142 Description of a Burning Mirror. 
simplify the calculations, I shall in the first place, suppose 
that, when Buffon made his experiments, each of the glasses 
of his mirror produced an effect equally great with what 
would have been done by a circular glass of the same.sur- 
face, on which the solar rays would fall perpendicularly. 
I shall afterwards suppose, that all the images reflected by 
the glasses of his murror were applied exactly upon each 
other, 
But it is beyond a doubt, that each of the glasses of 
Baffon’s mirror produced an effect smaller than that which 
would have been produced by a glass on which the solar 
rays would have fallen perpendicularly ; for, the solar rays 
falling obliquely on the glasses of his mirror, it is evident 
that the quantity of the rays reflected was smaller than it 
would have been if the solar rays had fallen perpendicularly 
on the glasses; and I shall presently show that with Buffon’s 
mirror it 18 impossible to throw precisely the images of the 
sun upon each other. It. follows therefore, that by taking 
as a basis the experiments of Buffon, my results will be too 
great. ; 
On the 23d of March, at mid-day, Buffon at a distance 
of 66 feet set fire to a plank of tarred beech-wood, with 
forty glasses, the mirror forming with the sun an angle of 
nearly 20 degrees of declination, and another of more than 
10 degrees of inclination. 
On examining the table tna preceding page, we shall find 
that at this distance the ivpage was quintuple ihe surface of 
the mirror. Thus the fifth part of 40 glasses, i. e. eight 
glasses, would have produced the same effect at a very small 
distance, But at avery small distance the heat of the 
image reflected is the half of the heat of the sun; there- 
fore, four times the heat of the sun would set fire to a plank 
of tarred beech-wood. [suppose in this experiment, as well 
as in those which follow, that the number of glasses only was 
employed necessary for producing inflammation or fusien. 
On the same day, the mirror being placed still more dis- 
advantageously, he set fire to a plank tarred and sulphured 
126 feet distant, with 98 glasses. 
At this distance, the image reflected was nearly twelve 
times as large. The heat necessary therefore to set fire to 
this plank would be the heat of the sun miutiplied by 
Pee z.¢. the heat necessary for that would be equal to 
tour times.and 1. the heat of the sun. 
On the 10th of April. in the afternoon, with a clear sun, 
a tarred plank was set fire to at the distance of 150 feet, 
and 
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