306 Prof. J. C. Maxwell on Stresses in Rarefied Gases [Apr. 11, 



they are warmer than the air in general, may be converted into attrac- 

 tion by heating the ring, so as to produce a flow of heat inwards 

 towards the axis. 



8. If a body in the form of a cnp or bowl is warmer than the air, the 

 distribution of temperature in the surrounding gas is similar to the 

 distribution of electric potential near a body of the same form, which has 

 been investigated by Sir W. Thomson.* Near the convex surface the 



dh 2 



value of — 2 is nearly the same as if the body had been a complete 



sphere, namely 2T- g , where T is the excess of temperature, and a is 



the radius of the sphere. Near the concave surface the variation of 

 temperature is exceedingly small. Hence the normal pressure on the 

 convex surface will be greater than on the concave surface, as Mr. 

 Crookes has shown by the motion of his radiometers. 



Since the expressions for the stress are linear as regards the tem- 

 perature, everything will be reversed when the cup is colder than the 

 surrounding air. 



9. In a spherical vessel, if the two polar regions are made hotter 

 than the equatorial zone, the pressure in the direction of the axis will 

 be greater than that parallel to the equatorial plane, and the reverse 

 will be the case if the polar regions are made colder than the equa- 

 torial zone, 



10. All such explanations of the observed phenomena mast be sub- 

 jected to careful criticism. They have been obtained by considering 

 the normal stresses alone, to the exclusion of the tangential stresses ; 

 and it is much easier to give an elementary exposition of the former 

 than of the latter. 



If, however, we go on to calculate the forces acting on any portion 

 of the gas in virtue of the stresses on its surface, we find that when the 

 flow of heat is steady, these forces are in equilibrium. Mr. Crookes 

 tells us that there is no molar current, or wind, in his radiometer 

 vessels. It may not be easy to prove this by experiment, but it is satis- 

 factory to find that the system of stresses here described as arising from 

 inequalities of temperature will not, when the flow of heat is steady, 

 generate currents. 



11. Consider, then, the case in which there are no currents of gas, 

 but a steady flow of heat, the condition of which is 



o-_v*)=<>: 



dx 2 dif dr 2 



(In the absence of external forces, such as gravity, and if the gas in 

 contact with solid bodies does not slide over them, this is always a 

 solution of the equations, and it is the only permanent solution.) In 



* Reprint of Papers on Electrostatics, p. 178. 



