BLR 



BUli 



minute. There are but two species ; of 

 which B. disticha has the root composed 

 wholly of capillary fibres, very small. The 

 plant 'has the appearance of an antheri- 

 cum ; root-leaves six, grass-like, or ensi- 

 form, two inches long 1 , quite entire ; stem 

 upright, simple, a span and a half in height, 

 having six or seven small alternate leaves 

 an inch long; two equal divaricating 

 spikes, each composed of about nine flow- 

 ers, terminate the stem ; the flowers are 

 sessile, in a single row ; they are blue, 

 very elegant, and do not fall off'. It is a na- 

 tive of Ceylon. B. biflora, has strong 

 fibrous roots, with several oblong oval 

 leaves arising from it, which are smooth 

 and entire, four or five inches long; among 

 these springs the flower stem, six or eight 

 niches high, terminating by blue flowers, 

 growing together in each sheath. It is a 

 native of Virginia and Carolina. 



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-j-fcws, 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 of Vitalian,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 Buffbn. 



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 

 bv Dr. Harris and Dr. Desaguliers, arc, 



that a silver sixpence melted in 7" ; 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 Buffbn 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 ic 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 Buffbn, 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 a large 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 8^ 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, 



