462 BURNING GLASS. 



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and Lactanlius have also spoken of glasses that burnt bj refrac- 

 tion. The former calls them balls or globes of glass, or chrystal, 

 which expo>ed to the .-un, transmit a heat suth'ci'-nt to sn fire to 

 cloth or corrode away the dead flesh of those patients who stand 

 in need of caustics ; and the latter, after Clemens Alexandrinus, 

 takes notice fhat fire may be kindled, by int. rposing glasses filled 

 with water, between tho sun and the object, so as to transmit the 

 rays to it. [Dutens. 



Among the moderns one of the earliest who devised a burning 

 mirror, was the celebrated Lord Napier, the in?entor of logarithms, 

 who, in a paper containing hints of secret inventions, dated June 

 2, 1596, (the original of which is now among the MSS. in the 

 Lambeth library, marked 658, anno 1596), says, 



" First, The invmtion, proof, and perfect demonstration, geo- 

 metrical and algebraical, of a burning mirror, which receiving of 

 dispersed brains of the sun, doth reflex the same beams altogether 

 unift d, and concurring precisely in one mathematical point, in the 

 which point, most necessarily it engendereth fire ; with an evident 

 demonstration of their error who affirm this to be made a parabolic 

 ection. The use of this invention serveth for the burning of the 

 enemy's ships at whatsoever appointed distance. 



" Secondly, The invention and sure demonstration of another 

 mirror, which receiving the dispersed beams of any material fire, 

 or flame, yieldeth also the former effect, and serveth for the like 

 use." 



Of the moderns, the most remarkable burning-glasses, are those 

 of Magine of 20 inches diameter ; of Sepatala of Milan, luar 42 

 inches diameter, and which burnt at the distance of 15 feet ; of 

 Settala, of Vilette, of Tchirnhausen, of Buffbn, of Trudaine, and 

 of Parker. 



That of M. de Villette was three feet eleven inches in diameter, 

 and its focal dis'ance was three f-et two inches. Its substance is 

 a composition of tin, copper, and tin. glass. Some of its effects, 

 as found by Dr. Harris and Dr. Desaguliers, are, that a silver six. 

 pence melted in 7{"j a King fieorge's halfpenny melted in 16", 

 and i-.n in 34"; fin melted in 3", and a diamond weighing 4 grains, 

 lo*f J'lis o f its we ; ht. 



1 .:.it of M. de Hnllbn is a polyhedron, six feet broad, and as 

 many high, consisting of itib small mirrours } or flat pieces of look. 



