PART II. POLAR MAGNETIC PHENOMENA AND TERRELLA EXPERIMENTS. CHAP. I. 389 



hour of day ; but it should also be remembered that there possibly exists another movement of the systems. 

 We might, for instance, imagine the positive system to be moved southwards, which would cause the 

 occurrence of phenomena such as those we now have before us; for the western stations, Godthaab and 

 Jan Mayen, are in the north of the auroral zone, while the eastern stations are in its southern part. 



The order of the systems is thus exactly such as we are accustomed to find at this time of day 

 during the most typical of the storms already described. 



With regard to the movement of the systems from time to time, we find apparent traces of a 

 westerly movement of the negative system in America, and possibly a less pronounced easterly move- 

 ment of the positive storm-centre. 



On the three charts following, //, ///, and IV, the development of the perturbations in this section 

 can be distinctly followed. That of the negative storm is the more marked. Between I5 h 2O m and i6' 1 

 20, it is distinctly developed only at Kingua Fjord; but at i6 h 50 there are also distinct, strong current- 

 arrows at Fort Rae and Uglaamie on the one side, and Cape Thordsen on the other. The most powerful 

 forces, however, are still at Kingua Fjord. 



At 17'' ao m the great current-circle has already formed, and we find the most abundant precipi- 

 tation in the district Kingua Fjord to Fort Rae, showing that the storm-centre has moved a little west- 

 wards. At both Godthaab and Jan Mayen, where previously the positive storm was the strongest, there 

 are now powerful negative forces. 



The storm is at its height from i8 h I5 m to i8 h 3o m , and we find very strong perturbing forces, 

 especially on the night-side. The most powerful are apparently at Ssagastyr; but as the deflections at 

 both Uglaamie and Little Karmakul are too wide to be measured, it is possible that they may be just 

 as powerful there as at Ssagastyr. The negative storm then decreases once more on Chart IV, while at the 

 same time the storm-centre moves back to the regions about Fort Rae and Uglaamie. It may be noticed 

 that this contrary movement of the system of precipitation, takes place after the sun has crossed the 

 meridian of the magnetic axis. 



The positive system can be followed in a similar manner. At first it extends from Godthaab 

 eastwards as far as Little Karmakul, as shown on Chart II. On Chart III the negative system breaks in 

 upon it, causing, in some cases, distinct reversals of the direction of the current, as, for instance, at 

 Little Karmakul and Jan Mayen ; while in others the current-arrow only swings backwards and forwards 

 as at Bossekop and Sodankyla. At Christiania, however, the effects are still chiefly those of the 

 positive system. 



At the end, we find again stronger effects of the positive system, the force at Little Karmakul, at 

 i8' 1 50'", for instance, being of remarkable magnitude. Its effect are also distinct at the more westerly 

 stations. At Jan Mayen, where, not long before, the effects of the negative system had been so distinct, 

 the current-arrow has once more begun to oscillate, and at 19'' 2o m is more indicative of the positive 

 system, although there are evident signs of the action of both systems simultaneously. 



The negative values of P, constantly found at Jan Mayen should also be noted. They show that 

 although at one time the positive system is the stronger, and at another the negative, this variableness 

 has no special significance as regards the vertical forces. As we have so often pointed out, the expla- 

 nation of the phenomenon is, that the negative system, whose area of precipitation must be assumed to 

 be chiefly to the north of Jan Mayen, and the positive system, whose storm-centre is certainly situated 

 to the south of that station, will both act in the same direction, namely a negative direction. 



The conditions at Little Karmakul are also somewhat variable, and on Chart IV we find both 

 distinct negative and distinct positive forces. 



The last current-arrow for 19'' 50 comes more properly in the next section of the perturbations, 

 which we shall now proceed to examine. 



Birkeland. The Norwegian Aurora Polaris Expedition, 1902 1903. 50 



