VOL. XLIV.] PHILOSOPHICAL TRANSACTIONS. 345 



glasses, has been considered as a thing impossible and romantic. Descartes posi- 

 tively denied the fact, which had been believed for so many ages; and our modern 

 philosophers, after many trials, and various reasonings, have been of the same 

 opinion. But M. de BufFon, being asked if it might be possible to invent a 

 phaometer, or machine for measuring the intensity of light, has discovered by 

 trial, that light was able to produce great effects in a focus at a great distance, 

 by using a great number of disks, which would reflect so many images of the 

 sun, and throw them all into one place. He put together therefore a sort of 

 polyedron, consisting of 1 68 small mirrors, or fiat pieces of looking-glass, each 

 6 inches square; by means of which, with the faint rays of the sun, in the 

 month of March, he set on fire some boards of beech wood at 1 50 feet distance. 

 By increasing the numbers of mirrors, he hopes to be able to do the same at QOO 

 feet distance. 



His machine has besides, the conveniency of burning downwards Or horizon- 

 tally, at pleasure ; and it burns either in its greater focus, or in any nearer interval, 

 which our commonly known burning-glasses will not, their focus being fixed and 

 determined. 



Perhaps this machine may afford a manner of measuring 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 comparison; for the point of 

 kindling will not determine it; because that chiefly depends on the greater or less 

 degree of inflammability of different combustible bodies. 



Mr. Maiipertuis, in a letter to the president, dated at Potzdam, May 20, 1 747, says, that hii 

 friend Buffon has recovered the burning-glasses of Archimedes; that with l68 plane glasses, each 6 

 inches square, he has melted a silver plate, at the distance of 60 feet, and fired pitched boards at 

 150. Each speculum is moveable, so as, by the help of 3 screws, to be set to a proper inclination 

 for directing the rays towards any given point. — Orig. 



On a Pirorganon, or Electrical Machine. By Jo. Henry IFinkler, Prof. Lips., 



F. R. S. N° 483, p. 497. 



This is an electrical machine, contrived to make several sparks and crackles, 

 from several different parts, and which sparks may exhibit certain imaginary 

 figures and shapes ; such as, a winged wheel, also Charles's wain, &c. But the 

 machine being extremely complex, and the uses merely fanciful and whimsical, 

 it is not deemed of any use to represent or describe it more particularly. 



On Gems or Precious Stones; more particularly such as the Ancients used to en- 

 grave on. By Robert Dingley, Esq. N° 483, p. 502. 



Gems or precious stones, of all species, are sometimes found of regular shapes, 

 and with a natural polish j and sometimes of irregular shapes, and with a rough 

 VOL. IX. Y ir 



