CHAP, iv.] APPLICATIONS OF THE LA WS OF HEAT. 367 



plane mirrors (he had 100) about half-a-foot square. Each of these 

 mirrors was furnished behind with three screws, by means of which they 

 might all be arranged in less than a quarter of an hour so as to reflect 

 the image of the sun to the same spot. M. Buffon, by means of this 

 compound mirror, had already been able to burn objects at a distance 

 of 200 feet." (Ejwyl.) In fact, at that distance he set fire to wood; 

 at 140 feet he melted lead, and at 100 feet silver. 



This illustrious naturalist and physicist attempted in this way to 

 realize the hypothesis of a Grecian poet Tzetzes who believed that it 

 was by this means that the Roman fleet had been destroyed at 

 Syracuse. It was intended to show, by ocular demonstration, the pos- 

 sibility of Archimedes's invention, and the patriotic act attributed to 

 the greatest geometer of antiquity. But Buffon had been anticipated 

 without his knowledge by P. Kircher, and earlier still by Anthernius, 

 the architect of S. Sophia, who must be considered as the original 

 inventor of plane jointed mirrors. 



Berniere constructed in 1759 a concave mirror of plate glass of 

 1*16 metres aperture, in the focus of which silver and even iron are 

 said to have been melted in a few seconds and flints were softened and 

 fell in drops like glass ; at least this is so stated by Daguin, in his 

 Traite (He Physique. 



We may now give some details respecting the heating effects pro- 

 duced by the refraction of a converging lens, or what is commonly 

 called a burning-glass. The same physicists who experimented with 

 mirrors tried also the effects of lenses of great dimensions. " The largest 

 lens of this sort/' says D'Alembert in his Encyclopaedia, " was that of 

 M. Tschirnhausen ; the diameter of the lens was between three and 

 four feet, its focus was at a distance of twelve feet, where the pencil of 

 rays converged to a diameter of one-and-a-half inch, and in addition, 

 in order to increase the power at the focus, the rays were converged 

 a second time by a second lens parallel to the first, placed where the 

 diameter of the cone of rays formed by the first lens was equal to the 

 diameter of the second lens, so as to receive them all." The effects 

 were similar to those of the burning mirror of the same experimenter. 



One of the most curious experiments made upon the refraction of 

 heat was that of Mariotte, who made a convex lens with a piece of 

 ice obtained by freezing pure water deprived of its contained air. 

 With this new kind of burning glass, Mariotte exploded gunpowder. 



