Thermal Emission from the Surface of a Body. 269 



Professor Ayrton's authority, in Everett's ' Illustrations of the 

 C.G.S. System of Units ' (1891 edition, p. 133) *. 



Experiments on thin wires by Messrs. Ayrton and Kilgour 

 have confirmed the fact that the emissivity can be expressed 

 empirically through a considerable range of radius in the form 

 given by Peclet ; and experiments on rods which have been 

 in progress in this Laboratory since 1891, an account of which 

 was read by Mr. Eumorfopoulos before the Physical Society 

 on the same day as this paper, lead to the same conclusion. 



It was these that first called my attention to the subject ; 

 and in order to account approximately for them and the 

 results elsewhere obtained, I propose here to examine the 

 results of supposing the loss to only in part follow the law of 

 radiation, the remainder being assumed to follow the law of 

 conduction. 



The rate of loss due to radiation will be proportional to the 

 excess of the temperature of the body above that of its 

 enclosure, and if we reckon temperatures from that of the 

 enclosure, we may write the rate due to this cause 



he , 



* A mistake occurs in Everett as well as in a paper on the same 

 subject published later by Professor Ayrton and Mr. Kilgour in the Phil. 

 Trans, for 1892, in which Everett's statement is quoted. The formulae 

 are given in Box as 



•421+ for the cylinder 



an( l 1 -047H 



•3634+ i^ZiP for the sphere, 

 r 



in which the units are the pound, foot, and hour, the radius being 

 huicever in inches. 



Translated into C.G.S. units, they become 



Cylinder : f-572+i^il X 10~ 4 , 



Sphere : -00004928+ ' 00()36Q9 . 



r 



In Everett, and in Ayrton and Kilgour's paper, the latter appears as 



■0004928+ - 0003609 

 r 



Further, it is not clear from Everett whether the formula he gives refers 

 to air-effect plus radiation or to one of these alone. The specification of 

 11 blackened sphere " would lead one to suppose that either the total effect 

 or else the radiation only is meant, since the air-effect has been shown to 

 be independent of the nature of the surface. On reference to Peclet, the 

 formula is seen to represent the air-effect alone. 



