134 Description of a Burning Mirror. 
manner. Tt is necessary, therefore, if the form of the mirrdr 
be invariable, that this mirror should turn entirely with the 
sun around the focus ; which appears impracticable: and if 
the elements of which it is composed are moveable, inde- 
pendently of each other, each of these plain elements must 
turn in such a way as to be constantly perpendicular to the 
straight line, which divides into two equal parts the angle 
formed by the ray of the sun and the corresponding vector 
ray. 
It seems difficult to give to the elementary mirrors the 
Motion in question, by means of a machine, Jess perhaps 
on account of the changes of declination of the sun ren- 
dering the machine complicated, than because the dilatation 
of the metallic rods which would transmit the motion, 
might change in a remarkable and unforeseen manner the 
directions of the elementary mirrors, and because the inevi- 
table shakings would give to these mirrors a vibratory mo- 
tion, which would keep the individual images in a perpe- 
tual agitation. 
‘There remains therefore no other reasonable method of 
composing a burning lens of several plain mirrors, than to 
confide each of them to a person individually instrocted 
how to keep it in the position in which it ought to be in 
order to reflect the image of the sun on any “determinate 
point, and to vary this position conformably to the motion 
of the sun. But M. Peyrard truly observes, that this me- 
thod is attended with an inconvenience which completely 
opposes iis success. It is very easy, in truth, fora single 
attentive person, conveniently placed, to aaa on a given 
point the image of the sun reflected by a mirror of middling 
size, and to keep it there in spite of the motion of the sun: 
nay, the difficulty would not be great for three or four per- 
sons to do the same thing at once. But if 50, 100, or 200 
persons were to form in this manner a burning focus, as 
none of them could distinguish the image which he sends 
from that which is sent by the others,—if a single one 
among those images was removed from the focus, each of 
the performers would wish to ascertain if it was hig: an 
agitation and confusion would hence ensue, which would 
hinder the focus from being formed. It is thisinconvenience 
which M. Peyrard has-set about remedying, and he obviates | 
jt ina very ingenious manner. With this view he fur- 
nishes each of ‘his mirrors with a simple apparatus which 
we shall now deseribe. 
A smal] glass supported on a trivet, and furnished wiithe 
two wires W shich cross each other at the focus of the glass, 
may 
