Instruments for concentrating upon a very small surface the rays of the sun, which fall upon a much more extended one, by which means such an intense heat may be produced as to fuse, burn, or volatilize most substances. Burning glasses are convex lenses, which, acting according to the laws of refraction, transmit the rays, but incline or refract them towards a common point in the axis, called the focus. These were not entirely unknown to the ancients, although we have no minute accounts of their construction or performances; but in modern times very powerful instruments of this description have been constructed. The most celebrated is that made by Mr. Parker, of Fleet-street, at an expense of 700f., and several years of labour and research. It consisted of a lens composed of flint glass, which, when fixed in its frame, exposed a surface of 32 inches diameter in the clear; the distance of the focus was 6 feet 8 inches, and its diameter one inch. The rays from this large lens were received and transmitted through a smaller one, of 13 inches diameter in the clear within the frame, its focal length 29 inches, and the diameter of the focus | of an inch.
Amongst other remarkable effects produced by this instrument were the following: 3 grains of cast iron were fused in ten seconds; 20 grains of gold in four seconds; 10 grains of steel in twelve seconds; and 10 grains of common limestone in fifty-five seconds. Ten cut garnets, taken from a bracelet, began to run the one into the other in a few seconds, and at length formed into one globular garnet. The clay used by Mr. Wedgwood, to make his pyrometric test, run, in a few seconds into a white enamel. We are sorry to add to the account of this wonderful instrument, that, after being offered for sale to several learned societies, we believe it was sent to China, either on speculation, or amongst the presents which accompanied Lord Macartney's embassy, and was there left. Burning mirrors are concave mirrors or reflectors, formed of polished metal or silvered glass, and which, acting by the laws of reflection, throw the rays back into a point or focus before the glass. These instruments were not only well known to the ancients, but, if we may credit the accounts which have come down to us, the effects of some of their contrivances were superior to any produced in modern times by the same means.
Archimedes, it is said, reduced the Roman fleet under Marcellus to ashes, by burning mirrors, at a bowshot distance; and Proclus is said to have burned the fleet of Vitellius at the siege of Byzantium by the same means. Of the moderns, the most remarkable burning mirrors are those of M. de Villette, and of Buffon. That of M. de Villette was 3 feet 11 inches diameter, and its focal distance 3 feet 2 inches. It was composed of tin, copper, and tin glass. Some of its effects, as found by Dr. Harris and Dr. Desaguliers, are, that a silver sixpence melted in 7 1/2 minutes; a halfpenny melted in 16 minutes, and ran in 34 minutes, and a diamond weighing 4 grains lost 7/8 of its weight. That of M. Buffon is a polyhedron, 6 feet broad, and as many high, consisting of 168 small mirrors, or flat pieces of looking glass, each 6 inches square, by means of which, with the faint rays of the sun, in the month of March, he set on fire boards of beech wood at 150 feet distance. At another time he burned wood at 200 feet distance; he also melted tin and lead at the distance of above 120 feet, and silver at 50 feet.