764 



BURNING MIRRORS BURNS. 



glasses produce an effect equal to that of the most in- 

 tense fire. They kindle wood which is both hard and 

 wet in a moment, and make cold water, in small ves- 

 sels, boil in an instant ; metals, placed upon a plate of 

 china, are melted and vitrified by them : tiles, slates, 

 and similar objects, become instantly red-hot, and 

 vitrified. As Tschirnliausen's glasses, however, are 

 nut perfectly clear, and the effect is 1 1m- considerably 

 lessened, Brisson and Lavoisier undertook, in 1774, 

 to put together two lenses, resembling those used for 

 watch glasses, filling up the space between them witli 

 a transparent fluid. In this manner, veins and im- 

 purities may be avoided, at less expense. They suc- 

 rrcdrd in making a burning-glass of four feet in 

 di:imeter, the greatest thickness of which, in the cen- 

 tre, amounted to eight inches, and which, of itself, 

 had a much greater power than the glasses of Tschirn- 

 hausen in connexion wilh a smaller lens, or collective 

 glass ; but produced an extraordinary effect if joined 

 to a collective glass. 



The experiments made by means of large burning- 

 glasses are important in chemistry and physics. The 

 power of a burning-glass, however, is almost four 

 times less than that ot a burning mirror, or reflector 

 (q. v.), of equal extent and equal curvature. This 

 reflects more heat than the glass allows to pass 

 through it ; has a smaller focal distance, and is free 

 from the dissipation of the rays, which takes place in 

 the. burning-glass, since it reflects them all nearly to 

 one point, while the burning-glass refracts them to 

 different points. On the other hand, the burning-glass 

 is much more convenient, on account of the place of its 

 focus, which is behind the glass. The burning point 

 (focus) is an image of the sun ; its diameter is equal to 

 the 108th part of the focal distance, and its centre is 

 the focus properly so called. In the higher branches of 

 geometry and conic sections, the/o'are points in the 

 parabola, ellipsis, and hyperbola, where the rays, re- 

 flected from all parts of these curves, meet. Several 

 accidents in modern times have shown, that confla- 

 grations may be caused by convex window-glasses or 

 water-bottles, c., which have the form or burning- 

 glasses, if the rays of the sun are concentrated by 

 them upon combustible substances lying within their 

 reach. Since the casting and polishing of large lenses 

 are attended with great difficulties, Buffon's plan of 

 casting them in pieces, or zones, and afterwards put- 

 ting them together, has lately been practised. Lenses 

 of this last kind have been ingeniously applied, by 

 Becquey, for augmenting the light on light-houses, 

 according to the suggestion of Fresnel. (See Pharos.) 

 For the history of burning instruments, see the article 

 Burning Mirrors. 



BURNING MIRRORS, OR REFLECTORS; mirrors, the 

 smoothly polished surface of which reflects the rays of 

 the sun that fell upon it in such a direction, that they 

 unite at some distance from the mirror, in a more 

 limited space, and act upon substances within this 

 space like the most powerful fire. Concave mirrors 

 cause the rays that fall upon them in a direction 

 parallel to their axes to converge. Spherical mirrors 

 of this kind are the most common ; but parabolic 

 ones are also used ; and even plane mirrors may be 

 employed like concave ones, if several of their are 

 combined in a proper manner. In order that a burn- 

 ing mirror should produce its whole effect, its axis 

 must be directed exactly towards the centre of the 

 sun's disk. This is the case, if the light, intercepted 

 ny a plane, perpendicular to the axis of the mirror, at 

 its focal distance, forms a circle. The focus then lies 

 in a straight line between the sun and the mirror 

 The ancients were acquainted with such mirrors, as is 

 manifest from several of their writings still extant. 

 It is impossible, from the nature of things, hat Archi- 

 medes, during the siege of Syracuse 



should have set on fire the fleet of the latter by means 

 of concave mirrors : it would be more credible, that 

 "t had been effected by a combination of plane mirrors. 

 Various experiments have shown, that great effect* 

 may be produced, at a considerable distance, by the 

 latter instrument. Kircher placed five plane mirrors, 

 of an equal size, in such a position as to reflect the 

 rays upon a spot one hundred feet distant, and thereby 

 produce a great heat. Buffon, in 1747, effected a 

 combination of 168 plane mirrors, each of which was 

 6 inches broad, and 8 long. With 40 of these mir- 

 rors he set on fire, almost instantaneously, a board 

 of beech wood, covered with tar, at the^distance of 

 66 feet; and, with 128 mirrors, a board of pine 

 wood, likewise covered with tar, at a distance of 

 150 feet. With 45 mirrors, he melted a tin bottle, 

 at a distance of 20 feet, and, with 117 mirrors, small 

 pieces of money. He afterwards burned wood with 

 this machine, at the distance of 200 feet, melted tin 

 at the distance of 150, lead at the distance of 130, 

 and silver at the distance of 60 feet. During the 

 last century, several large mirrors were made in Italy, 

 two of which are still in Paris and Cassel. Von 

 Tschirnhausen also manufactured one in 1687, 3 

 Leipsic ells (about 5A English feet) in diameter, and 

 the focal distance of which was 2 ells (3,' 8 English 

 feet).* It consists of a thin plate of copper, highly 

 polished, and is now in the mathematical hull in 

 Dresden. This mirror sets wood on fire, makes water 

 boil, melts tin three inches thick, as well as lead, 

 vitrifies bricks, bones, &c. Besides metals, wood, 

 pasteboard, and other materials serve for burning 

 mirrors, if their surface be polished. Burning mirrors 

 have of late been used as reflectors (q. v.), to throw 

 light at a great distance, and may be very usefully 

 employed in light-houses. If, for instance, a lamp is 

 placed in the focus of a parabolic mirror, the rays of 

 light which fell on it are all reflected in a direction 

 parallel to the axis ; thus the reflectors of Lenoir ap- 

 pear like stars of the first magnitude at the distance 

 of 80,000 feet. For further information on burning- 

 glasses and burning mirrors, see Priestley's History 

 and present State of Optics ; and the 5th vol. of the 

 new edition of Gehler's Physikalisches Lexicon, 

 Leips. 1825. 



BURNISHER is a blunt, smooth tool, used for smooth- 

 ing and polishing a rough surface by pressure, and not 

 by removing any part of the body. Other processes 

 of polishing detach the little asperities. Agates, 

 tempered steel, and dogs' teeth, are used for burnish- 

 ing. It is one of the most expeditious methods of 

 polishing, and one which gives the highest lustre. 

 The burnishers used by engravers are formed to bur- 

 nish with one end, and to erase blemishes with the 

 other. 



BURNS, Robert ; a very distinguished Scottish poet, 

 was the son of William Burnes or Burns, a gardener 

 and small farmer, near the town of Ayr, and was 

 born January 25, 1759. He was brought up to 

 rustic labour ; but his education was not neglected, as 

 he was, at an early age, instructed in English gram- 

 mar, by Mr Murdoch, (who died not long since in 

 London,) to which he added an acquaintance with the 

 French language and practical mathematics. Smitten 

 with a passion for reading, he devoted every moment 

 he could spare to the perusal of such books as fell in 

 his way, and, among them, meeting with the works ot 

 some of the best English poets, he was enabled to 

 cultivate and improve a taste for poetry and romantic 

 fiction ; which was, perhaps, first inspired by the 

 chimney-corner tales of an old woman in his father's 

 family, whose memory was plentifully stored with 



* Another account gives diameter, 4} French feet, focal 

 distance, 12 feet. 



