BURNING-GLASS. 



In the Sloankm library, N° 4CJ 7, is another letter, (bound 

 up in folio, with a great manv different MSS.) from Mr. 

 Wilham Btckttt to fir Han Sloanc, dated F^l . 4th 17!^; 

 w'lerein the principal fafts mentioned in the letter to Dr. 

 Doiiglafs are recited. But wc found nothing befidcs 

 which is worthy of notice, except the followini;; remark : 

 *' This triiining or burning, (being, upon the difnfe of thefe 

 " words, calltd gonorrhoea), is what we now call the clap : 

 " which word 1 take to be of a modern iifage ; for, in all 

 " my enquiries, I cannot find it was ever dcfigned to fignity 

 " the fame thing it does now, before the reigu of 

 " Charles I." 



BcRxiNG, in I^ait: See Arson, Branding, Trhason, 



l«^C. 



Burning of diaminds, is nfcd among the jewellers for put- 

 ting the DIAMONDS into a fiercs fire, in order to divell them 

 of a yellow or brown colour. 



Burn iNG en the forehead, Front'u inujllo, was anciently the 

 penalty of a calumniator. In the middle age, we find fre- 

 quent inllances of /'Kr;,»«j in the cheek ; a punilhmcnt allotted 

 to bondmen, or villains guilty of theft. 

 BuRMNG-^'yi'. See Bush. 



BuRM.SG ,fA//j, or luniing-mlrror, a machine whereby the 

 fun's rays are coUeiled into a point ; and by that means 

 their force and cffcft extremely heightened, fo as to burn 

 objefts placed therein. 



Burcing-glaffes are of two kinds : the firfl convex, called 

 letilet caitjl'icit ; which tranfmit the rays of light, and in 

 their paifage refract, or incline them towards the axis ; 

 having the property of Icnfes, and acting according to the 

 laws of refraction. 



Tiie fecond, which are the more ufual, are concave ; 

 very improperly called burning-glafles, being ufually made 

 of metal : thefe refle6\ the rays of light, and, in that reflection, 

 incline them to a point in their axis ; having the properties of 

 fpecula or mirrors ; and acting according to the laws of 

 refleftion. See Mirror and Reflection. 



The fii-ft, or convex kind, authors fuppofe to have been 

 unknown to the ancients ; but the latter they are gene- 

 rally allowed to have been acquainted with. Their mag- 

 nifying power is taken notice of both by Seneca and 

 Pliny ; and their burning power is mentioned in a treatife of 

 Optics, afcribed to EucHd, theorem 91. It is probable that 

 the Romans had a method of lighting their facrcd fire by 

 means of reflecting concave _/^f<:u/a. But Porta, and many 

 others, fuppofe, that the burning mirrors of the ancients were 

 made of metal, in the form of a feftion of a parabola, having 

 the vertex cut off'; it being the well-known property of this 

 curve to reflect all rays that fall upon it parallel to the axis 

 into the_ focus. Hiftorians tell us, that Archimedes, by 

 means o,' reflefting mirrors, burnt a whole fleet ; and though 

 the effeft related be very improbable, yet it fufficiently 

 proves, that fuch things were then known. The machines 

 then ufed, nobody doubts, were metalhc and concave, and 

 had their focus by reflexion: it being agreed, that the 

 ancients were unacquainted with the refrafted foci of convex 

 glafles. Yet M. de la Hire has difcovered even thefe in the 

 Clouds of Ariftophanes ; where Strepfiades tells Socrates 

 of an expedient he had to pay his debts, by means of a round 

 tranfparent ftone or glafs, ufed in lighting of fires, by which 

 he intended to melt the bond; which in thofe days was 

 written on wax. The glafs here ufed to light the fire, and 

 melt the wax, M. de la Hire obferves, could not be concave; 

 fince a refleiled/ocw coming from below upwards, would 

 have been exceedingly improper for that purpofe : and the 

 old fcholiaft of Ariftophanes confirms the fentiment. Phny 

 makes mention of globes of glafs and cryftal, which being 



expofed to the fun, burnt the clothes and flefh on people's' 

 backs; and LaCtantius adds, that a glafs fphere, full of 

 water, and held in the fun, hghted the fi-e even in the coldelt 

 weather; which incontelUbly proves the efTedls of convex 

 glafles. 



Indeed, there is feme difficulty i^ conceiving how they 

 (hould know fuch j/lafTes bnrnt, without knowing th;y 

 magnified ; which it is granted they did not, till towards 

 the clofe of the thirteenth centuiy, when fpeflacles were 

 thought of. For as to thofe palTages in Plautus, which 

 feem to intimate the knowledge of fpcttacles, M. de Is 

 Hire obferves, they do not prove any fuch thing : and he 

 folvcs this, by obferving, that their 'burning glaifes being 

 fphere=, either fo'id, or full of water, their/o./ would be one 

 fourth of their diameter dillant fiom them : if then their 

 diameter were fiippofed half a foot, which is the moll we 

 can allow, an objedt mufl; be pt the diftance of an incli and an 

 half, in order to its being perceived magnified : thofe at 

 greater diltances do not appear greater, but only more 

 confuted, through the glafs than oat of it. It is no wonder, 

 therefore, the magnifying property of convex glafles was 

 unknown, and their burning one known ; it is more" wonderful 

 there fliould be 300 years between the invention of fpcCtacles 

 and telefcopes. 



Every concave mirror, or fpeculum, collefts the rays dif- 

 perfcd though its whole concavity, after reflexion, into 

 a point or focus ; and is therefore a burning mirror. 



Hence, as the focus is there where the rays are the mod 

 clolely contracted, if it be a f'egment of a large fphere, its 

 breadth mufl; not fubtend an arch above eighteen degrees : if 

 if it be a fegment of a fmaller fphere, its breadth may be 

 thirty degrees ; which is verified by experiment. As the 

 furface of a mirror, which is a fegment of a larger fphere, 

 receives more rays than another of a lefs, if the latitude of 

 each fubtend an arch of eighteen degrees ; or even more or 

 lefs, provided it be equal ; the effeds of the greater mirror 

 will be greater than thofe of the lefs. 



And, as \.h& focus is contained between the fourth and fifth 

 parts of the diameter, mirrors thgt are fegments of greater 

 fpheres burn at a greater dillance than thofe which are feg- 

 ments of a fmaller. 



Since, laftly, the burning depends on the union of the 

 rays, and the union of the rays on the concave fpherical 

 figure ; it is no wonder, that even wooden mirrors, gilt, 

 or thofe prepared of alabafter, &c. covered with gold ; nay, 

 even that thofe made of paper, and covered with ftraw, 

 ftiould be found to burn. 



Among the ancients, the burning mirrors of Archime- 

 des and Proclus are eminent ; by one of which, the Roman 

 (hips befieging Syracufe, under the command of Marcellus, 

 according to the relation of Zonaras, Tzetzes, Galen, 

 Euftathius, Anthemius, &c. and by the other, the navy of 

 Vitalian befieging Byzantium, according to the fame Zonaras, 

 were burnt to aflies. 



Many have queftioned the faft recorded by feveral hif- 

 torians concerning the furprifing effeas of Archimedes's 

 burning mirrors ; and they have principally urged the im- 

 poffibihty of producing fuch effefts by means of concave 

 fpecula, the focal dift;ance of which muft have been much too 

 fmall for the purpofe. Des Cartes, particularlv, difcredited 

 the ftory, as fabulous, on this account ; but Kircher made 

 many experiments with a view of vindicating its credibility. 

 Apprehending that the concave fpecula of the ancients were 

 fedions of a parabola, he began with combining feveral of 

 this figure ; but, failing of fuccefs in this way, he tried the 

 eff^eCt of a number of plane mirrors ; and with five mirrors of 

 the fame fize, placed in a frame, he contrived to throv? the 



rays 



