PART II. POLAR MAGNETIC PHENOMENA AND TERRELLA EXPERIMENTS. CHAP. I. 325 



In conclusion, we must also consider the values that we find of P,. These, as we have said, will 

 sometimes be rather uncertain, inter alia on account of the construction of the measuring apparatus; 

 and we must therefore be careful not to think we can draw definite conclusions, especially where there 

 are only slight deflections. 



At Fort Rae, as we see, there are all this time positive values of P v , which would thus imply that 

 the main body of the current-system was situated slightly to the south of this station. At Uglaamie, on 

 the other hand, negative forces first appear in the vertical intensity. When the horizontal current-arrow 

 has assumed the more constant westerly direction, the vertical curve goes over to the opposite side, 

 and the positive deflections then last for the remainder of the period under consideration. 



Also on looking at P,, it seems thus, as though at first there were perturbing forces of a more 

 local character at Uglaamie. 



At Little Karmakul, the positive values of P, indicate that the positive system of precipitation must 

 lie a little to the north of the place. 



We will now pass on to consider the conditions that develop after the conclusion of this first period. 



It would be quite possible, in the succeeding part of the term day also, to mark off several divisions; 

 but such a marking-off would scarcely be advisable, as the perturbation-conditions, as a whole, are all 

 the time undergoing a more or less continuous change. 



Here, as in the preceding section, the perturbations admit of being arranged in two groups. On 

 the one side we have a negative polar storm, on the other side a positive. 



We will first consider the negative storm. This occurs, as will be seen from the plates, in the 

 district about Kingua Fjord, Fort Rae, Uglaamie and Ssagastyr, and furthermore at Cape Thordsen and 

 Fort Conger. In the preceding section, however, the storm-centre was in the vicinity of Fort Rae; and 

 now the perturbing forces there are considerable weaker than at the other stations. 



The storm-centre thus seems to have moved. In the first part of this last section, the most powerful 

 perturbing forces seem to be concentrated upon the districts about Uglaamie and Ssagastyr; but this 

 condition is not very apparent, as the forces round the auroral zone at these stations rarely vary much 

 in magnitude. 



Later however - - at about 20 h or ai h there is a distinctly defined storm-centre at Cape 

 Thordsen. At the other stations, where the negative storm occurred before, the perturbations at this 

 hour are practically over. 



It thus seems as if we here had a distinct westward movement of the negative storm-areas. 



There next occurs, as already mentioned, a positive polar storm, but in a much more limited area 

 than the negative, judging at any rate by the stations from which we have observations. 



We stated in the ' preceding section, that at the conclusion in Jan Mayen, a positive polar storm 

 began. In the present section, this positive storm developes greatly, and forms a system of precipitation, 

 which at first extends from Godthaab eastwards to the regions near Little Karmakul, but is afterwards 

 concentrated more upon the regions about Bossekop. 



These are conditions which are immediately apparent from the curves. Judging from the deflec- 

 tions in the horizontal-intensity curves for Jan Mayen and Bossekop, it would appear that the storm- 

 centre during this period, after lying in the vicinity of Jan Mayen while the storm is comparatively less 

 powerful, has subsequently moved eastwards to Bossekop, the storm, at the same time, attaining its 

 greatest strength. Whether the conditions do actually develope in this way, it is impossible to deter- 

 mine merely by the aid of the observations from these two stations, seeing that magnetically considered, 

 Jan Mayen lies considerably farther north than Bossekop. Observations from the southern border of the 

 auroral zone would here have been of great importance. 



Birkeland. The Norwegian Aurora Polaris Expedition, 1902 1903. 



