BURNING INSTRUMENTS. 



;, the space occupied by the reflected light is 

 always greater than the surface of the mirror, and 

 this s;>acr increases with the distance of the object. 

 The heat which is lost by reflection does not much 

 cd one-half of the direct heat of the lolar rays ; 

 so that two plain mirrors, which reflect the sun's 

 light upon an object moderately distant, will pro- 

 duce a degree of heat nearly equal to the direct neat 

 of the sun. By increasing, therefore, the number of 

 mirrors, the reflected heat of the sun may be increa- 

 sed to any assignable magnitude. If the distance 

 of the object is increased, the space occupied by the 

 reflected rays from each mirror will also be increa- 

 sed ; and, in proportion to this augmentation, the in- 

 tensity of the heat will be diminished ; but this dimi- 

 nution of the heat may be compensated by increasing 

 the number of the mirrors. Hence it is possible, 

 theoretically speaking, to produce any degree of heat 

 at any given distance. In practice, however, the 

 distance at which combustion may be effected is li- 

 mited by several causes. While the reflected rays 

 pass through the atmosphere, their force is consider- 

 ably diminished by the density of the medium ; and 

 een when the plain mirrors are formed with the ut- 

 most care, they are still by no means free from irre- 

 gularities on their surface. The angular deviation 

 produced by these irregularities increases with the 

 distance at which the reflected rays are received ; and 

 at the distance of a mile, the reflected light, instead 

 of forming one large image, would be separated into 

 various detached portions. 



Burning The next burning mirror that seems to have been 



f constructed after that of Archimedes, was contrived 

 ? ' by Anthemius of Tralles, who flourished towards 

 the end of the fifth century. This learned architect 

 was a favourite of Justinian, who employed him in 

 the construction of various edifices, but particularly 

 in the church of St Sophia at Constantinople, which 

 lie designed and executed. He was also the disciple 

 of Proclus, from whom he probably received some 

 information respecting the construction of burning 

 mirrors, of which he has given a full and distinct ac- 

 count in a fragment entitled n irot$*<j%ui ftr,^^- 

 utcrtuv. " Of Wonderful Machines," which has been 

 translated and illustrated by M. Dupuys, in the Me- 

 moires dc I' Academic des Inscriptions, 1777, torn, 

 xlii. p. 392 451. The following quotation from this 

 fragment, will give the reader a complete view of the 

 invention of Anthemius, who appears to have been 

 completely master of the subject upon which he wrote. 

 After acknowledging, that it was universally admit- 

 ted in his time, that Archimedes had destroyed the 

 Roman fleet by means of burning mirrors, Anthemius 

 observes, " Let u, therefore, bring and collect at 

 one point other different rays, by means of plain and 

 similar mirrors, in such a manner that all these rays, 

 united after reflection, may produce inflammation : 

 This may be effected by means of several persons 

 holding mirrors, which, according to the positions in- 

 dicated, send the rays to one point. 



" But, in order to avoid the embarrassment result- 

 ing from entrusting this operation to several persons, 

 (for we shall find, that the matter intended to be 

 burnt does not require less than twenty-four reflec- 

 tions,) the following construction must be followed: 

 Let there be a hexagonal plain mirror, and other 



adjoining similar mirror j attach, d to the sidel of the 

 hexagonal mirror by the smallest diameter, so that 

 they may be moved on these line* by means of plates 

 or bands applied, which unite them to each other, or 

 by means of what arc call. d lunges. If, therefore, 

 we bring the surrounding mirrors into the same plane 

 with the mirror in the centre, it is clear, that all the 

 rays will undergo a reflection similar, and conformable 

 to the common position of all the parts of the instru- 

 ment. But if, the centre mirror remaining as it were 

 immoveable, -we dexterously incline upon it all the 

 other mirrors which surround it, it is evident that 

 the rays reflected by them will tend towards the 

 middle of the place where the first mirror is directed. 

 Repeat the same operation, and around the mirror, 

 already described placing other similar mirrors, all of 

 which may be inclined towards the central mirror, 

 collect towards the same point the ray > which 

 send, so that all these united rays may excite inflam- 

 mation in the given spot. 



" But this inflammation will take place better, if 

 you can employ for this purpose four or five of these 

 burning mirrors, and even seven, and if they are all 

 at the same distance from the substance to be burnt, 

 so as that the rays which issue from them, mutually- 

 intersecting, may render the inflammation more con- 

 siderable : For, if the mirrors are all in one place, 

 the rays reflected will intersect at very acute angles; 

 so that all the place around the axis being h< .. 

 the inflammation will not take place at the single 

 point given. 



" It is therefore possible, by means of the burning 

 mirrors just mentioned, to carry inflammation to a 

 given distance. Those who have made mentio 

 the mirrors constructed by the divine Archinu- 

 have not said that he made use of a single bur 

 mirror, but of several ; and I am of opir.iu: . t'.-r. 

 there is no other way ef carrying inflammation tu 

 any distance." 



From the demonstration vrhich Anthemius pro- 

 ceeds to give, but which we have not room for in- 

 serting, it appears that his mirror was constructed on 

 exact geometrical principles, and was truly parabolic. 



About the end of the sixteenth century, when an Burninc 

 ardour for science began te prevail, we find the sub- mirror* f 

 ject of burning mirrors again brought into notice. LeonharA 

 Our countryman, Leonhard Digges, in a work en- 

 titled Pantometria, published in London in 1571, and 

 republished by his son Thomas Digges in 1591, men- 

 tions a burning mirror, which appears to have consist- 

 ed of a combination of plain mirrors. In the Preface 

 to the second edition, Mr Thomas Digges observes, 

 " Archimedes also (as some suppose,) with a glasse 

 framed by reuolution of a section parabolical!, firs. J 

 the Roman nauie ia the sea, commmg to the siege of 

 Syracusa. But to leave these celestial causes, and 

 things donne ofantiquitie, longagoe, my father, hath 

 at sundrie times, by the sunne beames, fired pofc der and 

 dischargde ordinance half a mile and more distante ; 

 which things I am the boulder to report, for that 

 there are yet liuing diuerse of these his doings (ocit~ 

 lad testes, eye witnesses,) ar.d many other matters 

 farre more strange and rare, which I omit as imper- 

 tinent to this place." 



In the 21st chapter of the first book, the subject 

 of burning glasses is resumed. " Sonae hi 

 4 



