BURNING-GLASS. 



r.Av? reflected from tticm to tlic fame place, at thediftance of 

 Tiuire than a hundred feet ; and hy their means he produced 

 1mc1> a dei^ne (if heat, as led him to conclude that, by in- 

 crcalr.ig their number, he could have fct fire to iiifianimable 

 iiiblbnces at a greater diUance. He likewife made a voy- 

 aTo to Syracufc, in company with his pupil Scliottus, in order 

 10 examine the place of the fuppofed tranfaftion ; and they 

 ■were both of opinion that the galleys of Maretllus could not 

 b;ive been more than thirty paces from Archmiedes. M. 

 BiifTon, thovigh ignorant of the particular ttdimonits of 

 sueient writers, rel.itive to the invention of Archimrdcs, and 

 (jf the attempts of Kirchor above mentioned, has more lately, 

 liv a fimilai- contrivance, fufficiently eviixed the praiticability 

 ot the operation. 



Dr. Wolfe, in the vear 1768, after having given an ac- 

 count of fome parabolic mirrors, conftiufted by M. Hoe- 

 fen of Drefden, offers a conjefture, that thofc of Archi:ncde9 

 might be of this kind, fince it is not difficult to dticribe in 

 parabola, whofe parair.eter is 230o feet ; and that rays re- 

 flcftcd from fuch a fpecuhim might be received by a lens, 

 after haviiig been brought to a focus, and tranlmitted pa- 

 rallel to any diftance : but he was not apprized, that Kepler 

 and Decluiles had fhewn, that no rays could be conveyed 

 parallel to one another, except thofe which procc'dc'd from 

 the fame points in the fun's dilk. Dutens du Miroir Ardent 

 d'Archimede. Paris, 175J. Pliil. Tranf. vol. xlviii. p. 621, 

 &c. Ibid. vol. lix, p. 8. 



Am.ong the moderns, the mofl remarkable bunn'ng .mirrors 

 are thofe of Magine, twenty inches in diameter ; of Septala 

 of Milan, which was near three feet and a half in diameter, 

 and which burnt at the diftance of fifteen or lixteen pacta ; 

 of Villette, and Tfchirnhaufen, the new complex one of 

 M. Butfon, that of Trudaine, and that ot Parker. 



Villette, a French artift at Lyons, made a large mirror, 

 which was bought by Tavtrnier, and prefented to the king 

 of Pcrlia; a fccond, bought by the king of Denmark ; a 

 third, prefented by the French king to the Royal Academy ; 

 a fourth has been in England, where it was publicly ex- 

 pofed. — The effeds of this mirror, as found by Dr. Harris 

 and Dr. Defaguliers, are, that a filver fixpence is melted in 

 j" and I; a halfpenny ot king George I. in 16", and running 

 with a hole in 34". Tin melts in 3", call iron in 16"; ilate 

 in 3''; a foffile ibcU calcines in 7"; a piece of Pompey's 

 Pillar in Alexandria vitrifies in the bhick part in 50", in the 

 white in 154" ; copper ore in S"; bone calcines in 4", and 

 vitrifies iu 33". An emerald melts into a fubftance like a 

 turquois Itoiie ; a diamond weighnig four grains, lofes 5 of 

 its weight ; the albedos vitrihes ; as all other bodies will do, 

 if kept long enough in ihe/ucus; but when once vitrified, 

 the mirror can produce no farther t ffeft. — This mirror is 

 forty-feven inches wide; and is ground to a iphere of feventy- 

 fix inches radius ; fothat its focus is about thirty-eight inches 

 from the vertfx. Its fubftance is a compofition of tin, cop- 

 per, and tin-glafs. Phil. Tranf. vol iv. p. 198. 



M. Tfchirnhaufen's rcfleciling mirror deferves next to be 

 mentioned. The following things are noted of it in the 

 Afta Eruditorum, for i^sj, p. r^2. I. Green wood takes 

 fire inftantaneoufly, fo that a ilrong wind cannot extinguifli 

 it. 2. Water boils immediately, and ecfgs in it are prefcntly 

 edible. 3. A mixture of tin and lead, three inches thick, 

 drops prciently, and iron or Heel-plate becomes red hot pre- 

 fently, and a little after burns into holes. 4. Things not 

 capable of melting, as ftones, bricks, 6<.c. become foon red 

 hot, Uke iron. 5. Slate becomes fiill white, then a black 

 glafs. 6. Tiles arc converted into a yellow glafs, and fhells 

 into a blackifh yellow one. 7. A pumice-ftone emitted from 

 a volcauo, melts into white glafs ; and, 8. A piece of crucible 



alfo vitrifies in eight luiniilci. 9. lioiies arctoori turned into 

 an opaque glafs, and earth into a black one. It is made of 

 copper, and its fubftance is not above double the thickntfs of 

 the back of a knife : this was about 4;'; French feet in dia- 

 meter, and it burnt at the diftance of twelve fctt. 



Every lens, whether convex, plano-convex, or convexo- 

 convex, colleds the fun's rays, difpcrfed over its convexity, 

 into a point by rcfraclion ; and is therefore a burning glals. 

 The moll confiderable of this kind known, is that made by 

 M. de Tfchirnlnufcn : the diameters of his Icnfes are three 

 and four feet ; the focus at the diftance of twelve feet, and 

 its diameter an inch and a half. To make the focus morr 

 vivid, the rays are coU.-Aed a fecond tim.e by a fecond len* 

 pnrallel to the fird ; and fituited in th.at place where the 

 diameter of the cone of rays formed by the tirll lens is cqnal 

 to tli;: diameter of the fecond ; fo that it receives thtm all ; 

 and the focus from an inch and a half, is contradled into tlv: 

 fpace of eight lines, and its force increafed prouortionably. 

 ',•- was purchafed by the duke of Orleans, who prefented it 

 to the French Academy. Its weight was 160 pounds. 



Its efferts, among others, as related in the Aiila Erudil. 

 LipliiE, are, that it lights hard woods, even moillcned with 

 water, into a (lame, iiiftantly ; that water, in a iittlc vcdel, 

 begins to boil prefently ; all metals are melted ; brick, 

 puniice-ftone, delft warts, and the ad^eftos done, are turned 

 into glafs; fulphur, pitch. Sec. melted under water: the 

 afhes of vegetables, woods, and other matters, tranfmuted 

 into glafs ; in a word, every thing applied to its focus, is 

 either melted, turned into calx, or into fmoke ; and the 

 colours of jewels, and all other bodies, metals alone excepted, 

 are changed by it. He obferves, that it fucceeds bed, 

 whfin the matter applied is laid on a hard charcoal, well burnt. 

 Though the force of the folar rays be here found fo 

 Ihipendous, yet the rays of the full moon, coUefted by the 

 fame burning glafs, do not exhibit the lead increafe of heat. 

 Wolfius tells us, that an artid of Drefden made burning 

 mirrors of wood, bigger than thofe of M. Tfchirnhaufen or 

 Villette, which had effefts at lead equal to any of them. — 

 Traberus teaches how to make burning mirrors of leaf gold, 

 viz. by turning a concave, laying its infide equally with pitch, 

 and covering that with fquare pieces of the gold, two or 

 three fingers broad, faftening them on, if need be, by fire. 

 He adds, that very large mirrors may be made, of thirty, 

 forty, or more concave pieces, artfully joined in a turned 

 wooden dilh or fcuttle ; the edefls of Vv'hich will not be 



much lefa than if the fnrface was continuous Zahnius adds 



farther, that Newman, an engineer at Vienna, in 1699, made 

 a mirror of pafte-board, covered withinfide with draw glued 

 to it ; by which all kinds of metals, &c. were readily melted. 

 Sir Ifaac Newton's burning miiTor confided of feven con- 

 (55ve glades, each of which was eleven inches and a half in 

 diameter, and fo ditpofed as to have one common focus: fis 

 of them were placed round the fev.-nth and contiguous to it, 

 in fuch a manner as to form the fegment of a fphere, whofe 

 fubtenfe is about thirty-four inches and a half ; the focu» 

 is about twenty-two inches and a half diftant, and about aii 

 inch in diameter. This fpecuhun vitrified biick or tile m 

 one fecond, and in about lialf a minute melted gold. M. 

 Zeiher, not long fince, made fome improvement of this in. 

 drument ; and formed panes of plain glafs into the neceifarj' 

 degree of curvature, by heating them fo hot on a di(h made 

 of metal, that they could all adume the fame form. M, 

 Buffon has conftruftcd furnaces lor converting plain into 

 concave mirrors ; the mirrors are expofed to a degree of 

 heat fufficicnt to foften the glafs, in coiifequence of which it 

 conforms itfelf to the fpherical figure of the mould on which 

 it is placed. This method is fubjcfl to many diiScultiea and 

 4 C I aw!. 



