Mr. Parker, an eminent glass manufacturer, in Fleet street, London, constructed the most powerful burning mirror ever made. He erected an out building at the bottom of his garden for the purpose of carrying on his operations. He succeeded in forming a most powerful burning lens. Its diameter was three feet. Platinum, iron, steel, flint, &c. were melted in a few seconds, on being exposed to its immense focus. A diamond weighing thirty grains was reduced to six grains, in the space of thirty minutes. It opened and foliated like the leaves of a flower, and emitted whitish fumes, (carbonic acid gas;) when close again, it bore a polish, and retained its form. Garnets, clay, &c. soon melted.
Seven hundred guineas were subscribed to indemnify the inventor, it having cost him seven hundred pounds. It was purchased, however, and presented by lord Macartney to the Chinese government, and remains now at Pekin.
M. Payard, (Archives des Découvertes, &c.) has invented a burning mirror, consisting of several plane mirrors so arranged as to concentrate the solar heat into a focus with great precision. The arrangement, it may be proper to state, is different from that heretofore used, and the effect is said to be very powerful.
A polygonal mirror, from a suggestion of the celebrated Buffon, was erected in the Botanic Garden at Paris, in 1747, and had also a very powerful effect. This mirror was composed of one hundred and sixty-eight plates of tinned or silvered glass, capable of moving in every direction and of being fixed at different degrees of inclination, so that there could be given to the whole, a form more or less concave, and the focus be thrown to different distances. This mirror set fire to wood at two hundred feet, and fused metals at forty-five feet.
As caloric, like light, follows the same laws with respect to its motion, and as the angle of incidence is equal to the angle of reflection, the radiation of heat, conducted after the manner of Pictet's experiment, has not only occasioned the combustion of gunpowder, but of other inflammable substances. But, for this purpose, the mirrors must be large and extremely bright. That heat radiates in all directions, and is reflected, and that the calorific rays may thereby be concentrated, are facts which are now universally admitted. The application of this principle, by using concave mirrors sufficiently large, has, we are informed, produced the explosion of gunpowder. They were placed about twelve feet apart. In the focus of one a live coal was put, which was constantly blown with a double bellows, and in the focus of the other some gunpowder. In all our experiments with the ordinary reflectors, we could never produce any thing like the heat necessary to inflame gunpowder. That the principle is substantiated by experiment is evident; for the rays of a heated body, or a substance which produces heat, as a lamp or candle, placed in the focus of a concave mirror, are reflected in parallel lines, and if another concave mirror be placed opposite to it at some distance, the calorific rays will be thus intercepted and reflected back in a focus. This focus, therefore, like the focus of a burning glass, is the concentration of all the parallel rays of heat.
Sec. XL. Of Incendiary and Poisoned Arrows.
The bow is a very ancient weapon of offence, made of steel, wood, horn, or other elastic substance, which, after being bent by means of a string fastened to its two ends, in returning to its natural state, throws out an arrow with great force. That the bow was a weapon of offence among the nations of antiquity, the inhabitants of Asia and Africa, and the Aborigines of this country, and that it was used in Europe, before the invention or use of fire-arms, are facts, of which we have abundant proof. Bows are much the same in all countries. It has generally two inflections or bendings, between which, in the place where the arrow is drawn, is a right line. The Grecian bow was adorned with gold or silver. The Scythian bow was distinguished from those of the Grecians and other nations, by its incurvation, which was so great, as to form a half moon, or semi-circle. The Persian bows were made of reed. The Indians used the same material, as well for their bows as their arrows. The Lycian bows were made of the cornel tree; and those of the Ethiopians, which surpassed all others in magnitude, were made of the palm tree. The Romans, although they did not admit bows in the infancy of their republic; yet they considered them as hostile weapons. They employed auxiliary archers in all their wars. The Amazonians, as well as the primitive Grecians, in drawing their bow, did not pull back their hand towards their right ear, according to the fashion of the ancient Persians, and of modern ages; but, placing their bow directly before them, returned their hand upon their right breast.
While noticing this subject, we may also observe, that Louis XI first abolished the use of bows in France, introducing, in their place, the halberd, pike, and broadsword. The long bow was much in use by the English archers, and many laws were passed encouraging its use. In the time of Henry VIII, the parliament complained of the disuse of long bows.
The bow is now laid aside altogether as a war weapon. The arrows, made use of, were armed with barbed iron, and, among the aborigines of this and other countries, with a stone, formed in a particular manner, many of which are picked up in this country. We have found them at West Point. The natives were in the habit of poisoning their arrows, by using a particular composition, not known; the effect of which, however, when the arrow penetrated into the flesh, is always destructive.