SUPPOSED RADIATION AND REFLECTtON' OF COLD. 341 



gent of any arch and of its supplement to the whole circum- 

 ference, or 360 degrees, are equal and contrary to one an. v 

 other, or the one negative of the other. ^ 



Let t, u, w, be put for the tangents of the three arches A, Demonstratloni 

 B, C respectively, and r for the radius, and for the whole 

 circumference. Then A + B-f-C=:0,andC = - A+F. 



By trigonometry, /, A+B=!^i±hi%nd the tang. C=tang. 

 ( -- A -f- B) = - tang. A-j-B, by what has been said above. 



Therefore^,A+if,B + ^, Cor^ + M + t.=^-}-«-!l^i±i 



r'^—tii 



— tux j-~-_ ; but f and ware the expressions for the tan. 



gents of A and B respectively, and - ^ '^"^" is the expres- 



r'^—tu 



sion for the tangent of C, or for iv. Therefore, r* x t-\-u-\.w^ 



or the square of the radius multiplied into the sum of the 



three tangents of A, B, andC = ^MR', or the product of 



the tangents. Q. E. D. 



On the apparent Radiation a?id Rejection of Cold by means 

 of two concave metallic Mirrors. In a Letter from Mr. 

 John Martin. 



To Mr. NICHOLSON, 

 SIR, 



A HERE are many phenomena, exhibited to the notice Some chemical 

 of the chemical philosopher in the course of his arduous ctemly^ex^'l 

 research, that are not so well understood as perhaps the ed. 

 present state of science might lead him to expect. Some of 

 these phenomena have hitherto been totally inexplicable; 

 others have not been explained with all the clearness and 

 perspicuity that could be wished. Among the number of Apparent radi- ,! 

 the latter may be ranked the apparent radiation and reflec- ^^^'^^^^^ /'^' . . I! 

 tion of cold by means of two concave metallic mirrors. 



This 



I 11 



