41 6 BIRKELAND. THE NORWEGIAN AURORA POLARIS EXPEDITION, igO2 1903. 



experience. The polar areas of perturbation are always manifested in the main in the same manner, and 

 every part of the day has, so to speak, its characteristic area of perturbation, which will always approxi- 

 mately form when there are any perturbations. 



At the close of the period, the conditions are, as we have said, almost normal everywhere, with 

 the exception of Kingua Fjord, where there are still some powerful forces. 



We will now look at the charts for this last section of the perturbation. 



Chart III represents the conditions from 5 h 2o m to 7'' 2o m . The storms are chiefly negative. 

 There is the powerful system in America, especially noticeable at Fort Rae, and one less powerful in 

 Jan Mayen, a westward continuation of which is indicated by the conditions at Godthaab and Kingua Fjord. 



Of the positive storms there is little observable here. At 6' 1 2o m there is an indication of one at 

 Little Karmakul, but the force is not great. 



The current-arrows at the southern stations at 7'' 20 are rather more difficult to include in a polar 

 field of perturbation answering to the systems of precipitation appearing here. The observations we 

 have are too few for us to determine the nature of the perturbing forces at work; we will only draw 

 attention to the simultaneous deflections appearing in the horizontal-intensity curves for Fort Rae and 

 Kingua Fjord on the one hand, and Christiania and Gottingen on the other: The maxima occur simul- 

 taneously, and there are also several coherent serrations. This is apparent chiefly until n h , after whii-h 

 hour the polar systems in the north of Europe also appear much more powerful, so that the phenomena 

 in Central Europe are mainly governed by this precipitation. 



This is certainly to some extent a phenomenon similar to that with which we meet at about 3 h on 

 this day. 



On Chart IV the positive storm appears more distinct, but has not yet extended farther than to 

 Little Karmakul. At Godthaab and Kingua Fjord, the same negative system is at work as in Chart III; 

 but it has now moved westwards, so that Jan Mayen is no longer in the district of precipitation. 



1 he current arrows in Central Europe may either belong to the area of divergence of the eastern 

 positive system, or to the area of convergence of the western negative system. It is rather doubtful 

 whether the system at Godthaab, and still more that at Kingua Fjord, can be regarded as a negative system 

 of precipitation. P ln it is true, is negative everywhere, so it therefore might be called so; but the direction 

 of the principal axis is more north and south than usual, a circumstance that is more conspicuous later on. 



On Chart IV, the arrows seem to be principally connected with the American system, while on 

 Chart V they form a transition between the negative system on the west and the positive system on 

 the east, or, as we might say, between the system at Cape Thordsen and the more southern system at 

 Jan Mayen and Bossekop. 



It is an unfortunate circumstance that on Chart IV there are no observations of horizontal intensity 

 for Fort Conger. If there had been a strong current-arrow there, directed southwards, it would seem 

 likely that a current-circuit had been formed from Fort Rae, through Uglaamie, Fort Conger, and Godt- 

 haab, and probably back to Fort Rae. When the system has moved a little, we find a circuit similar to 

 this, as there is negative precipitation at Cape Thordsen ; but this circuit does not appear at all distinctly 

 until Chart VI. If this could have been demonstrated as early as Chart IV, a very much better survey 

 of the perturbation-conditions would have been obtained, and a fact to hold to when seeking, by experi- 

 ments, for points of similarity. A fact such as this would have brought about some modifications in our 

 reasoning, but no essential simplification. 



As the observations, that we have at our disposal, seem to show, the negative system of pre- 

 cipitation developes by a more or less continual extension of its area westwards. 



In the second case we should have to imagine that a more or less momentary current-circuit was 

 formed, which increased somewhat during the course of the perturbation, while at the same time, owing 

 to the rotation of the earth, its position was changed. 



