COMPARISON OF ARCTIC AND ANTARCTIC DISTURBANCES. 249 



one selects a time when the phenomenon is near its maximum. This consideration most likely influenced 

 BIRKKLAND'S choice of disturbances. If a bay or bays such as those on p. 247 occurs in the curves of one 

 European station, it usually occurs simultaneously at other European stations, arid in the cases selected by 

 |!N;KI:I,.\M) this was true not merely of the European stations, but also to a greater or less extent of the 

 other non-polar stations. Thus when the bay answered to a deep depression on the Arctic curves 

 and not infrequently trace was lacking from one or more of the Arctic stations, so that only two 

 or three had to be considered uncertainties of time may have been without any very serious 

 influence on BIRKELAND'S conclusions. Another circumstance favourable to his measurements was 

 that all relate to what was Winter at nearly all his stations, while a large majority relate to what were 

 night hours at the Arctic and European stations. His 21 plates deal with a total of 207 hours, and of 

 these 104, or one half, fall between C p.m. and 2 a.m., G.M.T. Uncertainties from the diurnal inequality 

 were thus much reduced. When, however, one turns to the Antarctic curves which correspond to 

 BIRKELAND'S, one is met by the converse of all this. The season is the Antarctic Summer, when changes, 

 regular and irregular, are largest. The time in a large majority of cases falls in the Antarctic morning, 

 often during the hours when, as already explained, oscillatory movements were especially numerous and 

 rapid. A mistake of a few minutes in the time may make a huge difference to the result, and the curves 

 are so unlike those at any of BIRKELAND'S stations that no help is usually forthcoming from the identifi- 

 cation of peaks. If Arctic records exist synchronous with the occurrences of the " special type of dis- 

 turbance " dealt with in Chapter X which took place in the Antarctic Winter anyone who attempts to 

 apply BIRKELAND'S method to them will appreciate my difficulties. 



After carefully comparing the Antarctic curves with BIRKELAND'S plates in which the time-scale of the 

 originals is usually much reduced I decided that the application of BIRKELAND'S method was absolutely 

 impossible in a number of cases, and that, in general, the uncertainties attending it were too great to 

 justify the labour necessary. It will, however, I think, be found that the comparison which it proved 

 possible to make has led to results of no small interest. 



5. Professor BIRKELAND believes that he has succeeded in recognising several distinct types of 

 disturbance, to which he attaches specific names. The following table gives particulars as to the date 

 and duration of the disturbed periods dealt with in his 21 plates. It also gives the total range in 

 D and H at Kew during the time covered by each plate. Hours are counted from 1 to 24 or 24 

 signifying Greenwich midnight that being the plan adopted by BIRKELAND. An entry such as 

 October 11-12, hours 12-2, means that the time extended from 12 (i.e. noon) on October 11 to 2 a.m. on 

 October 12. The range recorded for Kew represents the difference between the highest and lowest 

 values of the clement during the hours covered by BIRKELAND'S plate. It generally owes a good deal to 

 the regular diurnal variation natural to the period of the day, except in cases where only night hours 

 are included. 



2 K 



