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



It is, as stated in the description of the preceding perturbation, comparatively seldom that th< 

 effects of positive systems of precipitation can be observed in these polar regions. This, however, 

 the most characteristic example of such effects, and therefore goes far towards confirming our previous 

 assumptions. Unfortunate!}', only the first half of the perturbation can be studied, as the period of 

 observation ends while the deflections are greatest. 



We have now briefly reviewed the development of the perturbation by considering the curv 

 and have found that in the main the same conditions are repeated, and the development takes place i 

 exactly the same manner, as in the earlier storms. 



We will now pass on to consider the charts in which we have represented the various fields of 

 perturbation. These fields are here slightly more complete, as we have also made use of observations 

 from Kasan, from which place we have entire series of observations of the two horizontal components 

 for the last term days from the t5th May onwards. 



For this day we have drawn 14 charts representing 15 epochs in all. 



As Chart I shows, it is the positive storm that first developes. It is especially noticeable that the 

 positive system of precipitation appears to be situated comparatively far south, judging from the condi- 

 tions at the southern stations ; for if it is principally only this positive system that is acting, the stations 

 that we have included here must lie to the north of the point of divergence of this system. There is 

 of course also a possibility that in addition there is precipitation of stiffer rays in rather lower latitudes, 

 these being here those with the greatest effect. 



The positive system has developed most fully in the district Godthaab to Jan Mayen, while its 

 effects farther east are comparatively slight. 



There is perhaps rather more uncertainty as to the manner in which the conditions at Kingua Fjord 

 are to be understood. The direction of the current-arrows there is almost due south. Judging from the 

 chart, it would seem likely that the conditions might be considered as a continuation of the positive 

 system of precipitation. When we considered the curves and compared them with those at Godthaab, 

 we found, it will be remembered, that the character of the deflections at the two stations was sufficiently 

 different to justify the assumption that they were not very closely connected with one another, but that 

 on the contrary a system was acting at Kingua Fjord that was scarcely noticeable at Godthaab. This 

 assumption also seems to be the most probable on looking more carefully at the charts. At first, however, 

 this system at Kingua Fjord is comparatively inconspicuous and rather limited in its effects; and the positive 

 system that has formed to the east of it sometimes seems to encroach upon it and get the upper hand. 

 This is the case at the time of Chart II, when there clearly seems to be a positive system of precipitation 

 right from Kingua Fjord eastward past Little Karmakul, possibly as far as Ssagastyr. No effects of a negative 

 system of precipitation are noticeable. The strong current-arrows at the southern stations also seem to 

 indicate now that in addition to the great precipitation in or about the auroral zone, there may be smaller 

 amounts of precipitation farther south. Without such an assumption it would be difficult to find a simple 

 explanation of these current-arrows. The jagged, disturbed character of the curves, especially the hori- 

 zontal-intensity curves, is moreover a circumstance that supports this view, this fact indicating that the 

 systems in operation cannot be very far from the station itself. At the same time, the oscillations at the 

 polar stations to the north Jan Mayen, Bossekop and Sodankyla as also at Cape Thordsen, are com- 

 paratively gentle, without any sudden, violent changes backwards and forwards. At Little Karmakul, 

 however, the curve is rather jagged. 



The negative system of precipitation does not appear distinctly until I5 h 30, (Chart III), either 

 at Kingua Fjord, where it is strongest, or at Fort Rae. The positive system is also well developed 

 here; but at Cape Thordsen the perturbing forces in the horizontal components are rather small, this 



