BURNING INSTRUMENTS. 



J35 



tiher's 

 thod of 

 aking 

 burning 

 'rrors. 



both may be in the same point according to tin 

 theory. But, perhaps, it is not necessary to be so 

 curious; for, it seems tome, that the effect would 

 scarce be sensibly less, if both sides should be ground 

 to tlit c-oncave and gage of the same tool, &c. &c." 



The attention of Sir Isaac Newtonbcing thus acciden- 

 tally directed to the subject of burning instruments, he 

 procured seven concave glass mirrors, each of which 

 was eleven and a half inches in diameter, and six of 

 these were placed round the seventh, and contiguous, 

 but SD as to have one common focus. The general focal 

 length was '2'2 inches and a half, and about an inch in 

 diameter. It melted gold in about half a minute, 

 and vitrified brick or tile in one second. The effect 

 of these specula was obviously much less than seven 

 times the effect of any one of them. The rays of the 

 sun could fall perpendicularly only on the one in the 

 middle; and, in consequence of this obliquity of in- 

 cidence, none of the specula intercepted a column of 

 rays of the same diameter, and the image formed in 

 the focus of each could not be exactly circular, f 



Burning mirrors, composed of glass, were con- 

 structed by M. Zeiher of St Petersburgh. His ob- 

 ject was to convert plates of plain glass into concave 

 mirrors, which he effected, by placing the glass 

 upon a convex tool, and exposing it to a strong heat, 

 till it assumed the exact curvature of the tool. Zei- 

 her made numerous trials with plates of various sizes, 

 and, after several failures, he succeeded in finding the 

 proper method of conducting the operation. No 

 particular difficulties occurred in giving the proper 

 shape to plates five or six inches in diameter ; but, in 

 forming one of 16 inches, the circumference was 

 moulded to the tool before the central parts, where a 

 number of vesicles of air had collected; and, in some 

 other cases, the glasses cracked after they had re- 

 ceived the proper shape. The following method is 

 that which Zeiher always found to succeed : 



A small bit of the glass to be used, must first be 

 exposed to the fire till it becomes red hot, and if, 

 after cooling, it has preserved its polish and transpa- 

 rency, the glass is fit for the required purpose, for it 

 sometimes happens that the glass becomes quite black 

 after the operation. The plate of glass is next pla- 

 ced on a concave iron dish of the required curvature, 

 and put into a furnace. Coals are placed below and 

 above the dish, and on all sides of it. The greatest 

 care must then be taken that the glass shall become 

 equally hot both at the circumference and at the 

 centre, for if the red colour should get deeper in the 

 middle, the glass will be in great danger. As soon 

 as the whole is red hot, the instant of its bending to 

 the shape of the mould must be carefully watched, 

 and when this happens, which may be observed from 

 the reflected images of the surrounding coals, all the 

 fire must be removed from above the glass, and also 

 a great part of the fire at its sides. The glass must 

 then be covered with warm ashes, that have been 

 passed through a sieve, and it must be allowed to cool 

 gradually. It is of the utmost importance to mark 



the precise moment when the glass applies itself to the 

 Mirface of the mould, for, if it remain too long, a 

 part of t , which separates from the mould, 



will adhere to the glass. When the glass is covered 

 with the hot ashes, the fire must still be allowed to 

 remain below the mould, lest the glass should crack 

 by being cooled too suddenly. When the glass is 

 taken from the furnace, its convex sides may then be 

 silvered for a burning speculum ; or, if a lens is re- 

 quired, two of the pieces of glass may be joined) so 

 as to contain a fluid. 



M. Zeiher also constructed burning glasses, by ma- 

 king a concave frame of wood, and covering the con- 

 cave surface with a paste made of flower, chalk) &c. 

 till it had the requisite degree of curvature. A num- 

 ber of pieces of silveri/cd glass mirrors, about half an 

 inch square, were then fixed upon the concave side, 

 so as to constitute a polygonal reflecting surface. 



About the same time that Zeiher was occupied 

 with these pursuits, the celebrated Buffon was enga- 

 ged in the improvement of burning instruments. This 

 distinguished naturalist directed the whole energy of 

 his powerful mind to this curious subject ; and when 

 we say that he has in a great measure exhausted it 

 by the ingenuity of his contrivances, we are not de- 

 tracting much from the merit of his successors. 



Before Buffon began to construct his great mirror, 

 he made a number of preliminary experiments, which 

 are well worthy of being recorded. He found that 

 silvered glass reflected light more powerfully than the 

 best polished metals, even than those which are em- 

 ployed for the specula of reflecting telescopes; that 

 at short distances, such as four or five feet, only one 

 half of the light was lost by reflection ; that almost 

 no light was lost by being transmitted through 100, 

 200, and 300 feet of air ; that more of the light of 

 candles was lost by reflection than the sun's light, 

 owing to the greater obliquity of the incident rays ; 

 that the image reflected from a plain mirror six 

 inches square, is at a short distance also six inche 

 square, but afterwards it augments, and then grows 

 deformed, till it at last becomes round at greater dis- 

 tances ; that a plain mirror six inches square loses 

 its square figure at the distance of 60 feet, and one 

 of a foot square at the distance of 120 feet ; that a 

 lens 32 inches in diameter and 6 inches focal length, 

 having the diameter of its focus 8 lines, melted copper 

 in less than a minute, while a small lens 32 lines in dia- 

 meter, with a focal length of 6 lines, and its focus Vr 

 or y of a line, was scarcely capable of heating copper, 

 though the two lenses had theoretically the same 

 burning power, and that a large focus was therefore 

 preferable to a small one, in the fusion of metals. Af- 

 ter having determined these preliminary points, Buf- 

 fon constructed, with the assistance of M. Passemant, 

 a very powerful mirror, which we shall now proceed 

 to describe. 



This mirror, which we have represented in Plate 

 CV. Fig. 1., was at first composed of 168 pieces of 

 plain silvered glass, six inches by eight, and having 



Uunnr.f 

 lutiru- 



I .I'!!' . 



Buffon'* 

 mirror* for 

 burning at 

 great & 

 lances. 



PLAT* 



CV. 

 Fig. 1. 



f No account of this burning glass of Sir Isaac Newton's is given in the Philosophical Transactions ; and we are informed, 

 .ipmi VITV ood authority, that no such instrument is in the possession of the Royal Society. Mr Derham, however, a Fellow 

 u the Royal Society, gives the same account which we have followed in the text. See Derhain's Astrothcologia, lib. viL 



cap i. Note. 



