NON-PERIODIC CHANGES. 



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published two maps sivinR the mean values for three summer (December, January and Febru- 

 ary) and for three winter (June, July and August) months. For comparison with those maps 

 the McMurdo Sound values have been given in column 5. The mean value for the three 

 summer months, •796"=20-2 m.m., and for the three winter months, 1-381"=35-1 m.ra., agree 

 fairly well with the lines entered on the maps, although the winter value is somewhat 

 high and the summer somewhat low. 



The value of the mean difference between the daily maximum and minimum pressure 

 has been calculated for many stations. The monthly values for McMurdo Sound are given in 

 column 6. It will be seen that this measure of the non -periodic changes gives the most 

 regular yearly variation, the least value being in January and the highest in June with a 

 steady change from month to month in between. The mean value for the year is -180 

 inch. From the following table it will be seen that the value in McMurdo Sound is smaller 

 than at stations further to the north. The highest value in the world is found at South 

 Georgia, from which the values decrease as one proceeds to the south. In other words, the 

 non-periodic pressure changes are greatest over the Southern Ocean and diminish as the 

 Antarctic Continent is approached. 



Table 105. 

 Average difference between the daily maxitnum and 7ninimmn of pressure. 



The non-periodic changes so far considered give no indication of the steadine.ss of the 

 pressure over long intervals of time. Thus a place might have a large difference between the 

 maximum and minimum pressure in a given month, yet the mean pressure of that month 

 might be the same year after year. For the weather of a given place the rapid barometer 

 changes which give a large difference in maximum and minimum pressure, whether the month 

 or the day is taken as the unit of time, are of the chief importance, but for the influence 

 which the pressure of that place exerts on the pressure of the rest of the world, the steadi- 

 ness of the pressure over a longer interval of time is of more importance. The variations 

 of the mean monthly pressure from year to year are therefore of great interest and columns 

 7 to 10 of table 104 give the data for McMurdo Sound. 



The mean variation of monthly pressure from normal ■137"=3-48 m.m. is very large, 

 being almost equal to that found in the neighbourhocd of Iceland, 356 m.m., where the 

 pressure is more unsteady than in any other part of the known world. As the mean value 

 for each month is derived from only four years' observations, the yearly variation is not 

 very reliable, but it is significant that the three summer months November to January show 

 larger variation than the three winter months May to July, being ■191" and 137" respectively. 

 It would therefore appear that although the barometer is more steady during short 

 periods in the summer than in the winter as shown by columns 5 and 6, yet the variations 



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