PRESSURE WAVES AND PRESSURE DISTRIBUTION. 



211 



the vast majority of cases the waves afiect the three stations in the same order. To bring 

 out this point with greater certainty the curves were examined and whenever a maximum 

 pressure could be said with the certainty to afiect all three stations a + was entered against 

 each curve, similarly a minimum which affected all three stations was marked — . During the 

 nine months April to December 1911, 38 maxima and 35 minima were recognised and marked 

 in this way. All are shown on the plates and can therefore be examined by the reader 

 to satisfy himself that the method used is free from personal bias. Of the 73 maxima and 

 minima marked in this way 31 occur at the stations in the order Framheim, Cape Evans, 

 Cape Adare, while all but 8 occur at Framheim before Cape Adare. This is conclusive evi- 

 dence that these waves are not due to depressions entering the Ross Sea after passing Cape 

 Adare while moving in a general west to east direction. By tabulating the pressure and 

 time when each wave passed each station it is possible to determine the mean amplitude 

 of the waves at each station and the mean time it takes for the waves to pass from station 

 to station. The results of such an investigation are shown in the following table : — 



Table 122.* 

 Height and times of the Antarctic Pressure Waves. 



It will be noticed that the average height of the waves is practically the same at all 

 three stations, the differences being too small to be considered real. 



The last line of table 122 gives the average time the waves take to pass from Framheim 

 to Cape Evans and Cape Adare, these being nine and eighteen hours respectively. There 

 can therefore be no doubt that the waves travel from the south-east to the north-west. A 

 significant fact is that the maxima travel from Framheim to Cape Evans more rapidly than 

 the minima. This can only be accounted for, if real, by a deformation of the waves as they 

 travel. AVe shall see later that as the waves pass over the Ross Sea they produce winds 

 which modify the pressure and this may be the cause of the deformation. 



The question now arises : what is the nature of these waves, are they associated with 

 travelling cyclones and anticyclones, or are they true waves of pressure ? 



This point is best investigated by examining the weather maps to see whether the passage 

 of a wave of low pressure is associated with a travelling cyclone or a wave of high jiressure 

 with a travelling anticyclone. 



Weather maps, method of construction. — For constructing isobaric charts, simultaneous, 

 or nearly simultaneous observations are necessary. As observations were made at Framheim 



* In this table only waves which can be recognized at all three stations are included and therefore the results are 

 somewhat different from those contained in table 108. 



