174 



Sir W. Siemens. 



[Apr. 26, 



to some power of the absolute temperature, the second niay for the 

 present be represented by m¥(t). Hence we have — 



Rate of conversion of energy = AT ,LJ rmY(t). 



According to Prevost's theory of exchanges, the hot body is itself 

 receiving radiant energy from the surrounding bodies ; hence the 

 radiant energy is more appropriately represented by A(T> — where 

 b is the temperature of the surrounding bodies. Similarly it would 

 appear probable that the conduction and convection will depend on 

 the difference of temperature. Hence 



Rate of energy=A(T w — *")+hiF(T— £). 



The constants A and m will depend on the nature of the radiating 

 body and on the surrounding medium. 



Although for theoretical purposes it is important to eliminate the 

 conduction and convection, yet in most cases a medium is present, and 

 it has been shown by Mr. Crookes that, within limits, variations in 

 pressure have only a very small effect on the amount of heat lost by 

 conduction and convection. 



I have not as yet been able to make any experiments on the deter- 

 mination of the term mF(T — f), but it is my intention to make further 

 investigations on this point. I am indebted to Professor Stokes for 

 suggesting a method which appears to me likely to yield useful results. 

 He proposes to construct a chimney of white paper, and to fix it over 

 the wire through which the current is passing. The chimney will 

 collect all the heated air ascending by convection, and by suitable 

 means its temperature and the rate of flow can be measured, and 

 hence the rate of loss of heat by convection estimated. 



It might be supposed that conducting the experiment in vacuo 

 would diminish the convection. According to the original researches 

 of D along and Petit, the rate of cooling diminished in a geometrical 



progression, whose ratio was — — , as the pressure diminished in a 



1*366 



second geometrical progression, of which the ratio was -. Mr. 



Crookes, in a paper communicated to the Royal Society (" Proc. Roy. 

 Soc," 1880, vol. 31, p. 239) described some experiments on this 

 point, and showed that a diminution of pressure from 760 millims. to 

 120 millims. had a very slight effect on the convection. From 120 to 

 5 millims. the effect was somewhat more marked. A reduction of 

 pressure from 5 millims. to 2 millims., however, produced twice as 

 much fall in the rate of cooling as the whole exhaustion from 

 760 millims. to 1 millim. Hence to eliminate the effect of convection 

 a very high exhaustion must be obtained. . 



It still remains to describe the experiments by which the constants 



