316 



Planetary A tmosph eres 



[CH. XVII 



379. To study the way in which these proportions change as we pass 

 upwards, we use formula (757). 



If T is the temperature (absolute) of the outer atmosphere, the index of 

 the exponential may be written in the form 



\ ni a q f uz \ 

 = 



eti a 



- 2hm a ga - - = ^ 



\a + */ RT \a + z) 



The value of T being unknown, we shall calculate the function in terms 

 of 7p {~r~j an d can then calculate values of z corresponding to different 



values for T. The result of the calculations, putting g = 981, and taking the 

 values of R/m from p. 113, is given in the following table: 



From these figures it appears that as soon as we get beyond a certain 

 distance from the earth's surface, the atmosphere may be supposed to consist 

 solely of hydrogen. It is difficult to say exactly how far we have to go to 

 reach this region, on account of our ignorance of the temperature of the 

 upper air. If we regard clouds at a height of 17 miles as evidence of an 

 adiabatic atmosphere extending to at least this height above the earth's 

 surface, then, since the temperature at a height of 17 miles would be about 



