CHAPTER XVII. 



PLANETARY ATMOSPHERES. 



367. IN the last six chapters we worked out certain problems connected 

 with the free path in gases and molecular transport. In the present 

 chapter we shall apply the results obtained to the discussion of problems 

 connected with the atmosphere. These problems will consist of various 

 problems of aerostatics, and an investigation into the question of the 

 dissipation of planetary atmospheres. 



AEROSTATICS. 



Atmosphere in Isothermal (Conductive) Equilibrium. 



368. In 79 we obtained the equation 



v a = D a e-* h * ......... rrr. .................... (744), 



connecting the molecular density of gas of type a with the potential of a 

 permanent field of force. To apply this result to a planetary atmosphere, 

 we replace ^ by m^gz, where g is the value of gravity, and z is the height 

 measured vertically upwards. The equation accordingly becomes 



v a = D a e-' 2hm *9 z .............................. (745). 



In this equation we neglect variations in the value of g, and also the 

 rotation of the planet. 



If we measure z from the surface of the planet, D a must be the molecular 

 density of gas of type a at the surface, say (v a \, so that we obtain the 

 equations 



, a =Wo*--^, \ 



etc. 

 for the different constituents of the atmosphere. 



