BURNING INSTRUMENTS. 



Burning 

 Instru- 



Pf.ATE 



cv. 



Fi. 10. 



Peyrard's 



burning 



mirror. 



position of the- line yy must also be such, that its 

 distance from the axis of the telescope is equal 

 to the distance of the line IK from the same axis. 

 When the plate zz is thus adjusted, the straight line 

 yy will always be in the same plane with the line 

 IK, whatever be the position of the mirror, and a 

 line drawn from a point at N, where the axis of the 

 mirror cuts IK, to the point where yy intersects 

 xx, will be parallel to the axis of the telescope. 



The spring QQ' is fixed at Q to the arm HH, 

 and by a screw R working into its other extremity 

 Q', the end H of the horizontal arm may be made 

 to press the pivot o upon the frame of the mirror. 



The horizontal branch HH, which is represented 

 separately in Plate CV. Fig. 10., is surrounded with 

 several pieces. The piece d b and pivot o are fixed 

 in an invariable manner. The pivot oo is inserted in 

 a square hole through the piece VV, and through 

 the extremity of the arm HH. The piece db may 

 be moved either before or behind, by turning the 

 screw ; and the piece VV may be moved from right 

 to left with the piece d b, by means of the screw S. 

 The apparatus being thus constructed, the next 

 thing to be considered is the method of adjusting it. 

 In order to effect this, the axis of the mirror must 

 be perpendicular to the axis of the telescope ; the- 

 line drawn from a point near N, where the axis of 

 the mirror cuts the line IK, to the point of in- 

 tersection of xx and yy, must be parallel to IK, 

 and the straight line yy must always be in the same 

 plane with IK. 



The mirror is first placed in such a manner that 

 the line IK is at right angles to the axis of the te- 

 lescope. By turning the screw T, the lower edge 

 of the frame is made a tangent to the circular sur- 

 face MM', which is parallel to the axis of the tele- 

 scope. The screw T is then turned, in order to fix 

 the piece d b in an invariable manner. 



The axis of the telescope is next directed to a 

 point on a plane surface, placed at a certain distance. 

 This point must be situated in a vertical plane, per- 

 pendicular to the plane surface, and passing through 

 the eye of the observer and the centre of the sun. 

 A horizontal line being drawn through this point, a 

 second point is taken, as far from the first as the 

 centre of the mirror is distant from the axis of the 

 object-glass. By unscrewing S, turning the tele- 

 scope on its axis, and the mirror also about its own 

 axis, the piece VV is moved backwards or forwards 

 until the centre of the reflected image falls upon the 

 second point. The square plate z z is then adjusted 

 in such a manner that the shadow of the line IK falls 

 on the line yy, and that the shadow of NN is bisect- 

 ed by the line x x. When this happens, the plate 

 z,z is firmly fixed. Hence it follows, that whenever 

 this adjustment is made, and when the intersection 

 of the cross wires in the telescope is directed to any 

 point, the rays reflected by the mirror will be paral- 

 lel to the axis of the telescope, and will always conti- 

 nue so while the shadow of IK falls on yy, and while 

 the shadow of NN is bisected by or x. 



In making use of the mirror, the intersection of 

 the cross wues must be first directed to any point of 

 the object which is to be inflamed : the telescope 



must next be turned round in the Collars CC, C'C', Burning 

 till the shadow of the line IK falls upon y ij ; and, '"^ru- 

 finally, the mirror must be turned about its own axis 

 till the shadow of NN is bisected by the line x x. 

 The centre of the reflected image will consequently 

 fall upon a point of the object as far distant from 

 the point to which the intersection of the wires was 

 directed, as the centre of the mirror is from the axis 

 of the telescope. The image may obviously be pre- 

 served in this position as long as we choose, by keep- 

 ing the shadow of IK and N in the same position. 



In this manner M. Peyrard proposes to construct 

 every individual mirror, so that if each of them is 

 managed by one person, and directed to the same point, 

 the heat of the solar beams may be multiplied to any 

 extent. As the motion of the sun, and consequently 

 the change of position in the image, is very slow, one 

 person may direct ten or more of the mirrors, with- 

 out any injurious enlargement of the focus. 



If the object required to be burnt is in motion, 

 each mirror must be managed by two persons, one 

 of whom is constantly employed in directing the in- 

 tersection of the wires upon the moving object, 

 while the other takes care that the shadows of IK 

 and N shall fall upon their proper places. 



The attention of M. Peyrard was next directed 

 to the effects which might be expected from any 

 given combination of mirrors ; and the following are 

 the data upon which he proceeds. They are derived 

 from the experiments of Buffon. 



1. The light of the sun reflected by a plane glass 

 mirror, does not lose more than one half by reflec- 

 tion at short distances. 



2. At great distances it loses almost none of its 

 force, from the thickness of the air through which 

 it passes ; and, 



3. Its force is diminished solely in the inverse ra- 

 tio of the augmentation of the spaces which the 

 image occupies upon a plane perpendicular to the re- 

 flected rays. 



From these principles, and from the consideration 

 that the rays reflected from a circular piece of mirror 

 on a line perpendicular to its surface, form a cone 

 whose angle is 32' the diameter of the sun, M. Pey- 

 rard has computed the distances at which the reflect- 

 ed image of a mirror five decimetres * in diameter, is 

 doubled, tripled, quadrupled, thus : 



The image being 



The distance in 

 Metres is 



Double 22,25 



Triple - 

 Quadruple 

 Quintuple 

 Sextuple 

 Septuple 

 Octuple - 

 Nonuple 

 Decuple - 



39,33 



53,72 



G'6,41 



77,86 



88,41 



98,22 



107,4-4 



116,16 



If these distances were double, triple, quadruple, 

 the diameters of the mirrors, instead of being five, 

 would be ten, fifteen, twenty, Sec. decimetres. 



It follows, from the preceding table, that if any 



We have not converted the French into English measure, as it is only the ratio of the numbers that is of any use. 



