d 



' 



796 B1RKELAND. THE NORWEGIAN AURORA POLARIS EXPEDITION, igO2 1903. 



In Germany there is only the one component, Berlin to Dresden; and this very nearly coincides 

 with the direction of the magnetic meridian. 



In both cases, at the beginning of the perturbation, there is a double oscillation, which should 

 indicate the influence of polar systems. 



The "starting impulse" observed in the magnetic curves at the beginning of the perturbation, 

 southwards in H, and eastwards in D, seems to be caused by the magnetic effect produced by the.se 

 induction-currents. 



The direct effect of this polar precipitation must be assumed to decrease very rapidly at rather 

 great distances, as the strength of the current in these systems of precipitation can only be compara- 

 tively small, but the changes take place with comparatively great rapidity. 



If, on the other hand, we look at Greenwich for these days, we see here too an indication of a 

 double wave; but the principal phenomenon at the beginning of the perturbation is a uniformly-directed 

 deflection in the current-component EI , that is to say, conditions that must have been produced by the 

 induction-effect of the equatorial system. 



The deflection in E^ answers to a current-direction from NE to SW, and is thus fairly what 

 we should expect, as the current-direction in the outer equatorial System is from W to E. 



The deflection in the E W curve for Pare St. Maur, on the nth January, 1894, answers to a 

 current direction from E to W also in accordance with what we should expect. 



We further find in all cases that the maximal deflection in the earth-currents occurs at the time 

 when the deflection in the //-curve increases most, that is to say quite in accordance with what we 

 should expect to find from our former experience. 



These examples may easily be multiplied, but in this connection I need only refer the reader to 

 the curves published from Pare St. Maur and Greenwich. 



I will, however, draw attention to a difficulty that might possibly sometimes lead to misunder- 

 standing. In certain cases the variation will be exceedingly strong, and both the magnetic and the 

 earth-current curves may then be very faintly reproduced upon the photographic papers, often so faintly 

 indeed, that it may be impossible to follow the curve in its sudden and most rapid movements. It will 

 therefore sometimes be very easy to overlook certain small serrations. 



At the beginning of the storm of the nth January, 1894, we have a case in which the photo- 

 graphic curves were very faint and difficult to follow; and here, in order to indicate the uncertainty 

 arising therefrom, we have represented these parts of the curves with dotted lines. If, therefore, in 

 certain cases, a disagreement may be found in this respect, this uncertainty should be kept in mind. 



During the positive equatorial storms also, we thus find confirmation of our previously expressed 

 view of the connection between the magnetic perturbations and the earth-current phenomena. 



THE DIURNAL VARIATION OF THE EARTH-CURRENTS. 



163. In the previous articles, we have studied the connection between the earth-currents and t 

 magnetic storms. In addition to these, however, there are certain other, more regular variations, one 

 of which in particular, the diurnal variation, has been carefully studied. As regards earth-current 

 Weinstein has made a very thorough investigation of the phenomenon, based upon his observations i 

 Germany. The principal result at which he has arrived is given in a series of curves and vector dia- 

 grams in his previously-mentioned work. 



In England too, similar investigations have been carried out (see Airy, Phil. Trans. 1870, p. 2 

 and PI. XXIV). 



