104« M. Maurice's Abstract of Fourier's Demonstrations 



text of M. Maurice, though I have not always translated it 

 word lor word. 



I am, my Dear Sir, yours most faithfully, 

 Greenhill, Edinburgh, Dec. 6th, 1832. James D. Forbes. 



Abstract, fyc, 



1. Law of Radiation. — "The rays of heat which issue un- 

 der different angles from the same point of the surface of any 

 body, have an intensity which decreases proportionally to the 

 sine of the angle formed by their direction with the plane tan- 

 gential to the surface, at the point of emission." 



Demonstration. — Let AB, (fig. I.) be the mathematical sur- 

 face of the body; and AC the thickness of its physical surface. 

 We shall consider a normal ray of heat CA, and conceive 

 that C is the point furthest from the surface capable of emitting 

 any heat whatever by radiation. Consequently, as weadvance 

 from C to A, the particles radiate heat more copiously. 



Fig. 1. 



Fi< 



B, 



Let us assume the quantity of heat to be known which is 

 furnished by each point « situated in the normal direc- 

 tion, and that it is represented by the respective abscissae /3 n 



.am referred to the axis AB: we may conceive such a 



curve as Cnmo . . . . passing through their extremities ; and 

 the sum of these abscissae, or the area of the curve, will ex- 

 press the intensity of the ray of heat normal to the surface. 



We must next* consider the intensity of a similar ray in the 

 direction a y, inclined to the mathematical surface at an an- 

 gle <p. Let us consider first the point a of the normal ray. 



