the following fpecimens of this Writer's experiments and reafoning.

Dr. Franklin, it feems, before his laft voyage to England, had fuggefted to Mr. Kinnerfley, that it would be worth trying, whether, by hanging a weight to the end of a piece of brais wire, and fending a great charge of electrical fire through it, the cohesion of the conftituent 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 fuggefted, but the wire abfolutely became red-hot; and on a fecond charge was fairly melted; a circumftance, of which, Mr. Kinnerfley fays, neither he nor the Doctor had entertained the leaft fufpicion.

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

As for example, I formed across of two pieces of wood of equal lengths, interfecting each other at sight angles in the middle; hung it, horizontally, on a central pin, and fet a light ho fe, with his rider, upon each extremity; whereupon, the whole being nicely balanced, and each courfer urged on by an electrifed point, instead of a pair of spurs, I was entertained with an electrical horfe-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, alfo 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 metal by a cold fufion, as was formerly fuppofed *; but that when it paffes through the blade of a fword, if the quantity be not very great, it may heat the point fo as to melt it, while the broadeft and thickeft part may not be fenfibly warmer than before. To this oblervation Mr. Kinnerfley adds several pertinent reflections on the effects of thunder-ftorms; with the means of prefervation against fuch awful and deftructive phenomena. The recent inftances we have had of these effects, and particularly on St. Bride's church in Fleet-ftreet, may render the following relation acceptable to fuch perfons as are defirous of taking thofe means of prevention, which have now for.fome years been found, from experience, highly efficacious on the continent of North-America.

As the fact related ferves to corroborate the above experiment of Mr. Kinnerfley's, we shall give the whole in his own words, extracted from his letter to Dr. Franklin.

We had four houfes in this city, and a veffel at one of the wharfs, ftruck, and damaged, by lightning laft fummer. One of the houfes was ftruck twice in the fame form. But I have the pleasure to inform you, that your method of preventing fuch terrible difafters, 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, fufpected that the lightning, in one of the thunder storms laft fummer, had paffed through the iron condu&tor, which he had provided for the fecurity of his houfe, I waited on him, to enquire what ground he might have for fuch fufpicion. Mr. Weft informed me, that his family and neighbours were all stunned with a very terrible explofion, and that the fath and crack were feen and heard at the fame inftant: whence he concluded, that the lightning muft have been very near; and as no houfe in the neighbourhood had fuffered by it, that it must have pafled through his conductor, Mr. White, his Clerk, told me, that he was fitting at the time by a window, about two feet from the conductor, leaning against the brick wall with

* Mr. Kinnerley, however, is not the first who hath drawn this con clufion of the effects of lightning. Dr. Knight hath made the fame reflection, in a paper inferted in the Philofophical Tranfa&tions, vol. LI. Part I.See Monthly Review, vol. XXIII. page 192.

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which it was in contact; and that he felt a fmart fenfation, like an electrical fhock, in that part of his body which touched the wall. Mr. Weft farther informed me, that a perfon of undoubted veracity affured him, that, being in the door of an oppofite houfe on the other fide of Water-ftreet, (which you know is but narrow) he faw the lightning diffufed 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 perfon, of very good credit, told him, that he, being a few doors off, on the other fide of the ftreet, faw the lightning above, darting in fuch direction, that it appeared to him to be directly over that pointed rod.

Upon receiving this information, and being defirous of farther fatisfaction, 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. Weft, our going to the top of the house, to examine the pointed rod; alluring him, that if the lightning had paffed through it, the point mult have been melted; and, to our great fatisfaction, we found it fo. This iron rod extended in height about nine feet and a half above a ftack of chimnies to which it was fixed; (but I fuppofe, three or four feet would have been fufficient). It was fomewhat 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, confifted of fquare 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 houfe, four ftory and a half, to the pavement in Water-ftreet; being faftned to the wall, in feveral places, by small iron hooks. The lower end was fixed to a ring in the top of an iron ftake, 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 inferted a brafs wire, about two lines thick, and when firft 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 miffing; the fendereft part of the wire being, as I fufpect, confumed into fmoke. But fome of it, where the wire was a little thicker, being only melted by the lightning, funk down while in a fluid ftate, and formed a rough irregular cap, lower on one fide than the other, round the upper end of what remained, and became intimately united therewith.

"

This was all the damage that Mr. Weft fuftained by a terrible ftroke of lightning: A moft 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, concerning an Obfervation made by him in Optics.

This letter being fhort, and the obfervation contained in it fomewhat fingular, we fhall give it the Reader verbatim.

I having lately accidentally difcovered, that the shadows of things floating in water, a little below its furface, are reflected from the air above the water, more ftrongly (to my apprehenfion) than objects above the furface of the water are reflected from the water; and, confequently, that fishes playing beneath the furface of a ftill water, may fee their images diftinctly 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 thadowed fides reflected from it, which renders the images obfcure. On the contrary, the inhabitants of the waters have almoft a hemifphere of light falling on their upper fides, which are the fides that are reflected from the air, which confequently renders fuch images lighter, and more ftriking to the eye, than reflections of obfcured things in air, when reflected from the waAs I have never heard of, or read, any account of this discovery, I imagine it may be new but you, Sir, in far more extenfive reading, may be acquainted with fuch a discovery. If fo, I acknowlege my ignorance of it; and afk pardon for giving you this trouble, and defire it may be laid afide; but, if it be thought worthy communicating to the Royal Society, I will be ready, in a very fimple and easy manner, to demonftrate the truth of the above difcovery. I do not fee any use of this discovery at prefent, more than an amufing fpeculation; tho', perhaps, when it is reconfidered by perfons fuperior to me in penetrating into the fecrets of Optics, fome real ufe may be made of it.'

ter.

Art. 51. Ratio conficiendi Nitrum in Podilia. Auctore . Wolf, M. D.

This paper contains an account of the method of making Salt petre in Poland; the procefs of which is particularly defcribed.

Art. 54. A Letter from Mr. B. Wilfon to Mr. Apinus of Petersburg.

We have been fometimes unjustly cenfured for fpeaking flightly of the abilities and labours of our modern Philofophers, as if we intended, in checking the prefumption of fome forward and ignorant Experimentalifts, to depreciate the merit or utility

of

of experiments themfelves. Nothing, however, can be farther from the intention of the Monthly Reviewers; who are not quite fuch Novices in Phyfics, as not to know the importance of the practical, to the theoretical, part of fcience. It must be owned, nevertheless, that it is with a mixture of contempt and indignation, they are fo often obliged to attend to the futile reafonings, and inconclufive arguments, of mere Experimentmongers. It is, indeed, pleasant enough, to fee the epithets learned, fcientific, ingenious, &c. reciprocally beftowed on each other, by the feveral Academicians of Europe; when it is notorious, that a confiderable number of them can boaft no better title to the appellation of Philofophers, than the merit of having made a few practical obfervations in Aftronomy, Micography, Meteorology, or Electricity.

The cafe however is very different with the truly ingenious Author of the letter before us, whofe 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 fubject of this his letter to Mr. Apinus, it is extremely curious, relating to the very extraordinary phenomena obfervable in the Tourmalin; but it will not admit of our making any abftract of it, fatisfactory to our Readers.

MECHANICS.

Art. 25. The Properties of the Mechanic Power's demonftrated, with fome Obfervations on the Methods that have been commonly used for that Purpofe. By Dr. Hugh Hamilton, Fellow of Trinity College, Dublin.

In regard to propofitions, whofe truth is daily obvious to our fenfes, it is feldom that we give ourselves the trouble to investigate a strict demonftration of them. This appears to have been the cafe, with refpect to the theory of Mechanic Powers, and particularly to that of an Equilibrium. In treating of the theory of fciences, however, which are ftrictly mathematical, the ftrictest demonftration is requifite. Dr. Hamilton, therefore, hath, with a good deal of propriety, taken a review, in this article, of the feveral methods in which former Writers have deduced the practical principles of mechanics from the laws of motion; methods which, he obferves, being very different, it may reasonably be fuppofed, that no one hath been looked upon. as fatisfactory and unexceptionable. The moft general and remarkable theorem in mechanics, fays the Doctor, certainly is this,

That when two weights, by means of a machine, counterpoife 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 fuch a refemblance

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