PART I. ON MAGNETIC STORMS. CHAP. II. 149 



CONCERNING THE CAUSE OF THE PERTURBATION. 



52. Notwithstanding its simplicity, this perturbation possesses rather peculiar properties, which make 

 it difficult to refer it to any of the other types. In the first place, the perturbation at Axeleen, owing to 

 the difference in its course, and to the direction of the force, must be ascribed to the effect of a rela- 

 tively independent system. In more southerly latitudes, the field forms, as we have seen, an area of 

 convergence. This immediately brings to mind the polar elementary storms. There are, however, strong 

 reasons against such a view. 



On account of the form of the field, we should expect to have the storm-centre somewhere about 

 the south of Greenland, and the current-arrow might here be expected to be directed westwards along 

 the auroral zone. In the ordinary polar elementary storms, we shall then find the strongest force-effect 

 in this current-arrow's line of direction, around the main axis of the system, while the effect should 

 become less inwards towards the area of convergence. This time we come upon a peculiarity, namely 

 that the effect at Kaafjord and Pawlowsk is very small in proportion to that, for instance, at Val Joyeux 

 and San Fernando, which should lie almost at the same distance from the storm-centre, but much nearer 

 the area of convergence. A knowledge of the conditions at Dyrafjord would have enabled us to settle 

 the question; for if the perturbation should be referred to the same type as the polar elementary storms, 

 we should have found the effect very strong at Dyrafjord. 



It might be thought, as an explanation of the smallness of the force at Kaafjord and Pawlowsk, 

 that the system that brought about the perturbation on Axeleen, counteracted the southern system. This 

 has, indeed, to some extent been the case, especially at Kaafjord. It does not, however, explain it en- 

 tirely; for then the counter effect of the northern system would be as great at Pawlowsk as at Axeleen. 

 But everything seems to indicate that the perturbation at Axeleen is of a very local character. The 

 vertical component, for instance, changes its direction. And at Matotchkin Schar, nothing at all is 

 noticeable. 



It does not thus seem possible to refer this perturbation to the polar elementary storms. In favour 

 of this conclusion, there is also the fact that if it were so referable, it would have its storm-centre in the 

 sun's meridian, while the storms that have the current-arrow directed westwards along the auroral zone, 

 generally appear about midnight. But this is not all. From the calm conditions at the stations round 

 the auroral zone, it does not even seem to be of a polar nature. The cause of the perturbation in 

 lower latitudes must also be sought in occurrences in those lower latitudes. 



The cause of the magnetic storms must however be sought in electric currents, of whose form and 

 kind we shall endeavour to obtain a clear idea by the aid of the experiments with the terrella. 



The system with two vertical current-portions connected by a horizontal part, cannot satisfy the 

 field of force. It is then most natural to seek an explanation of the phenomenon in currents moving for 

 long distances along the surface of the earth, either on it, or at some height above it. It here seems 

 natural to suppose, after glancing at the chart, that we have had a current that, at any rate in the North 

 Atlantic region, has assumed the character of a real current-vortex. 



The perturbing force in the south-west of Europe, as we see, converges greatly. If we were to 

 produce all the forces until they intersected one another, the district of the greatest density the point 

 of intersection -- would lie only a little to the north-west of Spain. The force in North America, on 

 the other hand, has not such a great convergence. If we imagined ourselves moving over the earth's 

 surface in such a manner that we always advanced in the direction of the current-arrow, we should 

 describe some sort of curve, which we might call a current-line. What these current-lines are like in 

 our case, our material does not allow us to judge with certainty. There can be no doubt that those 

 from North America turn east, and unite with the conditions in the south-west of Europe, always, as 

 they do so, curving to the right, and always, the nearer they approach towards Europe, with a greater 



