Description of a Burning Mirror. 143 
and with 128 glasses. The inflammation was very sudden, 
and it took place throughout the whole extent of the 
focus. 
At this distance the image was nearly 15 times as large. 
The heat necessary, therefore, for setting fire to this plank 
; bet 12 ; 
would be the heat of the sun multiplied by 3 z..€, the 
rx 
heat necessary would be equal to four times and 4; the 
heat of the sun. 
On the 11th of April, at a distance of 20 feet, and with 
21 glasses, a beech plank was set fire to which had been 
already partly burned. 
At this distance the image was nearly double. The heat 
necessary, therefore, for setting fire to this plank was the 
heat of the sun multiplied by a i.e, by 5 and i. 
The same day, at the same distance, with twelve glasses, 
some small combustible substances were set fire to. The 
heat necessary, therefore, for setting fire to them was the 
heat of the san multiplied by 3. 
On the same day again, at the same distance, and with 
45 glasses, alaree pewter flask was melted which weighed 
about six pounds. ‘The heat necessary, therefore, was the 
a , 2.e. by, 11 and 4. 
2x2 
With 117 glasses, some thin pieces of silver were melted, 
and a piece of shect-iron was made red hot. \ ‘To produce 
this cfiect, therefore, there must be a heat equal to that of 
: ne hie ; 
the sun multiplied by 53? he by 294. 
heat of the sun multiplied by 
‘* By subsequent experiments,”’ ‘says M. Buffon, “ T as- 
certained that the most advantageous distance to make con- 
veniently with these’ mirrors experiments on the metals, 
was 40 or 45 fect. The silver plates whieh I melted at this 
distance with 224 glasses were very clean, so that it was 
impossible to ascribe the very abundant smoke which jis- 
sued from it, to grease or to other substances which the 
silver might have imbibed, and as those persons persuaded 
themselves who were witnesses of the experiment: [ re- 
‘peated it however on plates of silver quite new, and had 
the same eflict. The metal smoked very abundantly, 
sometimes during eight or ten minutes before being melted. 
I had intended to collect this smoke by means of a head si- 
milar to what is used in distillation, and I always regretted 
that my other occupations prevented me; for this way of 
extracting water from the metal is perhaps the only one 
which 
