182 Description of a Burning Mirror. 
- Take a parabolic conoid, the axis of which is constantly 
directed to the centre of the sun: let us suppose that some 
plain glasses are tangent to this conoid, and suppose that 
this conoid is cut by a vertical plane which passes by its 
axis. If we cut this conoid by a plane perpendicular on 
the axis, we shall have, on the side of the summit, a burn- 
ing mirror composed of plain glasses which will not in- 
flame an object, except in so far as it is placed directly be- 
tween the mirror and the sun. If we cut the conoid by a 
plane which is perpendicular on the vertical plane, and 
which passes between the sun and the zenith, the upper 
segment will give a burning mirror which will set fire to 
an object from top to bottom; and the other segment will 
give a mirror which will set fire to it from bottom to top, 
provided this object is in the vertical plane of which we 
have been speaking. Lastly, let us suppose that the sect- 
ing plane is not perpendicular on the axis, and that it forms 
with the horizon an acute angle, whether the secting plane 
passes by the axis, or whether it cuts ordoes not cut the axis, 
one of the burning mirrors which will result from this 
section will inflame from top to bottom, the other from 
bottom to top, any object which is placed to the right or 
left of the sun, and this is the case with the mirror of An- 
themius as well as with that of Buffon. 
This being granted, let us return to the burning mirror 
of Archimedes. © Antheimius relates that in the descriptions 
which the ancient authors gave of burning mirrors, wé 
alwavs find that these mirrors faced the sun, when the 
inflammation was produced, and that the object set fire to 
was neither to the right nor to the left. Hence I con- 
clude, that the mirror of Archimedes was one of the seg- 
ments of the parabolic conoid just mentioned, when the 
secting plane is perpendicutar on the vertical plane. 
Tzetzes informs us that the mirror of Archimedes was a 
collection of hexagonal mirrors, which moved by means of 
their hinges and certain plates of metal, 7. e. that the mir- 
rors of Archimedes were arranged in such a way that each 
could be moved in every direction, as in Buffon’s mirror; 
and so far the mirror of Buffon does not differ from that of 
Archimedes, except in so far as the mirrors being rectan- 
gular in the former and hexagonal in the latter. ~ 
Tzeizes adds that Archimedes placed his mirror in the 
midst of the solar rays at noon day: 7. e. he placed his 
mirror perpendicular to the plane of the equator. If the 
mirror of Archimedes had nat been intended to produce 
inflam- 
