590 Mr. H. Redmayne Nettleton on a JS 



Suppose 6 m = temperature o£ room = zero. 

 Then, whatever be the value of E, we obtain :• 



/) Vp8li 



eiu 



or 



K=^ ...... (4) 



Hence the ratio of the temperatures above and below — at 

 equal distances from the point in the bar which is at the 

 same temperature as the enclosure — is independent of the 

 value of the emissivity. 



Equation (4) is used below for calculating the value of the 

 thermal conductivity of mercury. 



(3) Application to Liquids. 



There are experimental difficulties, probably not insuper- 

 able, in determining the temperature at a given point fixed 

 in space and situate within a moving solid, and it was thought 

 advisable to experiment in the first place with a moving 

 column of mercury. 



When a column of mercury at a constant temperature 

 moves slowly along a tube, the velocity is a maximum at the 

 centre and a minimum at the circumference, viscosity being 

 the main factor in determining the form of the movement. 

 When, however, the top of such a column is heated and the 

 lower end kept cold, the velocity of the mercury at the 

 central part of the tube being greater than near the walls, 

 the average temperature of the central parts is less than the 

 average temperature of the outside parts, and therefore 

 the average density of the central parts is greater than the 

 average density of the outside parts. Thus the gravity head 

 in the central part is greater than the gravity head else- 

 where, and this tends to diminish the differences in velocity 

 at various parts of the cross-section. A mathematical in- 

 vestigation leads to differential equations of an exceedingly 

 complicated type, but the author is of opinion that, neglecting 

 radial conductivity, the isothermal surfaces would be approxi- 

 mately horizontal to quite a considerable distance from the 

 centre of the tube. 



It is not unlikely, however, that radial conductivity will 

 play an important part, and if the diameter of the tube is 

 small in comparison with the length, and if the flow is. small, 



