1 88 



BIRKELAND. THE NORWEGIAN AURORA POLARIS EXPEDITION, 1902 1903. 



however, inasmuch as the normal line lies in such a position that while the perturbation is at its height, 

 H is almost normal. At one of the Norwegian stations, Kaafjord, the perturbation is only just percep- 

 tible, the reason of this probably being that only at Kaafjord are the conditions so quiet that the com- 

 paratively slight effect is observable. At Axeleen and Dyrafjord. the conditions are very disturbed 

 before and after. This disturbed condition is also observable in southern latitudes, and is instrumental 

 in making this perturbation less clearly defined. 



It is the conditions in southern latitudes in Europe and Asia that contribute to make those of this 

 period especially worthy of remark. A very well-defined perturbation makes its appearance there in H, 

 with a simple course. The force gradually increases to a maximum, after which it once more diminishes 

 to zero. Throughout this district, the deflection represents a diminution in H, 



The table below shows the hour at which the perturbation commences and terminates, and that at 

 which the maximum is reached, as also the value of P t at the last named hour. 



TABLE XXIX. 



It will be seen, that the conditions at Sitka are rather peculiar as regards the course of the per- 

 turbation. The three stations in the east of North America come nearest to Sitka. The simple conditions 

 found between San Fernando in the west and Zi-ka-wei in the east, and between Kew in the north and 

 Batavia in the south, form a strong contrast to these variable conditions. In the latter district, the 

 perturbation is throughout chiefly in H. It is well defined, and as far as we can determine, commences 

 everywhere simultaneously at about 8 h 35 m . The maximum is not very distinct, but the time of its 

 occurence nevertheless does not vary greatly. It terminates simultaneously at about io h 50. As the 

 force is practically constant for several minutes about the maximum, P t max. will represent simultaneous 

 perturbing forces. The strength, it is true, is throughout somewhat greater in Europe than in the Asiatic 

 district; but nevertheless, between Kew and Zi-ka-wei and Batavia it does not vary more than from about 

 30.7 y to 22.5 y. This time the force is comparatively great at Wilhelmshaven too, a circumstance that 

 may be due to local conditions. 



The conditions are represented in three charts for the hours 9'' 15, g h 36 and io h . 



From 9 h to g h 30 at Sitka, there is a great current-arrow directed almost due south, as shown 

 on Chart I. Subsequently the current-arrow becomes smaller and is directed westwards along the auroral 

 zone. This condition continues from g h 30 to the conclusion of the perturbation. 



