MOVEMENTS OF ATMOSPHERE GULDBERG AND MOHN I 25 



If we have 



1 £ - 1 



- = 0.623 =0.377 



e s 



Then for air at 76o mm we have the values in the following table: 



§2. Height of the atmosphere — mean pressure 



We can adopt either one of two hypotheses concerning the height 

 of the atmosphere. We can suppose that the atmosphere is limited ; 

 in this case the temperature of the exterior stratum must neces- 

 sarily be absolute zero, for at this temperature the tension of a gas 

 is equal to zero. The other hypothesis is that the atmosphere ex- 

 tends indefinitely into space and that space is filled with a gas whose 

 tension is extremely feeble. For meteorology it matters little which 

 hypothesis is chosen, because in both cases the tension of the air 

 at very great heights will be insensible. Suppose 76o mm be the 

 pressure at the surface of the earth and suppose the temperature of 

 the atmosphere constant and equal to zero centigrade, we shall find 

 the pressure at the height of 200 000 meters equal to 0.000 000 oi mm . 



If the atmosphere does not contain the vapor of water its mass 

 will be invariable; if we suppose, moreover, that gravity does not 

 vary with elevation, the weight of this mass will be constant and 

 by calculating the mean pressure on the entire surface of the earth 

 it will be found to remain always the same. Considering the pres- 

 ence of the vapor of water whose quantity varies from time to 

 time, we shall see that the mass of the atmosphere does not remain 

 constant and that, consequently, the mean pressure varies with the 

 seasons. 



We have assumed that gravity is constant. In truth it varies 

 with the altitude and consequently the pressure of the atmosphere 

 depends on the law of the distribution of the mass in a vertical direc- 



