356 BIRKELAND. THE NORWEGIAN AURORA POLARIS EXPEDITION, 1902 1903. 



The simplest conception of the matter might be, that this was a to some extent connected negative 

 system of precipitation, whose eastern part extended more or less along the auroral zone, but whose 

 western part curved more northwards; and that the whole of this district of precipitation moved west- 

 wards with the sun. 



Such an assumption also agrees with what we find by experiment. We may here, for instance, 

 refer to fig. 140, pag. 327, where we clearly see such a deviation of the area of precipitation towards the 

 N., and particularly to the subsequent chapter in which the terrella experiments are specially treated of. 



Having discussed the conditions of perturbation so thoroughly, we need now only briefly touch upon 

 the perturbation-areas that we find represented on the charts for this section. 



On Charts III, IV and V, we find the direction of the current-arrows for the period in question 

 shown for nine epochs. 



In its main features, the movement of the negative system of precipitation that we found and have 

 described above, can be distinctly followed. 



If we considered the three polar stations mentioned above, which are situated to the south of the 

 auroral zone, we see, on Chart III, distinct effects of the system only at the most easterly of these, 

 namely, Ssagastyr. At Little Karmakul, the negative storm does not gain the ascendancy until Chart IV; 

 on Chart III the current-arrow swings backwards and forwards. 



Lastly, at Bossekop it appears that it is not until the last epoch represented on Chart IV that the 

 negative storm is predominant. Before that, there are only more or less distinct effects of the positive 

 system. We further see on Chart IV that the negative storm appears earlier in Jan Mayen than at 

 Bossekop. As regards the negative storm in other respects, we see all the time at Cape Thordsen 

 strong westerly-directed current-arrows. East of Ssagastyr, the strength of the current-arrows diminishes 

 considerably, so that the boundary of the area of precipitation is probably between Ssagastyr and Uglaamie. 



At the close of the section, we find the negative storm-centre in the north of Europe or the north- 

 west of Asia. 



The positive system asserts itself distinctly only on Chart III, at Godthaab, Jan Mayen, Bossekop 

 and Little Karmakul. 



With regard to the conditions in southern latitudes, we see only slight, though sometimes fairly 

 distinct, indications that the stations are in an area of divergence. Nor is this unlikely; for, judging from 

 the observations from the northern regions, we should expect to find the area of divergence farther west. 



On the other hand, we find on Charts IV and V, quite certain indications of an area of con- 

 vergence. 



There is one circumstance, however, which to some extent seems to point in the opposite direction, 

 namely, the conditions in the vertical intensity at Pawlowsk. We have already noticed that first positive, 

 and then negative values of P v are found here; but now we see that the positive forces also appear 

 to last longer than the period in which the positive storm predominates, being even apparent at times 

 when there are fairly distinct indications in a horizontal direction that we are in the area of convergence 

 of the negative system of precipitation. It is not impossible that the conditions are actually like this; 

 but on the other hand it should be remarked that the position of the normal line during this period, 

 might very possibly be a little different from what it is here; and one must therefore not conclude too 

 much from this circumstance. There is, moreover, a great possibility that in southern latitudes perturbing 

 forces might be operating that are imperceptible here, but which may yet exert a disturbing influence 

 upon the perturbation-conditions that we are now considering. 



At Gottingen, as we have said, the vertical intensity also exhibited conditions similar to those at 

 Pawlowsk. Here, however, they were more easy of explanation, as the station lies so much farther 

 west, that one might well imagine the positive system to be acting as long as the positive deflections 

 appear to continue. 



