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BURNING MIRRORS.

quently with more certainty and truth, than had the ancients.AVe only know of one chief science where the moderns canhoast a decided superiority over antiquity, which is in thelight afforded by modern chymistry the better to compre-hend the component properties of nature. But this wederived from a silly circumstance, from the voyage in questof the philosopher s stone. Let it not be told that ourmost boasted superiority in wisdom, was generated by theextreme of our folly.

Every concave mirror, or speculum, collects the rays dis-persed through its whole concavity, subsequent to reflection,into a point or focus; and by this means it becomes a burn-ing mirror.

As the focus is found where the rays arc all concentratediu a certain point, should it be the segment of a largesphere, its breadth must subtend an arch above 18 degrees;should it be a segment of a smaller, its breadth may be30 degrees ; this is verified by experiment. The surface ofa mirror which is the segment of a larger sphere receivingmore rays than that of a less ; if this intuitive truth be ad-mitted, it follows, that if the arch subtend 18 degrees,or more, should it be equal, the effects of the greatermirror will be more potent than those of the less. And asthe focus is contained between the fourth and fifth parts ofthe diameter, mirrors that are segments of a greater sphere,burn at a greater distance than those which are segments ofa smaller.

Since the last consideration is that the burning must de-pend upon the union of the rays on the concave sphericalfigure, it will not be surprising, that wooden mirrors gilt, orthose prepared of alabaster, &c. covered with gold, nay,even if they be made of paper and covered with straw, shouldbe found to produce ignition.

Among the ancients, the burning mirrors of Archimedes and Rocqes, are eminent; by those of the first, the Romanfleet besieging Syracuse , under the command of Marcellus,as related by Zonaras, Tzetzes, Galen, Eustathius, Anthe-mius , See.; and by the other, the navy of Vatalian besieg-ing Byzantium, according to the same Zonaras , were burntto ashes.

The fact of the surprising effect of the burning mirrorsof Archimedes has been doubted by many of the scien-tific world among the moderns, and by none more thanDescartes , who positively discredited it, and principallyfrom his conviction of the utter impossibility of constructingconcave mirrors of a sufficient focal distance for the pur-pose. If the scepticism of Descartes carried him so far,