lit 



BURNING INSTRUMENTS. 



from the rest, the coincidence of its focus with that of 

 the other may be easily effected by varying its distance 

 from F. The whole spherical surface, whose section 

 is AMR, except a small opening for admitting the ob- 

 ject to be fused, may be covered with lenses, having 

 nil their foci coincident at F ; though it will, perhaps, 

 be more convenient to have the posterior part MN 

 without lenses, and occupied by a mirror of nearly 

 the same radius FA as the sphere. The object of this 

 mirror, is to throw back upon the object at F the 

 light that passes by it, without producing any ef- 

 fect. Each of the lenses, except the lens A, is fur- 

 nished with a plane glass mirror^ which may be ei- 

 ther fixed to the general frame of the sphere, or pla- 

 ced upon a separate stand. When this combination 

 is completed, the sphere is exposed to the sun, so 

 that its rays may fall at right angles upon the lens 

 A, which will, of course, concentrate them at F, 

 and produce a pretty intense heat. The plane mir- 

 ror PQ, when properly adjusted, will reflect the 

 sun's light perpendicularly upon the lens B, by 

 which it will be refracted accurately to the focus F, 

 and produce a degree of heat fully one-half of what 

 was produced by the direct refracted rays of the 

 aun through the lens A. A similar effect will be 

 produced by the mirror RS and lens J), the mirror 

 TU and lens C, the mirror VW and lens E, and by 

 all the other mirrors and lenses which are not seen 

 in the section. The Affect may be still farther in- 

 creased by the addition of a large lens at XX. As 

 the angle which the surface of each mirror forms with 

 the axis of its corresponding lens is a constant quan- 

 tity, the mirrors may be all fixed to the general 

 frame of the .sphere, and therefore the only adjust- 

 ment which the instrument will require, is to keep 

 the axis of the lens A parallel to the direction of the 

 solar rays. 



In order to estimate the advantages of this con- 

 struction, let us compare its effects with those of 

 a solid lens, which exposes the same area of glass to 

 the incident rays. 



1. In theburning sphere, almost the only diminution 

 of light is that which arises from reflection by the 

 plane mirrors, and which may be estimated pretty accu- 

 rately at one half of the incident light ; but this loss 

 can be amply compensated by adding a few more 

 lenses. 



2. In the solid lens, a great diminution of light 

 arises from the thickness of the eentral portions, and 

 from the obliquity of the parts at the circumference ; 

 which, we conceive, will be fully equal to the light 

 lost by reflection in the burning sphere. 



3. In the burning sphere, the lenses may be ob- 

 tained of much purer glass than can be got for a so- 

 lid lens ; and therefore, cceteribus paribus, they will 

 transmit more light. 



1. Owing to the small size of each lens in'the 

 burning sphere, the diminution of effect arising both 

 from spherical aberration and from the aberration of 

 colour, will be very much less than in the solid lens. 



5. In the burning sphere, the effect is greatly in- 

 creased, in consequence of the shortness of the focal 

 length of each lens, and the greater concentration of 

 the incident light. 



6. In the burning sphere, all kinds of lenses may 

 be combined. They may be made of any kind of 



glass, of any diameter, and of any focal length ; and Burning 

 the lenses belonging to different individuals may be Instru- 

 combined for any occasional experiment, in which a ' 



great intensity of heat is requisite. 



T iU < fit i- . , Portable 



In the application or the preceding principle to a burning 



portable instrument, the different mirrors may be all pyramid, 

 cemented together, so as to form the frustum of a 

 pyramid, having as many planes as there are lenses 

 surrounding the central lens. A section of this py- 

 ramid is shown in Fig. 6., and a plan of it in Fig. 

 7. Each plane glass which composes the pyra- 

 midal mirror is so placed as to reflect the incident 

 rays upon the lens B or C which is adjacent to it j 

 while the central lens A converges to the same focus 

 as the rest the direct light of the sun which traver- 

 ses the axis of the pyramid. Thus in the plan, Fig. 

 7., the direct light of the sun falling upon the lens 

 A, is refracted to the focus <p, in Fig. 6. ; the light 

 which falls upon the mirror B, is reflected perpendi- 

 cularly upon the lens b, which refracts it to the same 

 focus <p ; the light which falls on the mirror C, it 

 reflected upon the lens c, and so on with the rest ; so 

 that a whole column of rays, nearly equal in mag- 

 nitude to the polygonal surface BGDEFG, Fig, 7., 

 is concentrated to one point ^. Instead of the glass 

 mirrors, a conical frustrum of tin or of silvered cop- 

 per might be advantageously employed. 



For farther information upon burning instruments* 

 see Wolfii Opera Mathgma:ica,tom. ii. p. 165 167. 

 Traberus, In Nervo Optic, lib. ii. cap. 12. prop. 5. 

 cor. 1. Vilette, Journal des Savans, 1666, p. 125, 

 1st ed.; p. 71, 2d ed. Ibid, 1679, p. 281. Phil. 

 Trans. No. 6. p. 95. Do. No. 33. p. 631, Do. 

 No. 10. p. 795. Do. 1719, vol. xxx. No. 360. p. 

 976. Tschirnhausen, Phil. Trans, vol. xvi. 1687, 

 No. 188, p. 352; and Act. Erudit. 1687, Jan. p. 52. 

 Hoesen, Phil. Trans. 176'8, vol. lix. p. 1, La Ga- 

 rbuste, Mem. Acad. torn. i. p. 276. Buffon, Sup- 

 plement a llistoire Naturelle, torn. ii. 8vo. Par. 1771. 

 Sixicme Memoirs, p. 181 309- Dutens DH Miroir 

 Ardent d'Archtniede, Paris 1755. A Description of 

 the great Burning Glass mane by M. Vileiie and hts 

 two Sans, born at Lyons, with -some Remarks upon 

 the surprising and wonderful l^ffecls thereof, London, 

 1719. See also Mem. Acad. 1717. Zeihor, Nov. Com. 

 Peirop. torn. vii. 1758, 1759, p. 237- Riohman, 

 Nov. Com. Petrop. vol. iii. p. 310. Nollet, Mem. 

 Acad. Par. 1757, p. 896. Kircher's Ars Magna 

 Luc/s et Umbrce, p. 772. Courtivron, Mem. Acad. 

 Par. 1717, p. 659. Trudaine, Mem. Acad. Par. 

 1771, p. 62. Journal Encyclopcdique, 15th Sept. 

 1777. Cadet and Brisson, Mem. Acad. Par. 1777, 

 p. 541. Dupuy's Mem. Acad. Iiiscript. 1777. () 

 BURNLEY, a market town in Lancashire, stands 

 on a peninsula, formed by the rivers Colder and 

 Brown, 213 N. of London, and at a small distance 

 from the Leeds and Liverpool Canal. Some consi- 

 derable manufactories of linen aud cotton are esta- 

 blished on the adjacent rivers ; and the vicinity a- 

 bounds with excellent pit-coal and quarries of free- 

 stone, flags -and slate. It has a handsome chapel of 

 ease, and two dissenting meeting-housee. There are 

 five annual fairs held in Burnley, and a weekly mar- 

 ket on Monday. Number of houses 682, inhabitants 

 3305, of whom 910 are employed in trade and ma- 

 hilfactures. (/?) 



