80 



HEAT OR CALORIC. 



" Some cotton, imbued previously with phosphorus, is supported 

 by a wire over a candle wick, placed as nearly as possible, in the 

 focus of one of the mirrors. A lamp being similarly situated with 

 respect to the other mirror ; by receiving the focal image of the 

 flame on any small screen, it will be seen in what way the arrange- 

 ment must be altered to cause this image to fall upon the phos- 

 phorus." 



" The screen S, placed between the mirrors, is then lowered so 

 as to intercept the rays. The iron ball being rendered white hot, is 

 now substituted for the lamp, and the screen being lifted, the phos- 

 phorus takes fire, and the candle is lighted." 



" Description and construction of the mirrors. The mirrors rep- 

 resented by the figure, are sixteen inches in diameter, and were 

 turned in the lathe, the cutting tool being attached to one end of an iron 

 bar two feet long, which at the other end turned upon a fixed pivot." 



" Of course the focal distance, being one half the radius of con- 

 cavity, is one foot." 



" I designed these mirrors, and proposed to have them made out 

 of castings ; but pursuant to the advice of Dr. Thomas P. Jones, I 

 resorted to sheet brass, which was rendered the more competent by 

 strengthening the rims with rings of cast brass, about three fourths of 

 an inch thick each way. For the idea of these rings, and the execu- 

 tion of the mirrors, I am indebted to Mr. Jacob Perkins." 



" I believe there are none superior, as the face is reflected by them 

 much magnified, but without the slightest distortion." 



" For die rationale of the operation of the mirrors, I refer to the 

 preceding article." 



11. Diversity of radiating power in metals, wood, charcoal, glass t 

 pottery, fyc. 



E 



