250 



COMPARISON OF ARCTIC AND ANTARCTIC DISTUBBANCES. 



6. The principal object in mentioning the ranges at Kew is to bring out an important point which I 

 hardly think BIRKELAND himself quite realised, and which I am confident will not be realised by readers 

 of his volume who are not themselves experts in Terrestrial Magnetism. 



The mean value of the absolute daily range in D at Kew derived from all days of the eleven years 

 1890 to 1900 was 13' -57. This value, it will be observed, was exceeded on only six occasions in the 

 table. During the eleven years the absolute daily range at Kew exceeded 20' a value not once attained 

 during BIRKELAND'S disturbances on no less than 12 per cent, of the total number of days. 



It must not, of course, be forgotten that the average length of the period covered by one of BIRKELAND'S 

 plates is slightly under 10 hours, and that the majority of the periods do not include the hours at which 

 the daily maximum and minimum most frequently occur. Still, taking everything into account, the fact 

 remains that the great majority of the days selected by BIRKELAND were not what are ordinarily called 

 disturbed days. In the Arctic, it is true, there were movements which would rank at Kew or any other 

 non-polar station as magnetic storms, but there is not a single one of the occasions on which the phenomena 

 at Kew would ordinarily be dignified with that name. On perhaps three occasions, October 31, to November 1, 

 1902, November 23-24, 1902, and February 8, 1903, one would have little hesitation in describing the 

 day as disturbed, but on the other hand there is quite a considerable proportion of the days which one 

 would be likely to describe as quiet. It is not merely that the movements on BIRKELAND'S selected days 

 were small, but that they were few in number, and in many cases represented slow changes. In the case 

 of an ordinary magnetic storm at Kew, not only would the range bo much larger than in any of the days 

 selected by BIRKELAND a range of 30' in D and 300y in H represents what may be called a second class 

 storm but the large movements would be much more numerous and some of them much more rapid. If 

 we take a really first-class storm, like that recorded at Kew on October 31, 1903, it represents an altogether 

 different order of conditions. Not merely is the range five or six times larger in D, more in H, but there 

 are dozens of rapid oscillations, altogether without parallel in the most disturbed cases selected by 

 BIRKELAND. The expenditure of energy during a first-class storm may, for all we know to the contrary, 

 be 10, 100, or even 1000 times greater than that during BIRKELAND'S most disturbed day, and we cannot 

 even say with certainty that the ultimate source of the energy, or the way in which it is expended, is the 

 same in the two cases. What I have called a first-class storm is apparently experienced as a large storm 

 over all the world or at least over a very large part of it and is invariably, or almost invariably, 



