Dimensions of a Body on its Thermal Emission. 279 



•04:7 x 10~ s . As their thin wires blocked up the enclosure 

 less than the thick rod here employed, it is natural that the 

 value obtained from them should be greater than that given 

 above. 



The formula which I have given for a cylinder appears to hold 

 through a far wider range than that given by Messrs. Ayrton 

 and Kilgour. Take for example their formula for a wire 

 at 100°. The emissivity for it can never (for any radius) fall 

 lower than 



•00107. 

 Calculating from my formula we get 



•00036 

 for a rod of '483 centim. radius at 100°. On reduction to 

 60° it becomes about 



•00025, 



which is much more within sight of the value "000154: which 

 is obtained from fig. 3. Considering the violent nature of the 

 extrapolation here made, the agreement is probably as close as 

 could reasonably be expected. 



Conclusions. 



i. That the theoretic assumption made in this paper gives 

 results which are accordant through a wide range of radius of 

 rod with experimental results obtained under similar con- 

 ditions as regards enclosure : and in this respect is far 

 superior to the usual assumption which gives no account 

 whatever of variation of the value of emissivity with radius. 



ii. That if the freedom permitted to convection effects be 

 varied, as it will be if the enclosure be changed, it is necessary 

 to consider the convecto-conduction constant as varying with 

 the changed conditions according to a law which can only be 

 found by a complete series of experiments made with en- 

 closures of different dimensions. 



iii. I conclude that the enclosing boundary is as important 

 a factor in determining the value of the emissivity as the size 

 of the body itself : and that therefore in any collection of 

 data (such as Everett's) it is very necessary to specify the 

 exact nature of the enclosure in which the experiments 

 were conducted; and, further, that all determinations that 

 have been made of this constant and published with imperfect 

 description of all the boundaries are of little scientific value. 



Finally, I must express my thanks to Prof. G. C. Foster 

 for kindly advice and suggestions given from time to time in 

 connexion with this matter. 



University College, London, 

 January 1st, 1895. 



U 2 



