WAVES. 



187 



If one examines the plates given in Volume II, on which the pressures of five stations 

 are plotted, it will be seen that at each station there are large pressure waves with well 

 marked maxima and minima, but on these there are numerous smaller undulations like ripples 

 on a large sea wave. It is obvious that we cannot include the latter in a discussion of the 

 former, we must therefore define the magnitude of the pressure changes which we will consider 

 in our discussion. There have been several similar investigations made previously and it has 

 been the nearly general practice to consider only pressure changes which exceed 5 m.m. 

 The data we are considering are given in inches and as '2 inch is very nearly -5 m.m., this 

 has been taken as the corresponding limit in the following investigation. 



The tabulated data have been examined and all successive maxima and minima which 

 differ by more than -2 inch have been noted. The results for the four years' observations 

 in BIcMurdo Sound are contained in the following table. 



Table 107. 



Pressure waves in McMurdo Sound {jour years) 



The mean yearly values based on four years' observations are : average depth of wave 

 from crest to trough =-572 inches, average length =152 hours whicn gives an average change 

 per hour of -0075 inches. There is an appreciable yearly variation in the character of the 

 waves, the amplitude being greatest and the period shortest in the winter, thus giving in that 

 season rapid barometer changes. The summer is the period with the greatest steadiness of the 

 barometer. 



Pressure waves at Cajje Adare and Framheim. — At Cape Adare and Framheim we 

 have not a complete year's observations, but fortunately wc are able to obtain approximate 

 values by a comparison with the McMurdo Sound results. Table 108 contains the data 

 at Cape Adare, Cape Evans and Framheim for the period April to December, 1911. 



