^S' BURNING GLASS OF ARCHIMEBE5. 



in which he employed it, are well known. These thre© 



processes, which come to the same thing, are attended with 



serious iuconTeniences. 



Neros=ary con- For a mirror to reflect to one and the same point the 



striK-.iion. pgyg Qf ^]^Q syn_^ considcrcd as parallel to each other, the 



reflecting surface must make part of that of a paraboloid of 



reTolntion, the axis of which is parallel to the yays of light, 



and its focus their point of union, If this mirror were 



composed of a great number of plane mirrors of moderate 



size, the plane of each must be parallel to a tangent of the 



paraboloid at the point where it is cut by the corresponding 



radius vector. Now in consequence of the motion of the 



sun the position of the axis of the paraboloid changes with 



Impracticable some rapidity. If the form of the mirror therefore be un- 



with glasses changeable, the whole must turn round the focns with the 



fixed in a single ,. , , , . i- ri i -p ^i 



frame. ^""? which appears to be impracticable: and if the parts 



that compose it be movable independent of each other, 

 each of these parts must turn so as to be constantly per- 

 pendicular to the right line, that bisects the angle formed hy 

 the solar ray and the corresponding radius vector. 

 With several It appears difficult to give the component mirrors the 



glasses niova- movement in question by means of a machine, less perhaps 

 difficult. because the change in the sun's declination would render 



this machine complex, than because the expansion of the 

 metallic rods, used for imparting the motion, would change 

 in a perceptible and unforeseen manner the direction of the 

 component mirrors ; and because the action of the machi- 

 nery Avould imj^art to each mirror a vibratory motion, that 

 woiild keep the image in perpetual agitation. 

 Each glass ma- There remains no other reasonable way therefore of com- 

 naged by a dif- _Qgjp„ a burning mirror of several plane mirrors, but by 

 fereiit person. ^ ^ , , ^ , , .,..,,,,.. 



entrusting each of tiie latter to an individual, charged witn 



keeping it in the proper position for reflecting the image of 

 the sun to a determinate point, varying the position agree- 

 Inconvenience ably to the motion of the sun. But Mr. Peyrard justly 

 of this. observes, that this method is attended with an inconveni- 



ence, which must prevent its success. It is easy indeed for 

 a single person, attentive and conveniently placed, to direct 

 to a point the image of the sun reflected from a mirror of 

 moderate size, and to keep it there, notwithstanding the 



motion 



