BURNING INSTRUMENTS. 



1VJ 



given combination of mirrors produced' a certain de- 

 erree of heat at a small distance, i" order to produce 

 the same degree of heat at the diM '2jM 



metres, double tlic number of mirrors would b" 

 quired ; at the distance triple the 



number would he required, and no on. 



From an examination of liufloi/K experiments, M. 

 Pt-yrard lias obtained the follow-in;-; results, respect- 

 ing the multiplication of the direct heat of the sun, 

 which is necessary to burn different substances. 



1. On the 23d of March, at noon, a plank of 

 tarred beech-wood was set on fire by 4^ times the di- 

 rect heat of the sun. 



'2. On the l()th of April, in the afternoon, a tar- 

 red plank was set on fire by 4-/ t { the eun's heat. 



S. On the llth of April, a beech plank, partly 

 burned, was set on fire by 5-{ times the sun's heat. 



4. On the same day, some small combustible sub- 

 stances were burnt by three times the sun's heat. 



5. On the same day, a large pewter flask, weigh- 

 ing 6 pounds, was melted by 11 } times the sun's heat. 



6. Some thin pieces of silver were melted, and a 

 piece of sheet iron was made red hot, by 29^ times 

 the sun's heat.-f- 



7. Silver plates melted with 37y times the sun's heat. 

 By supposing that five times the heat of the sun 



is sufficient to set fire to tarred planks, Peyrard ima- 

 gines that eight times the sun's heat will be sufficient 

 to set on fire all kinds of wood, and upon this prin- 

 ciple he has computed the following table, suited to 

 his own mirrors of five decimetres in diameter. 



At the distance of 



It would require to set fire 

 to wood 



22,25 metres - - - . 16 mirrors 



39,33 24 



53,72 32 



66,41 40 



77,86 48 



88,41 56 



98,22 64 



107,44 72 



116,16 80 



1250, or a quarter of a league - 590 

 2500, or half a league - - 2262 



In order to find the number of mirrors x necessary 

 for burning wood at any distance d, we have the fol- 

 lowing analogy : 









53.72 : 53.72 + d =r 1 : **, and 



x 



(53.72)' 



By making the height and breadth of the mirrors 

 double, triple, quadruple, &c. it is obvious that they 

 would inflame wood at double, triple, and quadruple 

 distances. M. Peyrard is of opinion, that with 5{K> 

 glasses five decimetres in diameter, he could reduce 



a fleet to ashes at the distance of a quarter of a league ; 

 that if they were a metre in diameter, he could pro- 

 duce the same effect at the distance of half a league ; 

 and that if they were two metre* in diameter, the 

 same result would be obtained at the distance of a 

 league. In this calculation, however, M. Peyrard 

 has completely omitted to take into account the great 

 diminution of effect which must necessarily arise from 

 any deviation in the surfaces of the mirrors, from a 

 perfectly plain figure, and from a want of parallelism 

 in the surfaces. Even if all the mirrors were parallel 

 glasses ground with the same care aa thos? which are 

 used for reflecting instruments, the diminution of ef- 

 fect would be enormous at the distance of a league, 

 as the angular deviation increases exactly with the dis- 

 tance. An experimental proof of this operation may 

 be obtained, by looking into a mirror at different dis- 

 tances. At a short distance the worst mirror will 

 give a tolerable representation of the observer, but as 

 the distance increase!, the distortion becomes wy 

 great. Even in the finest mirrors that are now made 

 as articles of furniture, we are confident that the 

 image will be very perceptibly distorted at the dis- 

 tance of 50 feet. This cause is, we fear, the most 

 insurmountable obstruction to the process of burning 

 objects at a great distance. By a numerous combi- 

 nation of large mirrors, it may be possible to inflame 

 wood, at the distance of a quarter of a mile ; but we 

 think that this is the greatest distance at which ef- 

 fects of this kind will ever be produced. 



Having thus given a full account of the various 

 catoptric burning instruments which have been con. 

 structed, we shall conclude this part of the article 

 with some general remarks. 



In the present state of military science, it is very ob- 

 vious that burning instruments can never be employ- 

 ed as an engine of war ; and therefore any attempt to 

 construct mirrors for burning at a distance must be 

 considered more as a matter of curiosity than of use. 

 The formation of instruments, however, for burning 

 at short distances, is a subject of the first importance 

 to science ; and we have no doubt that they will, some 

 time or other, be employed as the most powerful 

 agents in chemistry and the arts. Though catop- 

 tric burning instruments of great power have been 

 constructed, yet their effects have never been so great 

 as those of lenses, and they are besides liable to 

 numerous disadvantages. The burning point must 

 evidently lie between the centre of the mirror and the 

 sun ; and therefore the operator cannot so easily per- 

 form his experiments as when the focus lies on the 

 other side of the instrument. All his operations, in- 

 deed, have a tendency to obstruct the light before it 

 falls upon the reflecting surface. This evil will not 

 admit of a remedy, and consequently we must have 

 recourse to instruments of a dioptric or catadioptrie 

 nature, which are alone capable of uniting great power 

 with great facility of management. 



f This result enables us, in some measure, to estimate the accuracy of these deductions. As only rAi'n pieces of silver were 

 melted, we ure entitled to say, that the heat did not much exceed 28 of Wedgewood, or 3u4O" of Fahrenheit, the tens* 



pcrature at which silver melts, according to Sir James Hall's experiments. But -=r 135, which is obviously too 



9j 

 great a result for the heat of the sun. If the heat of the solar rays were 90 in April, \\hich is a sufficiently high esti- 



3910 

 mate, we have ^ = **> tne number of times that the sun's heat appears to have been increased by supposing thr 



temperature to be 3940*. 



