252 COMPARISON OF AECTIC AND ANTARCTIC DISTURBANCES. 



the "sudden commencements," would be classified by BIRKEI.ANH as "equatorial" perturbations. In their 

 case, however, as will be remembered, the amplitude at Claba and Mauritius was usually less than that at 

 Kew, and much less than in the synchronous movements seen in the Antarctic. 



BIRKELAND believes an "elementary polar"* storm to be due to what he calls "precipitation" in a 

 compirativcly limited Arctic area. By this he apparently means an influx (and efflux) of charged ions. 

 In some calculations in the volume this is treated as equivalent to an electric current, approaching and 

 receding from the Earth in lines which, if produced, would intersect at the Earth's centre. The current is 

 . regarded as stopping short of the Earth, then travelling in a straight line to a point equidistant from the 

 surface, and finally receding. In some special cases on p. 103 which seem to be supposed to represent 

 probable actual conditions the height of the connecting (so-called "horizontal") portion is put at 

 200 or 300 kms., its length being taken as 1600 kms. and upwards. In the case of the " elementary polar " 

 storms the disturbance at any given instant is large over only a very limited polar area. In temperate 

 latitudes the disturbance is small and diminishes rapidly as the distance from the area of " precipitation " 

 increases. This area usually keeps shifting its position, so that the disturbance travels across the polar 

 regions. 



If Prof. BIRKELAND had seen what has been called in Chapter X the "special type of disturbance " he 

 would not unlikely have called it a south polar elementary storm. It seems, however, to present a much 

 greater definiteness of type than BIRKELAND'S " polar " storms, and its duration is usually much less than 

 the two or three hours which BIKKELAND speaks of. 



9. In his volume BIRKELAND does not take the disturbances in chronological order when discussing them, 

 but treats first the "equatorial," secondly the "elementary polar," thirdly the " cyclo-median," and finally 

 the "compound." One finds, however, that there are few of the occasions on which BIRKELAND failed to 

 detect at one stage or another the presence of both " polar " and " equatorial " disturbances. I have thus 

 thought it simplest to follow the chronological order, as BIRKELAND himself has done in the case of the 

 21 plates at the end of his volume. In what follows, the references are to BIRKELAND'S plates unless the 

 contrary is explicitly stated, and the reader is strongly advised to have these plates before him while 

 consulting the details given here. 



10. October 6, 1902 (hours 13J-15J, Plate I). 



Of this "cyclo-median" disturbance, BIRKELAND, p. 150, says: " Its chief characteristics are that it is 

 as great in medium as in high latitudes," also " the effect over the district Wilhelmshaven, San Fernando, 

 Stonyhurst, Pola is of about the same magnitude." The difference from the " equatorial " disturbances, 

 which otherwise it closely resembles, is that there appears hardly any movement in Asia or the Tropics. 

 BIRKELAND infers that it must be clue to electrical currents at a height " small in proportion to the Earth's 

 dimensions." 



The phenomenon, as presented at Kew and the other stations where it was best developed, had for its 

 most prominent feature a sudden change in D, commencing at about 14h. 14m. At Kew, in the course of 

 5 or 6 minutes, the Declination needle moved about 4' 5 to the west and then returned much more slowly 

 towards its normal position, taking nearly 30 minutes to reach it. H fell as D increased, but the total 

 fall was only about 6y ; the return to the normal position was slow as in the case of D. The curves were 

 exceedingly quiet for some hours before and after this movement. While the amplitude of the movement 

 was very trifling, the isolation of the D movement and its nature are certainly unusual. 



Whether there was or was not a corresponding movement in the Antarctic is open to doubt. In the 

 Antarctic, as elsewhere, the day as a whole was exceptionally quiet ; but, as was invariably the case in the 

 Antarctic, both the D and the H curves show numerous small oscillations. The V curve was certainly not 

 more disturbed between 14h. 10m. arid 14h. 50m. than it was earlier in the day, and decidedly less 

 disturbed than it was a few hours later. The same appears true of the D curve. There was, however 

 it may be a merely chance coincidence a distinct bay on the H curve, whose inception was at least very 

 nearly simultaneous with the commencement of the disturbance in Europe. Between 14h. 10m. and 

 14h. 21m. H fell 9y, two-thirds of the fall taking place between 14h. 13m. and 14h. 18m. H remained 

 below its previous value until 15h. 23m., the return movement being much the slower. 



* The terms "polar elementary " and " elementary polar" are applied indifferently. 



