270 



BIRKELAND. THE NORWEGIAN AURORA POLARIS EXPEDITION, 1902 1903. 



gence in the east of Europe, Asia and the west of North America, and an area of divergence in the 

 district from Western Europe to the east of North America. The storm-centre of the negative polar 

 storm seems to be situated in the north-east of Asia. The arrows at Matotchkin Schar and Axeleen 

 indicate a continuation of this system. Unfortunately we have no observations for this point of time from 

 either Dyrafjord or Kaafjord, as the curves in this periode of time, in the case of the latter station, have 

 disappeared, the points of light from the magnetometers having been too faint to act on the photographic 

 paper. It is however probable that there have been positive deflections here in the horizontal 

 intensity curve, judging partly from the course of the curve immediately after, when it is drawn once 

 more, and partly from the conditions we have previously met with, where the fields have shown them- 

 selves on the whole almost exactly similar. In any case, circumstances such as these would agree 

 exactly with the area of divergence found in the district Europe to America, as has already been 

 pointed out in the preceding description. If we imagine a positive polar system in the district extending 

 from the regions west of Greenland, across Dyrafjord, towards Kaafjord, we here recognise the form of 

 field with which we are continnally meeting during the storms that occur at that time of day, namely in 

 the afternoon, Gr. M. T., only that the positive system sometimes extends a little farther to the east. 

 In this connection we need only refer the reader to the storms on the gth December, 1902, the i5th 

 and 8th February, 1903 (see especially p. 191), and the 27th and 315! October, 1902. 



In this manner a close agreement with the first polar storm is arrived at. As may be seen, we 

 have only to assume that the old systems have moved a little westwards and have altered, the positive 

 storm having become less, and the negative greater, so that the latter is now the more powerful and 

 greater in extent. 



The third or main polar storm is shown on Charts V, VI, XI, XII, XIII and XIV. The form 

 of the various fields is here the same in all essentials, and bears no small resemblance to the field 

 during the preceding storm. We still seem to have a similar area of divergence in the same district as 

 before. On looking at the northern stations, we find that the arrow at Kaafjord has taken a westerly 

 direction, which would indicate that the positive polar system that is supposed to produce this area of 

 divergence does not now extend so far east as before, a circumstance which recalls conditions found 

 during the preceding perturbation of the 3151 October and ist November, 1902. We then found that 

 the reversal of the direction of P h occurred earlier at the eastern stations than at the western, as if 

 the cause of this reversal were in some way or other moving westwards with the sun. 



It now seems as though the negative polar system extends as far as Kaafjord; but if we investigate 

 matters in lower latitudes, we find no distinctly-defined area of convergence. We do indeed find cur- 

 rent-aiiows in Europe directed southwards as we should expect, and they are of considerable strength, 

 a fact which may possibly indicate that the two systems are here acting more or less in the same di- 

 rection. At Honolulu and Sitka, we also find current-arrows such as we should expect to find on the 

 east side of the area of convergence; but in the intermediate district we find no eastward-directed 

 current-arrows forming a transition between these two areas. The current-arrows in the south of Asia, 

 on the other hand, have a westward direction. 



It should here be remarked, however, that if the system in the north is not very powerful, the 

 effect in the extreme south of Asia will be comparatively slight; and if, at the same time, there occur 

 systems whose greatest effect is at the equator, they will there easily gain the ascendancy and obli- 

 terate the effects of the polar storm. We should therefore, in order to explain the conditions during this 

 period in such a manner, have to assume that simultaneously with the negative polar storm there occurred a 

 storm of a kind similar to the negative equatorial storms that caused the current-arrows in the south of 

 Asia to point westwards instead of eastwards; and there are actually circumstances that indicate that 

 this would be the case. In the first place, the character of the horizontal intensity curve at these Asi- 



