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BURNING MIRRORS.

emphasis, all people, particularly the fat, short-necked, andthe aged, from the indulgence of that habit, which hasbeen in every iustance, except one, the Italian one, thecertain precursor to this lamentable catastrophe;—refrainfrom inebriety; -and the use of ardent spirits in particular,when, it seems, the effect may be avoided.

BURNING MIRRORS, or SPECULA.

Burning Mirrors, or Specula, are of two kinds; but each, oreither, collecting the rays of the sun into one concentratedpoint; by which means a force of heat is produced, thatexceeds every other ignitious power known in nature, orwhich can be raised by art. The first description are con-vex, called lentes canstica, which transmit the rays, and intheir passage incline them by refraction to an axis, havingthe property of lens, and acting according to the laws ofrefraction. The second, and most usual, are concave, andthough, perhaps improperly, called burning glasses, areusually made of metal; these reflect the rays of light, and inreflection incline them to a point in their axis, possessing theproperty of specula, or mirrors; and acting likewise by thelaws w r hich regulate reflection. That both kinds w ere knownto the ancients, to a certain and limited degree, we thinkthere can be no doubt; judging from various expressions:although we know it has been often alleged that they wereonly acquainted with the concave species, or that speciesoperating by reflection. We accordingly find that both Senecaand Pliny take notice of the burning power of specula.

In a treatise on optics, usually bearing the scientificname of Euclid, their principle is recognised. See Theo-rem xxxi.

The theogonists of antiquity are thought to have lightedthe sacred fire by their means. It is very evident, that ifit ever was by accident extinquished, they never renewedit by the aid of vulgar fire; because a circumstance is men-tioned of a party who once went from Athens to Delphos, onfoot, and back in one day, to fetch some sacred fire; but whoexpired on his return the moment he set his foot in the temple.The circumstance of his monument is, if our memory servesus, mentioned by Pansanias, in his description of Athensand its statuary. But Porta, with some others, suppose,that the burning mirrors of the ancients were made ofmetal, in the form of a section of a parabola, with thevertex cut off; it being the well-known property of thiscurve to reflect all rays which fall upon it, parallel to its