:o8 



BIRKELAND. THE NORWEGIAN AURORA POLARIS EXPEDITION, 19021903. 



the variation in form being gradual. All the stations of Central and Southern Europe have the same cha- 

 racteristic form of curve. The //-curve at Tiflis forms the transition to that at Dehra Dun. The com- 

 paratively high value at Wilhelmshaven seems to have been due to local conditions, as this station 

 always shows a greater force than the surrounding stations. 



The conditions at Pawlowsk do not appear to allow of a similar explanation, the comparatively 

 small force there being accounted for by the peculiar nature of the perturbation in question, a circum- 

 stance to which we shall return later on. 



We must here mention one more peculiarity. Although at Batavia the perturbation is almost im- 

 perceptible, we find, on coming as far south as Christchurch, that there is a distinct perturbation in the 

 horizontal intensity, appearing simultaneously with that in the northern hemisphere, and resembling in its 

 course the perturbations at the American stations. 



It is usual for Christchurch to occupy a peculiar position such as this, and frequently the forms 

 appearing in these southern districts are quite different. This may be explained by the fact that the 

 perturbation in the arctic regions is often accompanied by simultaneous perturbations in the antarctic 

 regions, and it is the effect of these latter that is noticed in Christchurch. Our material does not, 

 however, allow of certain conclusions being drawn in this matter. 



THE PERTURBING FORCES. 



38. This perturbation, as we have said, has a great resemblance to the previously-described per- 

 turbation of the 1 5th December, 1902. This resemblance is also apparent in the perturbing forces. 

 If we compare the charts of the two perturbations, we find a great similarity, as for instance in the 

 direction of the horizontal and vertical components of the perturbing force. The chief difference is that 

 the force at Kaafjord and Axeleen on the 151)1 December was very small in proportion to that at the 

 other places. 



The perturbing force elsewhere in Europe moreover exhibits a similar though smaller turn clock- 

 wise. The smaller extent of the turn seems undoubtedly to be connected with the circumstance that at 

 the commencement of this perturbation, the direction of the perturbing forces coincides with that at 

 2 a. m. on the I5th December, at which time, on that occasion, the perturbation was far past its maximum. 



As the force in these perturbations does not seem to continue to turn after the current-arrows 

 in Europe have become almost uniform in direction with those at the arctic stations, it is evident that 

 the perturbing force in this perturbation of the loth February must have a smaller area to turn in. 



For this perturbation four charts have been drawn, at intervals of a quarter of an hour. They 

 give a clear idea of the distribution of the force, and its changes during the progress of the perturbation. 



TABLE XVII. 

 The Perturbing Forces on the loth February, 1903. 



