BURNING-GLASS. 



accident? j however, out of twenty-four mirrois treated thus, 

 M. U'lir.ii pituTved three; two of which arc thirty fcven 

 inches in diameter, and the other forty-lls inches. The la!l 

 was tiii-foikd, and prefcnttd to the French king ; and is 

 reprcfented bv tlie maker, as tlic mod powerful buriimg 

 mirror in Europe, and yet the moon's hglit refleacd from 

 this concave fpcciiUim, on a thermometer placed in the focus, 

 produced no fenfible dilatation of the included fluid. M. 

 Buffon has alio made attempts for conarufting convex lenfes 

 of great p»wer, by fixing two of the above mentioned concave 

 glalfcs together, and liUing the cavity between them with 

 water : one of the glaflcs broke in the trial ; but others have 

 fincc undertaken to conftruft thefe water knfes. He has 

 likcwife fuggefted another improvement in the conftruflion 

 of burning' glalTes, confiding in a contrivance for diminifhnig 

 the thicknefs of the middlcor central part of the lens from 

 three inches to one inch ; whereby be propofes to prevent 

 the diminution of light occafioned by its paffing through 

 the middle part of a lens of large diameter, and fhoit focus, 

 wh c'l mull be proportionably thick in that part. This new 

 kind of lens is compofed of three parts ground to the fame 

 radius, or of two circular zones, or ba:ids, furrounding a 

 central or middle part. He computes that the heat in the 

 focus of a lens of this kind, three feet in diameter, will be 

 about four times greater than that produced by any burning 

 glafi yet known. 



But the moll confiderable improvement which M. Buf- 

 fon lias introduced into the optical apparatus is his ma- 

 chine, confilling of a number of mirrors ; whereby he feems 

 to have revived ihe fecret of Archimedes, and to have 

 vindicated the credit of hiftory in this point. In the year 

 1747 he condructcd a machine, with a hundred and forty 

 plane mirrors, each about four inches long, and three 

 broad ; thefe are fixed at about a quarter of an inch diftance 

 frqm each other, upon a large wooden frame of about fix 

 feet fquare. Each of them has three moveable fcrews, 

 which are fo contrived, that the mirror can be inclined 

 to any angle, and in any direftion that meets the fun ; 

 and by this means, the folar image of each mirror is made 

 to coincide with all the reft. The experiment was firft 

 tried with twenty-four mirrors, which readily fet on fire 

 a combullible matter, prepared of pitch and tow, and 

 laid on a deal board, at the diftance of fixty-fix French feet. 

 He then farther purfued the attempt, and put together a 

 kind of polyhedron, confifting of 168 pieces of plane look- 

 ing glafs, each being fix inches fquare ; and by means of 

 this fomc boards of beech-wood were fet on fire, at the 

 diftance of a hundred and fifty feet, in the month of March, 

 and a filver plate was melted at the diftance of fixty feet. 

 This machine, befide other advantages, may be eafily moved, 

 fo as to burn downwards or horizontally ; and it burns 

 either in its diftant focus, or in any nearer interval, which 

 our common burning-glafTes cannot do, becaufe their focus 

 is wholly fixed and determined. This machine, in the next 

 ftage of its improvement, contained three hundred and fixty 

 plane mirrors, each eight inches long, and fix broad, 

 mounted on a frame eight feet high, and feven feet broad. 

 With twelve of thefe mirrors, liglit combuftible matters 

 were kindled at the diftance of twenty feet ; at the fame 

 diftance, A large tin veficl was melted with forty-five of 

 them, and a thin piece of filver with a hundred and fe- 

 venteen. When the whole machine was employed, all 

 the metals and metallic minerals were melted at the dif- 

 tance of twenty-five, and even of forty feet. Wood was 

 kindled in a clear fliy, at the diftance of two hundred 

 and ten feet. The focus, at the diftance of fifty feet, 

 is about fevea inches broad ; and at the diftance of two 



3 



hundred and forty feet, it b'comes t>vo feet in diimeter. 

 Mr. Buffon afterivards con.ftruflcd a machine, which con- 

 tained four hundred mirrois, each half a foot fquare, 

 with which he couid melt lead and tin at the diftance of a 

 hundred a d tortv f.-et. Phil. Tranf. abr. vol. x. p. 194. Sec. 

 M. de B:!:Ton H'ftoire Na'urelle, &c. Supplem. vol. i. 

 Montucla, Hilloire'dts M'.thcmaliques, vol. i. p. 333. 



There is no folid lubilance that refifts the efficacy of mo- 

 dern burning-glafTes. Mclfrs. Macquer and Ban-rc have 

 fucceeded in melting fmall portions of vlatin^a, by means 

 of a concave glafs, twenty two inches in diameter, and of 

 twenty-eight incites focus ; though this metal is not fufible 

 by the ftrongeft fires that can be excited in furnaces, or 

 fuftained by any chemical apparatus. Yet it was long ago 

 obferved by the Acad.'mi!;ians del Cimento, tliat fpirit of 

 wine could not be fired by any burning glafs whicii they 

 ufed ; and notwithllanding the great improvement which 

 thefe optical inftruments have fince received, M. Nolltt has 

 not been able, by the moft powerful burning mirrors, to fet 

 fire to any inflammable liquors whatever. 



The principle upon which M. Trudaine conftrufted his 

 burning lens was that of combining two circular fegments of 

 glafs fpheres, and applying them with their concave fides to- 

 wards each other, fo a? to form a lenticular Cavity of four 

 feet in diameter. The cavity was filled with fplrits of wine ; 

 of which it contained 140 French pints. The focus of pa- 

 rallel rays in this inftrument was 10 feet 10 inches and one 

 line dillant from the centre of the glafs ; and origimllv con- 

 tradled to a circle of about eight lines in diameter, by the in- 

 terpofition of a fecond and fmaller lens of folid glafs. The 

 maker prefented it to the Royal Academy of Sciences, 

 but it vi'as foon afterwards accidentally broken. The ex- 

 peuce of its conftruclion is faid to have amounted to loool. 

 fterling ; and yet its powers were not very great ; for, ac- 

 cording to Mr. Magellan, it required 20 minutes for coagu- 

 lating the particles of platina, which Mr. Parker's lens 

 could do in lefs than a tenth piirt of the time. This feems 

 indeed to be the moft powerful burning lens that has yet 

 been conftrniled. Mr. Parker, of Fleet-ftreet, London, 

 was induced, at an expence of upwards of 700!., to contrive 

 and at length to complete a large tranfparent lens, that 

 would ferve the purpofe of fufing and vitrifying fuch 

 fubftances as refift the fires of ordinary furnaces, and more 

 efpccially of applying heat in vacuo, and in other circum- 

 ftances in which it cannot be applied by any other means. 

 After direfting his attention for feveral years to this objeft, 

 and performing a great variety of experiments "in'^he profe- 

 cution of it, he at laft fucceeded in the couftruftion of a 

 lens, of flint glafs, three feet in diameter, which, when 

 fixed in its frame, expofes a furface two feet Si inches in the 

 clear ; without any other material imperfection befides a 

 disfigurement of one of the edges by a piece of the fcoria of 

 the mould, which unfortunately found its way into its 

 fubftance. This lens was double convex, both fides of 

 which were a portion of a fphere of 18 feet radius. From 

 the papers and drawings, with which the ingenious inventor 

 has obligingly furnilhed us, we are able to give the follow- 

 ing accurate account of this lens, illuftrated'by appropriate 

 engravings. The elevation of this lens is exhibited in P/ate 

 II. Optics, fg. I. The large lens is A, whofe diameter in the 

 frame is 3 feet, and in the clear 2 feet 8| inches; its 

 thicknefs at the centre is 3^ inches ; its focal diftance-e feet 

 S inches ; and the diameter of the burning focus I inch ; 

 its weight was 212 pounds. B is a fecond lens, whofe 

 diameter in the frame is 16 inches, and which fliews in the 

 clear 13 inches ; the thicknefs in the centre is l| inch ; 

 length of tlie focus 29 inches, and its diameter | of an inch ; 



its 



