366 



THE APPLICATIONS OF PHYSICAL FORCES. [BOOK iv. 



to that of Septala in size and power. We find the following in the 

 Ada Eruditorum of Leipzig upon this subject : 



" ' This mirror sets fire to green wood in a moment so that it cannot 

 be extinguished by blowing violently upon it. It can boil water 

 as to cook eggs very quickly or, if the water is left in the focus a 

 short time it evaporates. It can melt in an instant a mixture of tin 

 and lead three inches thick ; these metals commence to melt drop by 

 drop, then they run down continuously, and in two or three minutes 

 the mass is pierced. It can very soon bring a piece of iron or steel to 



a red heat, and a little after 

 can makes holes in it by the 

 force of the fire. Copper, silver, 

 &c. liquefy as soon as they ap- 

 proach the focus. It can also 

 heat to redness like iron such 

 substances as cannot melt, as 

 stone, brick/ &c." 



The mirror of Tschirnhausen 

 was three Leipzig ells (or T69 

 metres) across, its focus was 

 at a distance of 2 ells (1*13 

 metres), and. it was made of 

 thin copper ; 



A French workman at 

 Lyons, called Vilette, made 

 many large mirrors, one of 

 which fell into the possession 

 of the Academy of Sciences 

 of Paris. It was a segment of 

 a sphere of seventy-six inches 



radius, and therefore thirty-eight inches focus, its aperture was 1*27 

 metres ; the substance of which it was formed was a mixture of tin, 

 copper, and mercury. Its heating effects were comparable to those of 

 the burning mirrors above described. 



Buffon, too, in the last century, made some curious experiments 

 by using for the concentration of the solar rays, not a concave mirror, 

 but a series of plane mirrors so arranged as to reflect the rays of the 

 sun to one spot. "He formed a large mirror composed of several 



FIG. 250. A hurtling mirror. 



