62 BIRKF.LAND. THE NORWEGIAN AURORA POLARIS EXPEDITION, IQO2 1903 



have thus had an opportunity of studying a number of perturbations that do not belong to those spe- 

 cially mentioned in the circular. 



It would be impossible, if we are to treat the perturbations upon the lines we have laid down, to 

 take notice of all the deviations that might indeed be worthy of mention. We have had to confine our- 

 selves to the study of the greatest and longest, or at any rate to perturbations of a universal character. 



We will here mention a circumstance that confirms us in our opinion that we have succeeded in 

 treating a number of the most important of the perturbations that have taken place during this period. 



Being aware of the one-sidedness there might possibly be in our material, we wrote on the gth 

 March, 1907, to the Director of the Coast and Geodetic Survey of the United States with a request 

 that he would send us magnetograms of some of the greatest perturbations that had occurred at Sitka 

 and in North America during the period from the autumn of 1902 to the spring of 1903. The Super- 

 intendent, Mr. O. H. Tittmann, and the Director, Mr. L. A. Bauer, were good enough to comply with 

 our request. The perturbations, however, which had been selected with regard to Sitka for ten days in 

 which "the magnetic perturbations were remarkably distinct, powerful and simple", proved to be of no 

 very different kind or magnitude from those we had already studied. It was principally a series of per- 

 turbations in January that were comparatively great in those regions. We shall go more fully into 

 these conditions farther on, as, with the .aid of the material from the polar stations of 1882 83, we 

 may draw important conclusions regarding the position of the storm-centres about the auroral zone at 

 various times of the day. 



A similar request was sent to the Director of the Observatory at Christchurch (New Zealand), 

 whence we also once more received magnetograms for 19 days of the period observed, in which the 

 perturbations at that place were remarkably distinct, powerful and simple. In 16 cases, however, the 

 perturbations were coincident with some we had previously received and discussed. 



THE EQUATORIAL PERTURBATIONS. 



26. It appears that magnetic storms of any considerable strength, are most frequently of a kind 

 in which the force increases towards the poles. It also appears, however, that it is not unusual to find 

 perturbations that are best developed and most powerful at the equator. It has even been found that 

 these perturbations in the regions about the equator, act principally upon the horizontal intensity, in such 

 a manner that the current-arrows point along the magnetic parallels. 



As regards the lower latitudes, the circumstances of the perturbation often exhibit symmetry both 

 with respect to the magnetic axis and to the equator. Such perturbations we have chosen to call equa- 

 torial perturbations. 



Of these there are again two kinds possible, namely, such as produce an increase in the horizontal 

 intensity, and such as produce a diminution. Both of these occur. 



The first of these we have called positive equatorial perturbations; the second kind we have called 

 negative equatorial perturbations. 



The reason for this separation is not merely the more formal one that the force is in opposite 

 directions; but it goes deeper, the two perturbations having quite a different character and course. The 

 positive equatorial perturbation in particular is strongly characterised, so much so that if attention has 

 once been drawn to it, it will always be recognised with the first glance at the registered curves. Its 

 more detailed characterisation will come out best in the treatment of the separate typical cases. 



