PART I. ON MAGNETIC STORMS. CHAP. III. 



235 



assume, as we might with reason do, that these polar storms in North America, and perhaps also farther 

 west, surround themselves with a field whose properties resemble those during the series of polar 

 elementary storms already described, with centres near the Norwegian stations, it will be impossible to 

 explain the strength and direction of the force at Honolulu as a direct effect of correspondent polar 

 systems with centres in North America. The perturbation at Honolulu must mainly be conditioned by 

 the equatorial system. 



During the second part of the long storm, the Norwegian stations begin to enter the evening and 

 night side, and we see that the current-arrows turn round. This takes place earlier at Matotchkin Schar 

 than at Kaafjord, showing that the cause producing this change in direction moves westwards with the sun. 

 At the Norwegian stations the perturbations have a very local character, but the conditions on the whole 

 are almost alike at Kaafjord and Matotchkin Schar, that is to say the direction of the current-arrows; 

 but at Axeleen they are very different. There there is a great vertical component, but a small hori- 

 zontal component (e. g. Chart XVI). A possible explanation of this is, perhaps, that as the current on 

 this occasion lies rather far south, Axeleen comes near to the neutral area. 



In lower latitudes also, we see that the district of the most powerful field has moved westwards 

 or in other words, this perturbation is of such a kind that the greater part of it follows the sun. 



We have already mentioned that at the stations Dehra Dun, Bombay and Batavia, a long diminution 

 in the horizontal intensity ensues, continuing throughout the day and night following. 



At the Norwegian stations the polar storms cease, and comparatively quiet conditions supervene as 

 early as 3 h on the ist November. 



In this manner we see that the perturbations that have appeared at the equator make themselves 

 independent of the polar storms, and outlast them. It might indeed be argued that the perturbation is 

 due to an after-effect of the long storm, in other words, that after the polar storms have ceased, it is 

 not real current-systems with which we have to do, but only an induced and slowly-vanishing temporary 

 magnetism in the magnetisable masses of the earth. This would be in accordance with the quite character 

 of the curve on the following day. 



In reality we here have before us a question of a fundamental nature, the answering of which 

 would be of the greatest importance to our comprehension of terrestrial magnetism itself, but would 

 require an acquaintance with these magnetisable masses such as we do not possess. 



It is certainly not impossible that a storm such as this, which has been powerful and lasted long, 

 may have after-effects. But the after-effect cannot explain it entirely; for at 5'' on the ist November, at 

 a time when the storm in the north has ceased, H at Bombay still amounts to 33 y. It is true the force 

 at Bombay has passed a value of 89 y, and during several hours maintains a value of about 70 y; but 

 nevertheless an after-effect of half this amount seems improbable. 



If such an after-effect at the equator were due to a temporary remnant-magnetism in the earth, and 

 if we suppose the magnetisable masses to be arranged symmetrically with reference to the magnetic 

 equator, we should also expect to find the direction of this effect the reverse of that of the exterior 

 magnetising force. 



In treating of the first part of the perturbation, by considering the conditions at Honolulu, we arrived 

 at the conclusion that we must here assume the existence of a negative equatorial system (see Art. 

 32), as the perturbations at Honolulu did not harmonise either in direction or strength with the condi- 

 tions farther north, and took no part in the great variations undergone by the perturbations in North 

 America. According to this, we may conclude that this time there is the effect of a current-system 

 which acts most powerfully in the regions round the equator. We are naturally led to connect this 

 perturbation with a circular stream of electric corpuscles flowing round the earth, resembling the 

 luminous ring round the terrella in the experiment represented in fig. 37. On account of the universal 



