134 



BURNING INSTRUMENTS. 



Traberus' 



burning 



mirrors. 



Neuman's 



burning 



mirror. 



Hoeseifs 

 and Eh- 

 rard's inir- 



when held there a little time is in a perfect Jluor, so 

 that in 2 or 3 minutes it is quite pierced through. 



4. A plate of iron or steel becomes immediately 

 red hot, and soon after a hole is burnt through it. 



5. Copper, silver, &c. melt in five or six minutes. 



6. Stones, brick, &c. soon become red hot. 



7. Slate becomes red hot, but in a few minutes 

 turns into a fine sort of black glass. 



8. Tiles which had been exposed to the most in- 

 tense heat of fire, melt down into a yellow glass. 



9. Pot-shreds, that had been much used in the 

 fire, melt into a blackish yellow glass. 



30. Pumice stone melts into a white transparent glass. 



11. A piece of a very strong crucible melted into 

 a glass in 8 minutes. 



1 2. Bones were converted intoa kind of opaque glass, 

 and a clod of earth into a yellow or greenish glass. 



13. The beams of the full moon when at her 

 greatest altitude were concentrated by this speculum, 

 but no perceptible degree of heat was experienced. 



Zacharias Traberus mentions in his book/w Nervo 

 Optico, that burning mirrors may be constructed by 

 turning a large concave of wood, laying its surface 

 equally with pitch, and covering it with square pieces 

 of leaf gold about two or three inches broad. Large 

 mirrors, he says, may be made of 30, or 100, or more 

 concave pieces, artfully joined in a turned wooden 

 dish or scuttle. 



Zahnius, in his work In Oculo Artific. fundam. 3. 

 informs us, that one Neuman, an engineer at Vienna, 

 formed burning mirrors of pasteboard, covered on the 

 inside with straw glued to it, and that their effect was 

 so powerful as to melt metals almost instantly. 



Parabolic mirrors of a large size, and very consider- 

 able power, were constructed by M. Hoesen of Dres- 

 den, and afterwards by M. Ehrard. These mirrors 

 were composed of several pieces of solid wood, and 

 on the convex part were pieces of wood, both di- 

 verging from the vertex and transversely, nicely fit- 

 ted and strengthened. The concave part of this 

 framing was covered with copperplate -|- of an inch 

 in thickness, 4* feet long, and 2^ feet broad, so as to 

 resemble one piece, finely polished. The speculum 

 was so supported as to be easily managed, and the 

 anterior part of it was subtended by an iron arch half 

 an inch thick. The middle of this arch, which coin- 

 cided with the place of the burning focus, was per- 

 forated into a ring, which supported from both sides 

 an iron fork for receiving the body to be examined. 

 Four of Mr Ehrard's mirrors constructed in this way 

 Lad the following dimensions : 



Burning 

 Intru- 



The celebrated Wolfius, who had witnessed the ef- 

 fects of these mirrors, assures us, that in burning, 

 calcining, melting, and vitrifying, they far exceeded 

 any thing of the kind ever known. The hardest 

 stones scarcely resisted a few seconds. Metals were 



Gre- 



rapidly perforated, and vegetables and bones were 

 immediately burnt to a cinder and vitrified. 



Our celebrated countryman, Dr Jam^s Gregory, 

 turned his attention to the construction of burning 

 machines, about the year 1670 ; and in a letter to Mr 

 Collins, dated St Andrews, 7th March, 1673, he 

 states his views on this subject, and requests Mr Col- 



lins to communicate them to Sir Isaac Newton, who f 1 



. . . . - , . ' . burnin 



returns a favourable opinion ot the invention, in a m irror. 



letter to Mr Collins. The passages in these letters 

 are too interesting to be given in any other form than 

 in the original words of these distinguished authors. 



" Mr Newton's discourse of reflection," says Dr 

 Gregory, " puts me in mind of a notion I had of 

 burning glasses several years ago ; which appears to 

 me more usefull than subtile. If ther be a concave 

 speculum of glasse, the leaded convex surface having 

 the same center with the concave, or to speak pre- 

 ciselie, albeit perchance to little more purpose, let the 

 radius of the convexitie be c, the thicknes of the 

 glasse in axis transitu f, the radius of the convexitie 

 9c x 4. 18c/+5/* ,-. 



equal to o _L tif ' tnis speculum sal have 



Jc T y 



the/be/' of both the surfaces in the same point ; and not 

 onlie that, but all the rays which are reflected be- 

 twixt the two surfaces, sal, in their egresse, come, 

 quam proxime, to the common focus. The making 

 of such an speculum, requireth not much more airt 

 than an ordinar plane glasse, seing great subtiltie is 

 not necessar here: so that I believe they who mak 

 the plane miroir glasses, wold mak one of these, three 

 foot in diameter, for four or five pounds sterling, or 

 little more : for I have seen plane glasses, almost of 

 that bignes, sold even here for less money. Now 

 seing (as Mr Newton observeth) that al reflecting 

 metalls lose more than one-third of the rayes? this 

 concave glasse, even cceteris paribtis, wold have an 

 great advantage of a metall one ; for certainlie an 

 exactlie polished thin miroir- glasse, of good transparent 

 mater, after a few reflections, doeth not lose one- 

 fourth of the rayes: and, upon other accounts, this 

 hath incomparable advantages, seeing it is more por- 

 table, free from tarnishing, and, above al, hardlie 

 jiyth of the value. The great usefulness of burning 

 concaves, this being so obvious, and as yet (for quhat 

 I know) untouched by anie, makes me jealous that 

 there may be in the practice some fallacie. Ye may 

 communicate this to intelligent persons, and espe- 

 ciallie to Mr Newton ; assuring him that none hath 

 a greater veneration for him, admiring more his great 

 and subtile inventions, than his and yours. 



P. S. If'ye please, let me hear, with the first con- 

 venience, what may be judged the result of this burn- 

 ing concave ; for I am as much concerned to be un- 

 deceived, if ther be any insuperable difficultie, as to 

 be informed of an most surprising success. I have 

 spoke of it to severals here, but al wer as ignorant of 

 it as my self," &c. 



Sir Isaac Newton's reply to Mr Collins is dated 

 Cambridge, April 9th 1673, and contains the follow- 

 ing passage : 



" Thedesign of theburning speculum appearsto me 

 very plausible, and worthy of being put in practice. 

 What artists may think of it, I know not ; but the 

 greatest difficulty in the practice, that occurs to me, 

 is to proportion the two surfaces so, that the force of 



