PART I. ON MAGNETIC STORMS. CHAP. 111. 273 



whereas the new moon was on the 3oth, or at a time when the powerful storms had just ceased. 

 Although it seems probable that the proximity of the new moon has something to do with the strength 

 of the storms, other circumstances here seem to be of greater importance. We will not enter more fully 

 into this question, however, but merely suggest that the time between the two maxima of about twenty- 

 five days corresponds very nearly to the sun's period of rotation in low heliographic latitudes, a 

 circumstance that may possibly help to explain this condition. In the case of this series of perturbations 

 we find, moreover, a very striking harmony with the observations of the occurrence of sun-spots during 

 the same period. 



To represent this series of perturbations, we have selected those occurring during the period from 

 the afternoon of the 23rd to the morning of the 24th, having copied the magnetograms from J5 h on the 

 23rd to 7 h on the 24th (see PI. VIII). 



We have observations for this day from all the stations. Unfortunately, however, the horizontal 

 intensity curve for Matotchkin Schar has not been drawn, so that we have registerings only of the other 

 two elements. At Dyrafjord, moreover, the registerings are somewhat defective, as they were some of 

 the first that were made there, and can therefore only be regarded as trial registerings. The deter- 

 mination of the mean line is therefore a little uncertain; but as the conditions at about i7 h , or a little 

 earlier, judging by the other stations, are more or less normal, the uncertainty is not so great after all ; 

 and as the deflections, at any rate during the greater part of the period in question, are considerable, 

 the uncertainty will not seriously affect the current-arrows. 



THE DISTRIBUTION OF FORCE. 



71. The storms that occur here, as a close examination will show, may be referred to those types 

 of perturbations with which we have become acquainted in the preceding perturbations. In order to 

 distinguish them in some measure from one another, we will here, too, divide the perturbations into 

 three sections, 



the ist section from I5 h 20 to about i6 h , 



the 2nd section from i6 h to about 22 h , and 



the 3rd section from 22 h to 7 h on the day following. 



The first section comprises a slight, brief perturbation that is perceived simultaneously at almost 

 all the stations from which we have received observations. The effect is strongest at the equatorial 

 stations in the south of Asia. In low latitudes there are deflections only in H, and P k is positive every- 

 where. At the Central European and arctic stations, on the other hand, there are also deflections of 

 varied extent in the declination curve. This then is a typical positive equatorial storm, as Chart I 

 for the hour i5 h 48 m distinctly shows. 



There are a few peculiarities in this equatorial perturbation that are worth noticing. The first of 

 these is the shortness of its duration. Judging from the conditions at the stations in the south of Asia, 

 it ends at about i6 h , and thus lasts only a little more than half an hour. If, on the other hand, we 

 look at the district Tiflis to Stonyhurst, the storm appears to be going on for another hour and a half, 

 the perturbing forces there having the peculiarities that characterise these storms; but the conditions, at 

 any rate, are not so unmixed as to allow of its being on the whole characterised as such. 



In the second place, the conditions in America are somewhat peculiar. There is no sudden rise 

 of the horizontal intensity curve at about I5 h 30 as at the other stations. It is not until somewhat 

 later that the curve ascends, and its rise is comparatively slow. We may therefore reasonably assume 

 that here too, other perturbing forces come into play, perhaps polar precipitation of some kind or other, 

 acting with comparative strength. We have also previously found similar abnormal conditions during 



Birkeland. The Norwegian Aurora Polaris Expedition, 1902 1903. 35 



