126

BURNING MIRRORS.

to the grand secret of Archimedes , and to have vindicatedhistorical credit, by showing what was possible; for in anno1747, he constructed a machine with 140 plane mirrors,each about four inches long, and three broad; these fixedat a quarter of an inch distant from each other, upon awooden frame of six feet square. Each of them had threemoveable screws, so contrived that the mirror might be in-clined to any angle, and meet the sun in every direction; bywhich means, the solar image in each mirror was made tocoincide with all the rest. The first experiment was triedwith only 24 mirrors, which readily ignited combustiblematerials, consisting of pitch, tow, &c. at a distance of 66French feet.

He then put together a kind of polyhedron 168 pieces ofplain looking-glass, each six inches square, by which someboards of beech-wood were inflamed at 160 feet distance,in the month of March, and a silver plate was melted at 60feet distance. This machine, besides its other advantages,might be moved to burn upwards, downwards, or horizon-tally, burning in a near or distant focus, which commonburning-glass will not do, from their focus being fixed anddetermined. In its next stage of improvement, it had 360plane mirrors, each eight inches long and six broad, mountedon a frame eight feet high, seven broad, which vastly im-proved its power of combustion; but when the whole ma-chine in its most perfect stale was employed, it producedthe following wonderful effects: wood was kindled in a clearsky at 210 feet. The focus at 50 feet, about 7 inchesbroad; at 240 feet, it was 2 inches in diameter. M. Buffonafterwards constriu .pd a machine, which contained 400mirrors, each half a foot square, with which he could meltlead and tin, at a distance of 140 feet.— Phil. TransactionsAbridged, x. 194, et M. de Buffon Histoire Naturelle, 8fc.Sup. to Montuda Hist, des Malhemaliques, i. 232.

We have now to present a memoir of a burning-glassfabricated in this kingdom, said to have been the most pow-erful one yet constructed. It was made by Mr. Parker, ofFleet-street, London , and cost upwards of £700; it would,however, serve to fuse and vitrify such substances whichwould resist all common and chymical fires, how intensesoever, and more especially was found capable of applyingheat in vacuo. It consisted of a lens of flint-glass 3 feet indiameter, which, when fixed in its frame, exposed a snrfaceof about 81 inches in the clear; it was without defect, ex-cept a disfigurement in one of the edges,, by a piece of thescoria of the mould, which had procured access into the