BURNING MIRRORS.

55

To explain a phrase which must presently be used, it may be remarked, that the term focus is Latin, and literally means a fire-place: it is therefore appropriately employed to denote that point where the rays of the sun are concentrated by a mirror; and hence when focal distance is mentioned, it will be at once intelligible.

The power of mirrors of a large diameter has been fully proved in later times. By these instruments the hardest metals, on which common fires, and even glasshouse furnaces, could produce no effect, have been melted in a few seconds. M. Vilette, a Frenchman, nearly a century ago, constructed a mirror three feet eleven inches in diameter, and three feet two inches in focal distance, which melted copper ore in eight seconds, a fish's tooth in thirty-two seconds, iron ore in twentyfour seconds, cast iron in sixteen seconds, a silver sixpence in seven seconds, and tin in three seconds. This mirror condensed the solar rays 17,257 times; a degree of heat which is about four hundred and ninety times greater than that of a common fire.

Mr. Parker, of London, also constructed a lens three feet in diameter, and six feet eight inches focus, which weighed 212 pounds. It melted twenty grains of gold in four seconds, and ten grains of platina in three seconds. The broader the mirror or lens, and the shorter the