296 THE HEIGHT OF THE HARRIER AND THE SOUTH POLAR PLATEAU. 



vertical temperature gradient being fixed at 5°C. per 1,000 metres the temperature at all 

 points in the atmosphere is known. It is therefore a simple matter to calculate the height 

 at which the pressure has any given value. In figure 88 horizontal lines are drawn in column 

 A to represent the height at which the pressure has the consecutive values bj, b2, h^, h^, etc. 



In the same figure column B represents the same column of air in July. The pressure 

 at sea- level is again 745 mm., but the temperature is — 26''C. Lines have again been drawn 

 to represent the height at Avhich the pressure has the same values bj, 1)2, h^, h^, etc. It wall 

 be noticed that in B each of these lines is lower than in A and the difference is greater 

 in the upper than the lower part of the figure. This is because the pressure decreases 

 more rapidly in a cold air column than in a warm one. At the height h represented by 

 the dotted line in the figure the pressure has changed from bx in January to by in Julyi 

 and it is a simple matter to calculate the value bx — by . 



Conversely it is as easy to calculate the height at which b^^ —by has any given value. 

 Under the conditions assumed Meinardus calculates that at 1,350 lupties above sea-level 

 the pressure is 11 mm. less in July than in January. 



If therefore the air below this height over the Antarctic did not exist the total pressure 

 over the Antarctic would be that required to make the total pressure over the whole globe 

 the same in January as in July. The obvious way to account for the absence of this air 

 is to assume that the average height of the laud within the Antarctic Circle is 1,350 metres. 



From this he concludes that there does actually exist within the Antarctic Circle a 

 continent so high that its mass spread uniformly over the Polar cap T^-ithin the Antarctic 

 Circle would '^ive an average level of 1,350±150 metres. Assuming that one-third of the 

 area in question is at sea-level, the average height of the continent over the remaining two- 

 thirds works out as stated above to be 2,000±200 metres. 



We do know that there is high land within the Antarctic, the Pole itself being on a 

 vast plateau over 2,.500 metres above sea-level, and there is no reason to doubt that this 

 hio-h land does act as suggested by Meinardus. There can therefore be little doubt that 

 qualitatively his theory is correct and that it is a most important contribution to our know- 

 ledo'e of meteorology and geodesy ; but whether reliance can be placed on the accuracy 

 of his numerical results which he assumes by the ±150 metres attached to the estimated 

 average height remains to be examined. 



The accuracy of the numerical calculation depends on two main considerations: 



(a) The physical principles on which the relation between meteorological data and height 



is calculated, and 



(b) the accuracy of the meteorological data on which the calculation is based. 



The 'physical jJrinciples.— The physical principles underlying Meinardus's calculation have 

 already been stated, but it is advisable to restate them in the form in which they are 

 actually applied. The area considered is that within the Antarctic Circle and is therefore 

 concentric with the South Pole. Part of this area is at sea-level and part is occupied by 

 high land, the relative sizes of the two parts are unknown. Meteorological data, all of which 

 have been obtained at sea-level, are available from cei-tain stations in or near the area under 

 consideration. By making the assumption that these observations are typical of the whole 

 Antarctic at sea-level and that meteorological conditions depend on latitude only, the average 

 pressure and temperature at sea- level over the whole area are determined.* The pressme at 

 different heights in the free atmosphere over the area at sea-level is then calculated for 

 January and July, and the height is determined at which the pressure difference between 



* In this section we are not considering tiie nature or accuracy of the data used by Meinardus, the argument 

 proceeds independently of the actual values used. 



