184 



PRESSURE. 



agree so well with tlie calculated values. Thus the theory of the two natural oscilla- 

 tions of the atmosphere receives very great support from the Antarctic observations. 



Non-periodic pre.ssure changes. 



• It is much more difficult to discuss the non-periodic pressure changes than the periodic, 

 for there is no simple way in which they may be reduced to numerical values. 



Numerous methods have been suggested and used for comparing the pressure changes at 

 different stations and each has its peculiar advantages and disadvantages.* The data for 

 McMurdo Sound have been analysed by several of these methods and the results included in 

 table 104. 



Table 104. 



The highest pressure recorded in the four years was 30-2.5 inches (May 19, 1902) and the 

 lowest 27-82 inches (.June 23, 1912), the difference being 2-43 inches. The month with the 

 greatest absolute range of pressure is June with 2-12 inches and the month with the least 

 range January with 1-13 inches. 



Koppen t considers that the best measure of the non-periodic pressure changes is obtained 

 from the mean difference between the highest and lowest values in each month. He has 



* Koppen. Met. Zeit. 29, p. 97, 1912. 



t Koppen. Met. Zeit. 29, p. 501, 1912. 



