252 BIRKELAND. THE NORWEGIAN AURORA POLARIS EXPEDITION, 19021903. 



During this first section, polar storms also occur at our Norwegian stations ; but they are not very 

 considerable, although of sufficient strength to explain the partial loss of the typical character of the 

 equatorial perturbation, especially as regards the northern stations. 



Between I2 h 25 and I3 h I5 m , however, a considerable polar perturbation sets in. 



It should be especially noticed that, as the curves show, during this interval of time there is a 

 perturbation at Sitka that, for that place, is rather violent. The direction of the perturbing force is very 

 nearly west all the time, and its greatest value is reached at about I2 h 50. It is also noticed at the 

 Norwegian stations, most distinctly at Matotchkin Schar; at Axeleen it is less, but still noticeable, and at 

 Kaafjord it is almost imperceptible. If we look at the curve for Matotchkin Schar, we see that the force 

 there is uniform in direction along the magnetic meridian; and we notice particularly that the maximum 

 does not occur until I3 h i8 m almost half an hour later than at Sitka. This must either be explained 

 by a movement of the current-system, or we must assume that the perturbation at Matotchkin Schar is 

 due to a relatively different system. 



The farther we go from the polar regions, the less perceptible does this brief polar perturbation 

 become. It is distinctly noticeable at Baldwin and Cheltenham, but not at Honolulu. At the European 

 stations, it is only just perceptible. At Zi-ka-wei and Dehra Dun it is distinctly noticed, at Batavia it is 

 almost imperceptible. At Christchurch on the other hand, there is a rather violent perturbation in 

 relation to the place, only noticeable in the //-curve. The perturbing force is here directed northwards 

 along the magnetic meredian, corresponding to a current-arrow from west to east. The effect at Christ- 

 church cannot have been produced by the same system as that which acts in the northern hemisphere; 

 for the effect of the latter is imperceptible even at Honolulu and Batavia. 



The explanation of this seems to be that simultaneously with the descent in the north, a similar 

 phenomenon appears near the south pole, and it is the effects of the latter that we observe at Christchurch. 



On Chart I, for I2 h 50, only the current-arrows corresponding to the polar storms are shown, 

 as we have endeavoured to separate their effect from that of the equatorial system by a decomposition 

 of the total perturbing force. 



The second section includes the interval from i7 u 20 to about i8 h 30, and it commences with 

 the appearance of violent storms in the arctic districts. The effect is especially strong at Matotchkin 

 Schar, but less so at Axeleen. At Sitka, on the other hand, it is very marked. 



Chart HI at i8 h o m . The distribution of force seems on the whole to be conditioned by this 

 polar storm. Judging from the serrated character of the curves, however, it seems that the effect of 

 the equatorial storm is still perceptible. 



Of arctic stations, Matotchkin Schar is the one at which the force is strongest; and its direction 

 is there south-east. At Axeleen it is less, and is directed south-west. 



If we look at the European stations from Pawlowsk to San Fernando, we find that at all of them, 

 with the exception of Pawlowsk, the forces are rather small. Even at Stonyhurst it is less than at 

 Tiflis and Dehra Dun. The direction of the current-arrow at Pawlowsk is about south, in the district 

 Potsdam to Wilhelmshaven and Munich, south-west, and at Stonyhurst and Kew, almost west. If we go 

 right across to North America, we find the direction at Cheltenham NNW, at Toronto still more 

 northerly, and at Baldwin almost north. They form, as we see, a harmonious continuation of the direc- 

 tions in Europe, becoming more and more northerly as we pass from the European stations across the 

 Atlantic to North America. Thus the current-arrows should indicate the existence of current-vortices 

 with a clockwise motion in the North Atlantic. In reality there is something like a divergence of the 

 horizontal component of the perturbing force out from a point in these districts. Somewhere or other, 



