114 BIRKELAND. THE NORWEGIAN AURORA POLARIS EXPEDITION, 19021903. 



its volume increasing to a maximum and decreasing to zero. This furnishes a direct proof that the 

 primary cause of the perturbation is to be found in currents above the earth, since the current in the 

 earth is evidently an induced current produced by the magnetic storm. The latter must therefore have 

 its cause in a current-system above the surface of the earth, if, as may be considered certain in the 

 case of these perturbations, it is conditioned by electric currents at all. 



Owing to the rapid weakening of the effect southwards, these horizontal currents must lie at a com- 

 paratively little height above the earth. The perturbations must be of a local character in the north, a 

 fact that is immediately apparent from the already-mentioned great variation in the nature of the per- 

 turbation from place to place. The perturbations in the southern districts are in strong contrast to 

 this, as they there show a slow, continuous change in their character. 



The perturbations in the southern districts are not of such a character that they can be regarded 

 as the effect of adjacent systems; their cause must necessarily be sought in the average effect of that 

 which takes place in the more distant systems, a circumstance which explains the quiet, regular character 

 of the curve. 



In discussing the perturbation of the I5th December from to some extent other points of view, we 

 arrived at the same result, as the explanation of the effect of the force outwards at great distances from 

 the arctic regions, must be sought in that of vertical currents in an opposite direction, connected with 

 the low-lying, horizontal portion of the current, which gave rise to the powerful perturbations in the north. 



By a generally continuous movement of this system, the turning of the perturbing force is precisely 

 explained. On that day, the sphere of action in the north being more than ordinarily local, this move- 

 ment may be clearly proved by the fact that the perturbation made its appearance much later at Axeleen 

 than at Dyrafjord. 



This perturbation is greater, and its influence is almost equally strong at all the four stations. It 

 commences quite as early in the regions about Matotchkin Schar as at Dyrafjord. 



Thus, although we cannot prove, from the times at which the perturbation began in the north, 

 that there was any movement eastwards along the auroral zone, the current-arrows on Charts II and III 

 at Dyrafjord indicate that such a movement really took place there, as already mentioned in the descrip- 

 tion of the charts. Outwards there is also the same distribution of force and turning of the perturbing 

 force, as in the perturbation of the I5th December. 



As we have said, the distribution of force at a' 1 on that day answers to that at 23** 15 on this 

 loth February. If we now imagine the system to be moving on eastwards, it will be easily seen that 

 the European stations would be passed by the magnetic field in a district in which the direction of the 

 perturbing force alters only slightly, and the turning would be with the hands of a clock. 



In this perturbation the current-system may be assumed on the whole to have a more easterly 

 position than in that of the previous I5th December, in accordance with the fact that the latter appeared 

 later in the day. 



The field of force on the surface of the earth indicates that our current-system should generally 

 have two symmetrically-situated points, the points of convergence and divergence, one on each side of 

 the horizontal portion of the current, for the horizontal component, two neutral districts in which the 

 horizontal component was very small. 



We have not yet seen both these points during the same perturbation; for when one of them 

 is in Europe, e. g. in the neighbourhood of Pawlowsk, as we shall generally find it during the polar 

 elementary storms that have their storm-centre near our Norwegian stations, the other should be situated 

 symmetrically on the other side of the auroral zone, or in the most northern parts of Greenland. 



During the perturbations of the i5th December and the loth February, we have found the area 

 of convergence, and during that of the gth December we have found the area of divergence. In our 



