BURN, in medicine and surgery, an in jury received in any part of the body, in consequence of the application of too great heat. See SURGERY.
BURNING-glass, a convex or concave glass, commonly spherical, which, being exposed directly to the sun, collects all the rays falling thereon into a very small space called the focus ; where wood, or any other combustible matter, being put, will be set on fire. See OPTICS We have some extraordinary instances and surprizing accounts of prodigious ef fects of burning-glasses. Those made of reflecting mirrours are more powerful than those made with lenses, because the rays from a mirrour are reflected all to one point nearly; whereas, by a lens, they are refracted to different points, and are therefore not so dense or ardent. The whiter also the metal or substance is, of which the mirrour is made, the stronger will be the effect.
The most remarkable burning-glasses, or rather mirrours, among the ancients, were those of Archimedes and Proclus ; by the first of which the Roman ships, be sieging Syracuse, according to the testi mony of several writers and by the other, the navy ofVitalian,besieging Byzantium, were reduced to ashes. Among the mo derns, the burning mirrours of greatest eminence are, those of Villette and Tschirnhausen, and the new complex one of M. de Buffon.
That of M. de Villette was three feet eleven inches in diameter, and its focal distance was three feet two inches. Its substance is a composition of tin, copper, and tin-glass. Some of its effects, as found by Dr. Harris and Dr. Desaguliers, are, that a silver sixpence melted in 71'i ; a King George's halfpenny melted in 16", and ran in 34", tin melted in 3" and a diamond, weighing 4 grains, lost seven eighths of its weight.
That of M. de Buffon is a polyhedron, six feet broad, and as many high, consist ing of 168 small mirrours, or flat pieces of looking-glass, each six inches square ; by means of which, with the faint rays of the sun in the month of March, he set on fire boards of beech wood at 150 feet dis tance. Besides, his machine has the con veniency of burning downwards, or hori zontally, as one pleases, each speculum being moveable, so as, by the means of three screws, to be set to a proper incli nation for directing the rays towards any given point : and it turns either in its greater focus, or in any nearer interval, which our common burning-glasses can not do, their focus being fixed and deter mined. M. de Buffon, at another time,
burnt wood at the distance of 200 feet. He also melted tin and lead at the dis tance of above 120 feet, and silver at 50. Mr. Parker, of Fleet-street, London, was induced, at an expense of upwards of 700/. to contrive, and at length to com plete alarge transparent lens, that would serve the purpose of fusing and vitrifying such substances as resist the fires of or. dinary furnaces, and more especially of applying heat in vacuo, and in other cir cumstances, in which it cannot be applied by any other means. After directing his attention for several years to this object, and performing a great variety of experi ments in the prosecution of it, he at last succeeded in the construction of a lens, of flint-glass, three feet in diameter, which, when fixed in its frame, exposes a sur face two feet 81 inches in the clear, with out any 'other material imperfection be sides a disfigurement of one of the edges by a piece of the scoria of the mould, which unfortunately found its way into its substance. This lens was double con vex, both sides of which were a portion of a sphere of 18 feet radius. It is diffi cult to form an accurate estimate of the burning power of this lens ; inasmuch as it is next to impossible to discover what should be deducted for the loss of power, in consequence of the impediments that the glass of which it was made must oc casion, as well as the four reflections, and two more by way of diminution ; but we will endeavour to appreciate it, after mak ing a full allowance for these deductions, which must necessarily result from every means of concentrating the solar rays, and which must be considered to be as the friction of an engine, of which nature they really partake. The solar rays re ceived on a circular surface of 2 feet 8i inches, when concentrated within the di ameter of an inch, will be 1056.25 times its intensity, or this number of times greater than the heat of the sun, as felt on the surface of the earth. We will suppose that, as the heat of the air, in ordinary summer weather, is 65°, and in sultry weather is 75., the average of which is 70°, and that we take this degree as the average effect, the accumulated power of the lens, on the supposition of an equal effect over the whole surface of the focus, will be equal to 73938°.