PART III. EARTH CURRENTS AND EARTH MAGNETISM. CHAP. I. 7 8 9 



We have here, therefore, a capital means of making exact comparisons of the points of time of 

 the deflections in the two sets of curves. 



The reader is further referred to MOUREAUX'S description in "Annales du Bureau Central Met6oro- 

 logique de France", 1893, P- B. 25. 



If we here compare the two earth-current curves, we at once discover that they do not, as at 

 Wilhelmshaven, go together in every detail. As a rule, however, the resemblance is very close in the 

 principal features, but it is frequently found, especially in the smaller details, that the character of the 

 deflections differs a good deal in the two curves. 



Nor, in accordance with this, is the relation between corresponding deflections in the two curves 

 constant 



From this it would appear, in the first place, that the direction of the current in these regions is 

 ,iot so constant as at Kaafjord or in those parts of Germany in which Weinstein made his obser- 

 vations. The conditions, indeed, are more in accordance with those at Bossekop. The cause of the 

 greater constancy in the direction in east Germany than in France, is probably to be found mainly in 

 :he different natural character. It may possibly be assumed that the considerably shorter length of the 

 :ircuits at Pare St. Maur may play a decisive part; but such an explanation is certainly not sufficient, 

 is in the curves published in Blavier's previously cited work, we find a similar disagreement between 

 ;he circuits that make different angles with the meridian. We here too, however, in more powerful 

 storms, find a marked principal direction for the earth-currents (cf. Bosler, Comptes Rendus, 6 fevrier, 

 1911, or his Dissertation, Paris 1912, p. 67). 



In the next place we find throughout a very striking resemblance between the E W curve and 

 he H curve. This condition is thus in accordance with what we found in Germany, and, as in the 

 ase of that country, we may conclude from this that the influence of the earth-current upon the hori- 

 zontal intensity is comparatively great, although possibly other conditions during certain storms may act. 

 refer here to the changes that are caused by displacements of the systems of precipitation along the 

 uiroral zone. 



Another circumstance that may also possibly cause the earth-current conditions in these two districts 

 o be somewhat different is that as we have often pointed out -- the polar districts of precipitation 

 requently have quite a definite geographical position, e. g. at about midnight (Greenwich), when the 

 .torm-centre is situated, as a rule, between the four Norwegian stations. 



In relation to this storm-centre or to the corresponding area of convergence, the two districts here 

 inder discussion will have a somewhat different position, and it might be imagined that this had some- 

 hing to do with the matter. Possibly too, the distribution of land and water has some significance, and 

 his should then be more evident in France than in Germany. 



A comparison between the D and N S curves reveals throughout conditions that clearly point 

 o induction-phenomena, for in the great majority of cases there exists, as a closer investigation shows, 

 . more or less approximate proportion between the rate of change in the Z>-curve and the deflections 

 n the N S curve. The direction of the current is reversed as the Z?-curve attains its maximum or 

 ninimum; and the current-curve reaches its extremes at the time when the Z>-curve varies most. This 

 ondition is here very clearly marked. In the smallest serrations, however, we think we again find an 

 mdoubted synchronism. At the same time we may remark that if we imagine the N S curve, for 

 nstance, moved a little to the right, its resemblance to the D-curve will in many cases be striking. 



I need here only refer the reader to the perturbation of March 30 31, 1894, where there are 

 pecially-marked variations in both curves, or to November 24, 1894, where the perturbation-conditions 

 re simpler. We also meet with similar examples, of which we can easily convince ourselves, in a 

 lumber of the other storms given. 



Birkeland. The Norwegian Aurora Polaris Expedition, 19021903. 100 





