BURNING MIRRORS.

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bones changed into opaque glass, and eartli into a blackone. It was made of copper of about of an inch thick,4j French feet diameter; it burnt at a distance of 12 feet,performing the above operations.

It should be observed, that every lens, whether convex,plano-convex, or double-convex, collects the sun’s raysdispersed over its convexity, into a point by refraction,which constitutes a burning-glass. The most considerableof these known are what have been previously mentioned,and the following one made by M. de Tschirnhausen: thediameters of his lenses were three and four feet; their focushappened at a distance of twelve feet; its diameter oneinch and a half: to make the focus more vivid, the raysbeing collected a second time by a second lens, parallel tothe first, and situated in that place where the diameter ofthe cone of rays, formed by the first lens, is equal to thediameter of the second, so that it received them all, andthe focus from an inch and a half was contracted into aspace of eight lines, and its force increased proportionally.It w r as purchased by the Duke of Orleans, who presentedit to the French Academy. Weight 160 lbs.

Its effects are partly enumerated as follows: it lights hardw r ood, moistened with water, instantly; boils w r ater directly;melts all metals; brick, stone, .earthen-ware, and asbestos,are transmuted into glass; sulphur, pitch, and other inflam-mable substances, set on fire beneath water; ashes of vege-tables, woods, and other matters, transmuted into glass : inbrief, every thing applied to its focus was either melted,turned into calx, or smoke. The colour of jewels, and allother matters, except metals, were changed. Succeedingbest when the matter applied is laid on hard well burntcharcoal.

Sir Isaac Newton ’s burning mirror, consisted of sevenconcave glasses, each 1H inches in diameter, so disposedas to have one common focus; six of them were placedround the seventh contiguous to it, so as to form the seg-ment of a large sphere, whose subtense was about 34|inches, the focus 22£ inches distant, and its diameter aboutone inch. This speculum vitrified brick or tile in onesecond; in 30'' melted gold; which instrument has beensince improved by M. Zeicher, by forming panes of plainglass to a necessary degree of curvature, by heating themso hot ou a dish made of metal that they all readily assumedthe same form. The most considerable improvement wasmade by M. Buffon, by constructing his machine of such anumber of mirrors, by which he appeared to have aspired