144 Description of a Burning Mirror. 
which we could employ: and if it is said that this smoke; 
which appeared to me to be humid, does not contain waters 
it would at any rate be useful to know what it is, for it 
may be merely volatilized metal : besides, I am persuaded 
that, by making the same experiments on gold, we shall 
sce it smoke like silver, perhaps more, perhaps les.” 
At the distance of 40 feet the image is triple : the heat 
necessary, therefore, for producing this effect id equal to 
that of the sun multiplied by ai i.e. by 37 and 4. 
Thus, - setting out from the imperfect experiments ‘of 
Buffon, five times “the heat of the sun would be more than 
sufficient for setting fire to tarred planks. I suppose that 
eight times this heat is sufficient for setting fire to all kinds 
of wood, and sutely so much heat would not lye requisite. 
It follows from this supposition : ate 
Ist, That at a distance of 22™,25, it would require 16 of 
my glasses to set fire to rete 
2d, At a distance of 39",33, it would require 24, 
, Ata distance of 53,72, “it would require 32. 
od At a distance of 66",41, it would require 40. 
5th, At a distance of 777,86, it would require 48. 
6th, At a distance of 88,41, it would require 56. 
7th, At a distance of 98™,22, it would require 64. 
8th, At a distance of 107,44, it would require 72 
oth, At a distance of 116™,16, it would require 80., 
JOth, At a distance of 1250 metres, é. c.. a quarter of 3 
league, it would require 590%. 
llth, At half a league, it would require 2962. 
If the height and breadth of the lasses Geuhite double, 
triple; quadruple, &c. it is evident that they would inflaine 
at double, triples quadruple distances. Thus 590 glasses of 
ametre in height would produce the same effect .at half 4 
Jeague, and glasses of two metres in height at one league : 
but | deceive myself, the effect would be much greater. 
If we used glasses of a metre in height, the focus at a 
distance of a quarter of a league would be 24 metres in 
height and in breadth. I am “of opinion, that with, 59u0 
glasses five decimetres high we might reduce to ashes a 
fleet at the distance of a “quarter of a league; at half a 
league, with 590 glasses of a metre in height ; and ata 
league, with 590 glasses two metres in height. 
* In order to calculate how miny glasses are requisite at this distance, we 
form the followi ing PIPRPEHION fy 
2 
m 2 
(58 79) (53 y724-1250) 2: Lj. 
and we find for the four ‘th term 590 minus 4 fraction. ‘ 
Instead 
