The seventeenth century was distinguished by the pro duction of various burning mirrors, of different construc tions. The principal of these were made by M. Vilette, a French artist at Lyons, wino appears to have construct ed no fewer than five of considerable magnitude. One of them was bought by M. D'Alibert for 1500 livres: another was purchased by Tavernier, and presented to the King of Persia : a third was sent by the French king to the Royal Academy : a fourth was bought by the King of Denmark ; and the fifth was brought to England for public exhibition. The first of these mirrors was thirty inches in diameter, and weighed above a hundred weight.
Its focal length was about three feet, and the size of the image was about half a Louis d'or. It was mounted on a circular frame of steel, and could easily be put into any required position. This mirror was made in I 67o, and ha. ing been brought to St Ger-mains by the order of the king, his majesty was so well pleas...C1 with it, that he rewarded Vilette with a hundred pistoles for the sight of it, and afterwards purchased it, and placed it ill the royal observatory at Paris. The effects were the following : Seconds. A small piece of pot iron was melted in. 40 A silver piece of fifteen pence was pierced in . 24 A thick nail (le clod de paydan) melted in30 The end of a sword blade of Olinde burnt in . 43 A brass counter was pierced in6 . .
A piece of red copper was melted in42 A piece of chamber quarrystone was vitrified in 43 Watch-spring steel melted in. 9 . .
A mineral stone, such as is used in harquebusscs a'rouet, was calcined and vitrified inI A piece of mortar was vitrified in. 52 . .
Green wood and other bodies took fire instantly.
The mirror of M. Vilette which was brought to Eng land was put into the hands of Dr Harris and Dr Desa guliers, who made several trials with it. It was a composition of copper, tin, and tin glass ; and its reflection had something of a yellow cast. There were only a few small flaws in the concave surface, but there were some holes in the convex side which was polished. The diameter of the mirror was 47 inches, its radius of curvature 76 inches, and its focal length 38 inches. The following results were ob tained in July 1718, 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.
.
.
and was ready to drop . 64 .
Chalk taken out of an cchinus spatagus filled with chalk, only lied away in. . 23 .
A fossil shell calcined in7
. .
.
.
and did no more in. 64 .
.
The black part of a piece of Pompey's pillar melt ed in. 50 .
. .
.
.
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 . 29,1 The mirror now became hot, and burned with much less force.
Seconds.
Iron ore fled at first, but melted in. 24 .
.
Talc began to calcine at. 40 and held in the focus. 64 n Calculus hum was calcined in . .,, .
and only dropped off in 60 --)1 The tooth of an anonymous fish melted in . . The asbestos seemed condensed a little in . . 2ti 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 71 .
.
Seconds.
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 melted 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 Tur quoise stone, and a diamond that weighed 4 grains lost .ths of its weight.
This mirror was made by M. Vilette, some years af ter the first, and with the assistance of his two sons. It came into the possession of 111 Vilette the son, engineer and optician to his electoral highness of Cologne, bishop and prince of Liege, where he commonly 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 Scptala, a canon of Milan, do not appear to have pro duced such powerful effects as those of M. Vilette. The mirror of Maginus was only 20 inches in diameter, and it would appear from the different accounts in the Phi losophical Transactions, that Septala had constructed two mirrors. The first, which was five palms, or about 32 feet in diameter, had a long focal length ; hut there is no particular information respecting its effects, which seem to have been far from powerful, as a gentleman who had seen it, declared, "that it could not set wood on fire but after the time of saying a MISERERE !" a method of measuring time not much in use among experimental philosophers. The other burning mirror of Septala scents to have been about seven feet in diameter, and to have had a focal length of 50 palms, or about 33 feet. See Schottus Magia Univers. Part I. vol. vii. § 6. p.