PART I. ON MAGNETIC STORMS. CHAP. II. 87 



This then, in an idealised form, is the appearance of the field which has a tendency to develope 

 during the polar elementary storms. It is not founded upon any sort of hypothesis, but is merely a 

 collocation of what almost invariably takes place, and of which demonstration will be given in the treat- 

 ment of the separate storms, when we shall also have an opportunity of going into the question of 

 the forms of current that may be assumed to have produced a field such as this. 



In comparing the above with the charts, we must remember that we there employ current-arrows. 

 We must then compare these with the current-lines in fig. 40. 



THE PERTURBATION OF THE 15th DECEMBER 1902. 



35. This magnetic disturbance makes its appearance upon an otherwise very calm day. It begins, as 

 the copies of the curves show, without any preceding equatorial perturbation, with a great storm in the 

 north, about Dyrafjord and Axeleen, and is accompanied by a perturbation, small indeed, but well- 

 defined, which is observed in northern America and Europe. The effect increases as we approach the 

 above-named Norwegian stations. It is only just perceptible at Dehra Dun, and not at all at Zi-ka-wei, 

 Batavia and Honolulu. There are unfortunately no magnetograms for that day from Christchurch. 



The perturbation is of rather short duration. It is first observed at Dyrafjord about o h io m , 

 and reaches its maximum at i 1 ' 8 m with a perturbing force of 386 y. At about 3'' 15 the storm is 

 over; but for a little while there are still slight oscillations to the opposite quarter. 



On Axeleen the storm does not make its appearance until about 35 minutes later than at Dyra- 

 fjord, reaches its maximum at i h 46 with a perturbing force of 193 y, and is over at about 3'' 45. 



The strange thing is that the oscillations at Kaafjord and Matotchkin Schar are comparatively so 

 small. At the first-named station, the perturbation begins at about the same time as on Axeleen, and 

 reaches its maximum at i h 4511 with a perturbing force of only 39.6 y. At Matotchkin Schar it 

 begins at about o h 5i m . The perturbing force reaches its maximum at about i b 9, with 27 y. 



At the stations Toronto, Baldwin and Cheltenham, a peculiarity is apparent, in that the perturba- 

 tion is not of equal duration in the horizontal intensity and the declination. In the horizontal intensity 

 it takes place between o' 1 40 and 3 h 3, a period which coincides almost exactly with the time of the 

 storm in the north. In the declination, on the other hand, the oscillation is of shorter duration, as it 

 begins at o h 55.5, but is well-defined and by no means inconsiderable. The oscillation in declination 

 thus takes place at the time when the storm in the north is at its height. 



In Europe, on the other hand, it begins rather suddenly at o 1 ' 45", and simultaneously in the 

 horizontal intensity and the declination. It lasts about 3 hours, but the time of its termination cannot 

 be exactly fixed, as the oscillations decrease little by little. 



This perturbation, as will appear from the above, has its origin in the northern regions. Its sphere 

 of action, which is rather limited, is concentrated about the neighbourhood of Dyrafjord and Axeleen. 

 The shortness of its duration, as also the comparatively calm character of the curves even during the 

 perturbation, seems to indicate that this is a polar-elementary storm of the most typical nature; it 

 appears to be produced by a coherent impulse, which increases to a certain size, and then again decreases 

 to during the course of the perturbation. At the same time, as the perturbation does not make its 

 appearance at all places simultaneously, the perturbing cause must be supposed to move with a some- 

 what continuous motion. 



The perturbing forces for the various places are calculated for a series of times (see the table), 

 and there is a series of charts representing current-arrows answering to simultaneous perturbing forces. 

 In studying the charts, the significance of the multiplier beside the current-arrows must always be kept 

 in mind (see Art. 23). 



