278 BIRKELAND. THE NORWEGIAN AURORA POLARIS EXPEDITION, igO2 1903. 



south of Asia have begun to move into the day-side; and we have repeatedly seen that these systems 

 are more readily formed there. In the second place, the current-arrows in the east of North America 

 differ a good deal in direction from the general one. Their main direction is south-east, and they thus 

 appear to be instrumental in forming the most easterly part of the area of divergence, which we should 

 therefore expect to find on the day-side of the globe. 



Finally, in the third place, the course of the horizontal intensity curve during this period, indicates 

 quite distinctly at Kaafjord the effects of an intermediate positive polar storm, which, however, are a 

 little weaker than those of the long negative storm acting simultaneously in that district. A similar 

 effect seems to be traceable at Sitka, as also at Dyrafjord. It is therefore not improbable that this is 

 also a similar effect. 



In this case, as so often before, Honolulu occupies rather a peculiar position as regards the per- 

 turbing forces. If, however, we assume that the centre of the positive storm lies comparatively far 

 south, the conditions at Honolulu might be explained, if it were imagined to be in proximity to the 

 point of divergence. The more northerly negative storm might then also produce current-arrows directed 

 eastwards. It may also, and perhaps with more probalility, be imagined that purely local conditions might 

 exert no little influence. 



In addition to the great area of convergence that we have found throughout this section, the 

 current-arrows in Western Europe and the east of North America indicate an area of divergence in 

 that district until 2 h on the 24th. In accordance with this, we here also find positive values of P v . 



Thus the conditions do not seem to differ essentially from those we find in the second section of 

 these storms. The systems acting appear to be on the whole the same as before, only altered as regards 

 their strength and displaced a little. The area of divergence, which at first appeared on the day-side 

 of the earth, has thus, during this storm, remained for a considerable time, continuing indeed on to the 

 evening and night side. Charts VI and VII, for the hours i h 20 and 2 h 40, clearly show, however, 

 how this area of divergence now rapidly moves westwards, until at 2 h 40 it is in the district of 

 North America and the east of Asia. In accordance with this, the positive vertical arrows in Europe 

 disappear, some becoming zero, as at Val Joyeux, some turning round to the opposite side, as at Pola. 



The last chart for this period, Chart VIII, shows the conditions as they appear at 6' 1 30"* shortly 

 before the termination of the storm. At Axeleen and Dyrafjord we find about this time increased strength 

 in the deflections, and simultaneously in southern latitudes corresponding deflections in the magnetic 

 elements. The forces on the whole are small, and from several stations we have received no observa- 

 tions; nevertheless there seems to be an area of convergence in the district extending from Europe to 

 the east of North America, with a point of convergence a little south of Iceland and Greenland. The 

 arrows, moreover, in the east of North America, together with Honolulu and Zi-ka-wei, possibly indicate 

 an area of divergence in those districts; but as we have so few stations there, we can draw no certain 

 conclusions in the matter. 



According to this, we again appear to have the effects of the two polar storms as before, only that 

 the storms have moved considerably westwards. 



We have thus, by going through this perturbation in its various phases, succeeded in explaining 

 all the fields that occur, from the previously-mentioned simple points of view. The conditions here have 

 been simpler, in so far as there appear to be no particularly marked effects of equatorial systems, but 

 on the whole only of polar systems. Although we have not, as before, thought it expedient to attempt 

 a decomposition of the forces that appear, into the separate elementary phenomena, we have been able, 

 by observation of the fields, to make such a separation. We thus obtain, through the study of this 

 perturbation, a further support to our theory of the simple elementary laws that govern the apparently 

 complicated conditions found in the great compound storms. 



