IN KAUIFIED OASES 



Since the expNMOM for the stress are linear as regards the temperature, 

 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 wroe, 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 equatorial zone. 



10. All such explanations of the observed phenomena must be subjected 

 to careful criticism. They have been obtained by considering the normal 



alone, to the exclusion of the tangential stresses, and it is much 

 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 

 gat 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 is not easy 

 to prove this by experiment, but it is satisfactory 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 



(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 this case the equations 

 of motion shew that every particle of the gas is in equilibrium under the stresses 

 acting on it. Hence, any finite portion of the gas is also in equilibrium ; 

 also, since the stresses are linear functions of the temperature, if we superpose 

 one system of temperatures on another, we also superpose the corresponding 

 systems of forces. 



Now the system of temperatures due to a solid sphere of uniform tem- 

 perature immersed in the gas, cannot of itself give rise to any force tending 

 to move the sphere in one direction rather than in another. Let the sphere 



