[ 495 ] 
to do any. more for the Dimenfions of themfelves 
are purely arbitrary. If you have a clear Idea of the 
Conftrudion of the Machine by this general Defgrip? 
tion, it is all I have aimed at ? 
XV. ExtraEl of a better from the Marquis 
Nicolini F.R, $. to the Proficient, concern- 
ing the fame Mirror burning at 150 Feet 
Di fiance. 
SIR, 
* * % 
Read April 30. YOU know that the Affair of Archi- 
* 747 ' medes fctiin° the Roman Fleet on Fire by 
means of Burning-Glaffes, has been look'd upon as a 
Thing impoffible and romantic. R)efcartes pofi- 
tively denied the Fad, which had been believed for 
fo many Ages 5 and our modern Philofophers, after 
many Trials, and various Reafonings, have been of 
the fame Opinion. But M. de Buffon , being asked 
if it might be poflible to invent a Rhaometer, or 
Machine for mcafuring the Intenfity of Light , hath 
difcovered by Trial, that Light was able to produce 
great Effeds in a Focus at a great Diftance, if one 
made ufe of a great Number of Disks, which would 
refled fo many Images of the Sun, and fling them 
all into one Place. . He pur together therefore a fort 
of Rolyedron , confiding of 168 fmall Mirrors, or 
flat Pieces of Looking-glafs, each 6 Inches fquare 5 
by means of which, with the faint Rays of the Sun, 
in the Month of March, he fet on Fire feme Boards 
