BURNING INSTRUMENTS. 



tn-itru- 

 ntl. 



England was put into the hands of Dr Harris and 

 J)r Desa^ulirrs, who made several triaU with it. It 

 was a composition of copper, tin, and tin glase ; 

 and its reflection had something of a yrl!<>w cast. 

 Tin- re were only a few small flaws in the concave 

 surface, but there wore some holes in the convex 

 side which was polished. The diameter of the n.ir- 

 ror was 47 im ' 'ins of curvature 76 incites, 



and its focal length MH inches. The following re- 

 sults were obtained in June 17 IN, between 9 and 12 

 o'clock in the morning, and the time was measured 

 by a half-second pendulum. 



Seconds. 

 A red piece of Roman patera began to melt in 3 



and was ready to drop in 100 



A black piece of the same melted in ... 4 



and was ready to drop in G t 



Chalk taken out of an echinus spatagus filled 



with chalk, only fled away in . ... 23 



A fossil shell calcined in 7 



and did no more in 64 



The black part of a piece of Pompey's pillar 



melted in SO 



And the white part in . . . .... 54 



Copper ore, with no metal visible, vitrified in 8 

 Slag or cinder of the iron work said to have been 



wrought by the Saxons, was ready to run in 29^ 



The mirror now became hot, and burned with 



much less force. 



Seconds. 



Iron ore fied at first, but melted in .... 24 

 Talc began to calcine at 40 



and held in the focus 64 



Calculus humanub was calcined in .... 2 



and only dropped off in 60 



The tooth of an anonymous fish melted in . 32^ 

 The asbestos seemed condensed a little in . 28 



But it now became cloudy. M. Vilette says 

 that the mirror usually calcines asbestos. 

 A golden marchesite broke, and began to melt in 30 



A. silver sixpence melted in 7 4 



A King William's copper halfpenny melted in 20 



and ran with a hole in it in . . . . 31 

 A King George's halfpenny melted in . . . 16 



and ran in 34 



Tin tnellcd in 3 



Cast iron melted in 16 



Slate melted in 3 



and had a hole in 6 



Thin tile melted in 4 



and had a hole and was vitrified through in 80 

 Bone calcined in 4 



and vitrified in 33 



An emerald was melted into a substance like 

 Turquoise stone, and a diamond that weighed 

 4 grains lost jjths of its weight. 

 This mirror was made by M. Vilette, some years 

 after the first, and with the assistance of his two sons. 

 It came into the possession of M. Vilette the son, 

 engineer and optician to his electoral highness of Co- 

 logne, bishop alid prince of Liege where he com- 

 monly resides. At the desire of several learned men, 

 M. Vilette brought it to London, where its effects 

 were exhibited in Priory garden, Whitehall. 



The burning mirrors of Maginus, and of Manfredi 



Septala, a canon of Milan, do not appear to have 

 produced such powerful effect! as those of M. Vilette. 

 The ir.irror of Maginus was only 20 inches in diame- 

 ter, uld appear from the different accounts 

 in. the Piiilotwilni-ul T''iinxrn-tions t that Septala had 

 constructed two mirrors. The first, which was five 

 palms, or about 3| feet in diameter, had a long focal 

 length ; but there is no particular information respect- 

 in/ its effects, which seem to have been far from power- 

 ful, M a gentleman who had seen it, declared, " lli' 

 it could not set tvood on Jire but after the time of say 

 ing n MISKKKKE!" a method of measuring time m.t 

 much in use among experimental philosopher*, 

 other burning mirror of Septala seems to have been 

 about seven feet in diameter, and to have had a focal 

 length of 50 palm, or about 33 feet. See Sch 

 [.'/livers. Part I. vol. vii. $ 6. p. 418. 



In the year 1685 M. I)e La Garouste presented 

 to the Academy of Sciences, a large metallic mirror, 

 five feet two inches in diameter, and five feet in focal 

 length. It was not equally polished, and a piece was 

 inserted in the middle of it where the metal had fail- 

 ed. This circumstance, however, did not seem to 

 diminish its force. Several trials were made with 

 this mirror in the Academy, by order of M. De 

 Louvois, but the precise effects which it produced 

 have not been detailed. It is merely stated, that those 

 who tried it were satisfied with the results, and that 

 its effects would have been much greater had it been 

 better polished, and mounted upon a proper stand. 



On the 27th of February, 16678, Francis 

 Smethwick, Esq. produced before the Royal Society 

 two burning concave glasses, ground of a neixly in- 

 vented jigure, which was probably that of a parabola. 

 One of them was six inches diameter, with three 

 inches of focal length ; and the other was of the same 

 diameter, with its focus ten inches distant. When 

 these were brought towards a large lighted candle, 

 they somewhat warmed the faces of those that were 

 four or five feet distant ; and when held to the fire, they 

 burnt gloves and garments at the distance of about 

 three feet from the fire. At another experiment 

 made in the presence of Dr Seth Ward, the deeper 

 of the two burned a piece of wood into flame in the 

 space of ten seconds, and the shallower one iu five 

 seconds. This experiment was made in autumn, at 

 nine o'clock in the morning, when the weather was 

 gloomy. By exposing the deeper concave to a north- 

 ern window on which the sun did not shine, it was 

 found to warm the hand by " collecting thewarmed air 

 in the day time, which it would not do after sunset." 



The burning mirror constructed by the celebra- 

 ted M. Tschirnhausen, was formed of copper plate, 

 scarcely twice as thick as the back of an ordinary 

 knife. It was about three Leipsic ells in diameter. 

 It was well polished; and burnt at the distance of 

 two Leipsic ells. * The following results were ob- 

 tained with it : 



1. A piece of wood held in the focus, flames in a 

 moment, so that a fresh wind can hardly put U. out. 



'_'. Water applied in an earthen vessel immediately 

 boils ; and the vessel being keptahere some time, the 

 water evaporates all away. 



3. A piece of tin or lead three inches thick melts 

 away iu drops as soon as it is put in the focus ; and 



La Ca- 

 roline's 



burning 



Smetli- 



wick'* 



burning 



mirror*. 



Tsehirn- 



burniug 

 mirror. 



* In another account its diameter is said to be 4J feet, and its focal length 12 feet. 



