Description of a Burning Mirror. 183 
inflammation, except at the moment when the sun was ina 
plane perpendicular upon the plane ofthe mirror and the plane 
of the horizon, it is evident that it would have been indif- 
ferent whether this mirror was or was not placed perpen- 
dicularly on the plane of the equator. But wherefore did 
Archimedes place his mirror perpendicularly on the plane 
of the equator? It was in order that his mirror might re- 
flect the solar rays on the same object during the whole 
time that the sun was on the horizon, and I shall show 
that the mirror thus placed was capable of producing this 
effect in two different ways. 
Let AB (fig. 8) be an iron rod parallel to the axis of 
ihe globe. Let CD be an iron rod perpendicular upon 
AB; let EF be the mirror of Archimedes, and let it 
be placed in such a way that the iron branch CD is per- 
pendicular on its plane wheu prolonged. It is evident that 
this mirror, when so placed, will be perpendicular on the 
plane of the equator. Let us suppose that by means of a 
screw, as we see in fig. 9, we can move the iron rod AB 
on itself. This being done, let any person, on turning the 
screw, be desired to keep the mirror in a position perpen- 
dicular on the vertical plane which’ passes by the axis of the 
iron rod AB and by the centre of the sun, and that an- 
other person is employed to adjust the mirror in such a’ way 
that the images reflected may be carried to a point D, 
taken on the iron rod CD. 
If during the whole day we retain by means of the 
screw the mirror in a position perpendicular on the vertical 
plane which passes by the axis of the iron rod AB and by 
the centre of the sun, itis evident that the images reflected 
at the point D will remain there fixed, without vibrating and 
without displacing the focus : for, if the contrary happened, 
it would be because in the space of twelve or fifteen hours 
the sun would approach or remove from the equator ina 
sensible manner; which is not the case. ; 
In the second place, let there be a piece of iron ACDEB 
(fig. 9): let its extremities AC, EB, be cylindrical, and let 
the part CDE be flattened and bent into a semicircle; let 
the axes of the cylinders AC, EB, be in the straight line 
AB, and let this straight line be parallel to the axis of the 
earth; let the piece of iron ACDEB be moveable around 
the axis AB, and let LI be a screw; let DK be the mirror 
of Archimedes; let this mirror be placed parallel to AB 
and perpendicular to the ag which passes by the axis of 
4 the 
