BURNING GLASS, 



463 



ing.glass, each six inclips square ; by means of which, with the 

 faint rays of the sun in the month of March, he set on fire boards 

 of beech wood at 150 feet distance. Besides, his machine has 

 the conveniency of burning downwards, or horizontally, as one 

 pleases ; each speculum being moveable, so as, by the means of 

 three screws, to be set to a proper inclination for directing the 

 rays towards any given point ; and it turns either in its greater 

 focus, or in any nearer interval, which our common burning-glasses 

 cannot do, their focus being fixed and determined. M. de Bnffon, 

 at another time, burnt wood at the distance of 200 feet. He also 

 melted tin and lead at the distance of above 120 feet, and silver 

 at 50. 



Mr. Parker, of Fleet. street, London, was induced, at an ex. 

 pence of upwards of 7001. to contrive, and at length to complete 

 a large transparent lens, that would serve the purpose of fusing 

 and vitrifying such substances as resist the fires of ordinary fur- 

 naces, and more especially of applying heat in vacuo, and in other 

 circumstances in which it cannot be applied by any other means. 

 After directing his attention for several years to this object, and 

 performing a great variety of experiments in the prosecution of it, 

 he at last succeeded in the construction of a lens, of flint-glass, 

 three feet in diameter, which, when fixed in its frame^ exposes a 

 surface of 32 inches in the clear ; the distance of the focus is 6 

 feet 8 inches, and its diameter 1 inch. The rays from this large 

 lens are received and transmitted through a smaller, of 13 inches 

 diameter, in the clear within the frame, its focal length 29 inches, 

 and diameter of its focus |ths of an inch : so that this second lens 

 increases the power of the former more than 7 times, or as the 

 square of 8 to the square of 3. 



From a great number of experiments made with this lens, the 

 following are selected to serve as specimens of its powers : 



