Description of a Burning Mirror. 177 
in which he gives a description of it, was translated by 
M. Dupuy, and is to be found in the Memoirs of the 
Academy of Inscriptions and _ Belles-Lettres for 1777. 
The following are his own words : 
“ Construct a Machine capable of burning at a given Distance 
- by means of Solar Rays. 
“© This problem appears impossible, if we adhere to the 
idea of those who have explained the method of construct- 
ing what are called burning mirrors: for we always find 
that these mirrors look towards the sun when inflamma- 
tion is produced ; so that if the given spot be not on the 
.game line with the solar rays, if it incline one way 
or the other, or if it be in an opposite direction, it is im- 
possible to execute what we propose by means of burning 
mirrors. Besides, the size of the mirror, which onght to 
be proportioned to the distance to which it is required to 
send the fire, forces us to acknowledge that the construc- 
tion, as explained by the ancients, is almost impracticable. 
Nevertheless, as we cannot take away from Archimedes the 
glory which is due to him, since it is unanimously allowed 
that he burnt the enemy’s ships by means of the solar rays, 
reason compels us to’ admit that the problem is possible. 
For my part, after having examined the subject, after . 
having considered it with all the attention of which I am 
capable, I shall explain the method which theory has sug- 
gested, after offering a few preliminaries. ' 
“© Al a given point of a plain Mirror, find a position, such 
as a Solar Ray coming from any inclination whatever 
strike this point, and let it be reflected at another. point 
which is also given. 
“Tet A (fig. 5) be the point given, BA the ray given, 
according to any direction ; and let it so happen that the 
ray BA, falling on a plain mirror and attached to the point 
A, is reflected at the point given IP. 
- Draw from the point A to the point T the straight line 
AY: ‘divide into two equal parts the angle BAT by the 
straight line AA, and conceive the plain mirror EAZ in a 
situation perpendicular to the line AA, it is evident, from 
what has been demonstrated, that the ray BA falling on 
the mirror EAZ will be reflected at the point ; all which 
must be executed....... 
«¢ Consequently all the solar rays equally inclined, and 
falling parallel to AB on the mirror, will be reflected by 
arallel lines at Ar. It is therefore demonstrated that, on 
Vo). 37. No, 155. March 1911. M what- 
