[ 496 ] 
of Beech Wood at 1 50 Feet Diftance. By increafing 
the Numbers of Mirrors, he hopes to be able to do 
the fame 900 Feet off. 
His Machine has befides, the Conveniency of 
burning downwards or horizontally, as one pleafes 5 
and it burns either in its greater Focus , or in any 
nearer Interval, which our commonly known Burn- 
ing Glaffes have not, their Focus being fix'd and de- 
termined. 
Perhaps this Machine may afford a Manner of 
meafuring either Light, or the different Degrees of 
Heat of burning Bodies. The Difficulty is to find the 
Method of marking the Degrees, and of fixing a 
Point of Comparifon ; for the Point of kindling 
will not determine it; becanfe that chiefly depends 
upon the greater or lefs Degree of Inflammability of 
different combuftible Bodies, 
Mr. Maupertuh , in a Letter to the Prefident , dated at Potz - 
dam , May 20. 1747. fays, that his Friend Buffon has recover’d 
the Burning-Glafles of Archimedes ; that with 168 plane GlafTes, 
each 6 Inches fquare, he has melted a filver Plate, at the Diftance 
of 60 Feet, and fired pitch’d Boards at 150. Each Speculum is 
moveable, fo as, by the Help of 3 Screws, to be fet to a proper 
Inclination for dire&ing the Rays towards any given Point. 
XVI. 
