BURNING INSTRUMENTS. 



137 





rard'i 

 ruing 

 iror. 



consequently .in incurvation of tlu- plain glass. See 



Mirrors of this kind, with a moveable focUs, were 



(It-it by BufTon as of great use for measuring the 



the solar nys, \vlien concentrated into foci 



of different sixes. As the quantity of incident light 



and heat is nearly the same to whatever curvature 



the glass is successively bent, we might thus detrr- 



mine the size of focus by which a maximum effect 



roduced. 



liufion likewise made a number of concave mir- 

 rors, by bending plates of glass on moulds of a sphe- 

 rical form. Some of these were so large as 3, 4, 

 i feet 6, and 4 feet 8 inches in diameter ; but the ut- 

 most care is requisite in the formation of those of 

 such a largo diameter. After these glasses were 

 moulded to the proper shape in appropriate furnaces, 

 their concave and convex sides were carefully ground 

 so as to be perfectly concentric, and the convex, 

 side was afterwards silvered by M. de Bernieres. 

 Out of twenty- four mirrors of this kind which Buf- 

 fon had moulded, he was able to preserve only three, 

 the- rest having broken, either by exposure to the 

 air, or in the operation of grinding. One of these 

 three, which was 46 inches in diameter, was present* 

 cd to the king of France, and was regarded as the 

 most powerful burning mirror in Europe. The 

 other two were 37 inches in diameter, and one of 

 them was deposited in the Cabinet of Natural His- 

 tory in the Jardin du Roi. Bnffon concentrated 

 the rays of the moon by means of the mirror of 46 

 inches diameter, but, though his thermometer was 

 very sensible, no heat was perceived. 



The great defect in the large burning mirror of 

 Buffbn, and, indeed, in every combination of plain 

 mirrors, is the difficulty of adjusting each mirror, 

 so as to be always perpendicular to the straight 

 line which bisects the angle formed by the ray of 

 the sun, and the line which joins the mirror and 

 the focus of the paraboloid. The necessary motion 

 might easily be given to all the elementary mirrors, 

 by attaching each of them to a machine resembling 

 the Heliostata of Gravesende ; but the dilatation of 

 its parts, and the shakings to which it Would inevi- 

 tably be exposed, might occasion an agitation in the 

 different images, which would greatly diminish the 

 power of the machine. The only method, therefore, 

 of combining plain mirrors, is to confide each of 

 them to an individual, properly instructed, to keep 

 it in such a position as to reflect the solar rays 

 to a determinate point, and to vary this position ac- 

 cording to the motion of the sun. M. Peyrard, 

 however, properly observes, that though this might 

 be easily done by three or four persons, yet, if 100 

 or 200 individuals were thus employed, none of them 

 could distinguish the image of his own mirror from 



Burning 



!:inr<.-' 



Instead of grinding the central part of the glass plate into a convex form. Zither proposes that a small burning glass 

 Mould l)o applied to inflame the sulphur; or, what is still better than either of these pluns, a convex lens might be fastened 

 f>y the balsam of Tolii, or any transparent cement, to tho centre of the glass plate. 



M. Zuiher employed a more effectual method of bending circular plates of glass, than that which was used by Buflon. The 

 .rcular piece of glav< was placed in an iron ring, across which was fixed a thin piece of iron, with a hole containing a female 

 -rcw, so placed as to be above the centre of the glass. A strong bar of brass was also placed a. T.-S the centre of the speculum, 

 and a screw working in the centre of this, and in the female screw already mentioned, pressed the- thin iron bar against the 

 .mil bent it into the proper curvature. A plate of Venetian glas*, two Jincs thick and 20 Hbinland inches in diameter, 

 n.ts bent in this way till it protruded two lines in the middle, so as to have a focal length of 15 feet, which was a greater 

 m vature in proportion than any of Buffon's. The glass wa kept in this state for several days, without suffering any inj'irv 

 Sir A"o. Comau-nt. Pctrop. 1758, 1759, p. 250, Vote. 



TL. V. PART I. 8 



Fig. 9. 



that of the rest ; and, therefore, if a single imago were 

 removed from the focus, each of the inirror-lioUvrt 

 would naturally wish to know if it was his, and a 

 general agitation and confusion of the images would 

 be the consequence. In order to remedy tins incon- 

 venience, M. Peyrard proposes to furnish each mir- 

 ror with a suitable apparatus, so that, when it is 

 once directed to the point where the image* are to 

 be collected, the individual who has the charge of it 

 can always adjust it to the motion of the sun, with- 

 out looking beyond the apparatus i: 



The apparatus with which ach mirror is connect- Pejrrard'i 

 ed, is represented in Plate CV. Fig. ( J., where AB l>urmug 

 is a common telescope, with only one tube, contain- mi p^^ rt 

 ing the object-glass at B, and the eye-glass at A. ^V. 

 This tube is moveable on its axis, between the two 

 collars CC, C'C', which are fixed to a piece of metal 

 DD. This piece of metal is supported on a stand 

 like a common telescope, having a #ertical and hori- 

 zontal motion, by which the axis of the telescope 

 may be directed with facility to any given point. 

 The axis of the instrument is marked out by the in- 

 tersection of a pair of cross wires placed in the ante- 

 rior focus of the eye-glass ; and when this point of 

 intersection is directed to any object, the whole in- 

 strument is kept steady in its place by the screws 

 F and G; the former of which prevents any motion 

 in a vertical direction, and the latter in a horizontal 

 direction. From the middle of the tube AB rises a 

 cylindrical piece of metal MMj and upon the eye- 

 glass extremity a branch of iron HHH, wrought 

 square, is fixed firmly in a direction parallel to the 

 axis of the cylindrical piece MM. 



A plain silvered glass mirror IL, inserted into a 

 proper frame, is made to turn on two pivots, one of 

 which MI r rests on the cylinder MM, while the other 

 oo is inserted in the horizontal part of the branch 

 HH. The straight line which passes through the 

 centres of these pivots, must be exactly parallel to 

 the silvered buck of the mirror, and at right angles 

 to the axis of the telescope, and the black mark N, 

 produced by a scratch upon the silvered surface, 

 must be bisected by the axis of the mirror. 



Above the object end B of the tube is fixed a 

 plate of metal, seen in the figure, and behind this 

 plate is seen another square plate zz, on which are 

 drawn the lines xx,yy, crossing each other at right 

 angles. By means of a piece of brass fixed to the 

 last of these plates, and traversing a square hole 

 made in the other plate, the square plate may br 

 moved up and down, and from right to left ; and it 

 is kept in any position which is thus given to it, by a 

 screw on the back of the fixed platr. The moveable 

 square plate must be adjusted in such a manner that 

 the line xx may intersect the axie of the telescope, 

 and be parallel to the axis o m of the mirror. The 



