418.
In the year 1685, Al. De la Garouste presented to the Academy of Sciences, a large metallic mirror, five feet two inches in diameter, and five feet in focal length. It was not equally polished, and a piece was inserted in the middle of it where the metal had failed. This circum stance, however, did not seem to diminish its force. Se veral trials were made with this mirror in the Academy, by order of M. De Louvois, but the precise effects which it produced have not been detailed. It is merely stat ed, that those who tried it were satisfied with the re sults, and that its effects would have been much greater had it been better polished, and mounted upon a proper stand.
On the 27th of February, 1667-8, Francis Smeth wick, Esq. produced before the Royal Society two burn ing concave glasses, ground of a newly invented figure, which v as probably that of a prrabola. One of them was six inches diameter, with three inches of focal length ; and the other was of the same diameter, with its focus ten inches distaht. When these were brought towards a large lighted candle, they somewhat warmed the faces of those that were four or live feet distant ; and when held to the fire, they burnt gloves and garments at the distance of about three feet from the fire. At ano ther experiment made in the presence of Dr Seth Ward, the deeper of the two burned a piece of wood into flame in the space of ten seconds, and the shallower one in five seconds. This experiment was made in autumn, at nine o'clock in the morning, when the weather was gloomy. By exposing the deeper concave to a northern window on which the sun did not shine, it was found to warm the hand by " collecting the warmed air in the day time, which it would not do after sunset." The burning mirror constructed by the celebrated M. Tschirnhausen, was formed of copper plate, scarcely twice as thick as the back of an ordinary knife. It was about three Leipsic ells in diameter. It was well polish ed ; and burnt at the distance of two Leipsic ells.* The
following results were obtained with it : 1. A piece of wood held in the focus, flames in a mo ment, so that a fresh wind can hardly put it out.
2. Water applied in an earthen vessel immediately boils; and the vessel being kept there some time, the water evaporates all away.
3. A piece of tin or lead three inches thick melts away in drops as soon as it is put in the focus ; and when held there a little time is in a perfcctfluor, so that in 2 or 3 minutes it is quite pierced through.
4. A plate of iron or steel becomes immediately red hot, and soon after a hole is burnt through it.
5. Copper, silver, &c. melt in five or six minutes.
6. Stones, brick, &c. soon become red hot.
7. Slate becomes red hot, but in a few minutes turns into a fine sort of black glass.
8. Tiles which had been exposed to the most intense heat of fire, melt down into a yellow glass.
9. Pot-shreds, that had been much used in the fire, melt into a blackish yellow glass.
10. Pumice stone melts into a white transparent glass.
11. A piece of a very strong crucible melted into a glass in 8 minutes.
12. Bones were converted into a kind of opaque glass, and a clod of earth into a yellow or greenish glass.
13. The beams of the full moon when at her greatest altitude were concentrated by this speculum, but no per ceptible degree of heat was experienced.
Zacharias Traberus mentions in his book, In ./Vervo Optico, that burning mirrors may be constructed by turn ing a large concave of wood, laying its surface equally with pitch, and covering it with square pieces of leaf gold about two or three inches broad. Large mirrors, he says, may he made of 30, or 100, or more concave pieces, artfully joined in a turned wooden dish or scut tle.
Zahnius, in his work In Oculo Artific. fundam. 3. in forms us, that one Neuman, an engineer at Vienna, form ed burning mirrors of pasteboard, covered 011 the inside with straw glued to it, and that their effect was so power ful as to melt metals almost instantly.