PART I. ON MAGNETIC STORMS. CHAP. III. 269 



at the bottom of the screen, which undoubtedly turns off in a manner resembling that shown in 

 fig. 50 a. 



The two systems will now produce, in southern latitudes, their respective areas of convergence 

 and divergence ; it is these areas that are represented on our charts, and which justify us in also drawing 

 conclusions respecting those parts of the auroral zone in which we have no stations. 



These two types of perturbations thus seem to be those which characterise the polar storms; and as 

 we are constantly meeting with them, we will give them different names. It will perhaps be practical to employ 

 the same terms as in the equatorial storms. The characteristic difference in the polar regions between the 

 two types, which instantly strikes the eye, is the direction there shown by /\. We will then designate 

 those storms which produce in their field of precipitation negative values of PI,, negative polar storms, 

 and those that produce positive values of PI,, positive polar storms. These names are not chosen with 

 any regard to the actual rays which we imagine will produce these fields, but only on account of the 

 effect we find on the earth. On the other hand, however, we also see the agreement between, for in- 

 stance, the positive polar and equatorial storms by comparing the figures and experiments just mentioned 

 (figs. 39 for O>JO ~ -9> anc ^ 5 b) 38 b and 68 [i, 4, 7]). In these cases the rays pass the 

 earth in a westerly direction. A similar agreement exists between the negative polar and equatorial 

 storms, as will be easily seen from the corresponding figures and terrella-photographs (figs. 39 for 

 y <C i and 503, 37 & 47 b). In these last, according to our assumption, the corpuscular current 

 passes the earth in an easterly direction, in a manner already frequently indicated. 



With this circumstance before us, we shall also find that during the present perturbations all the 

 fields formed can be explained comparatively easily They will, of course, not be polar systems alone 

 that act. At the outset it is more or less probable that rays will also descend in lower latitudes, and 

 thus have an effect, that will possibly sometimes obliterate the effects of the polar systems. 



As the probable cause of the first-occurring positive equatorial perturbation has been already 

 sufficiently discussed, we need here only refer the reader to our previous remarks in Article 31. 



We will first look then at the first polar storm, represented on Chart I. The time is I2 h 50, not 

 long, that is to say, after noon Greenwich; and we do actually find on the day-side what appears to be 

 an area of divergence. We have here endeavoured to distinguish the effects of the polar storm from 

 those of the equatorial, and the arrow-directions shown on the chart answer only to the former. The 

 certainty with which the perturbing forces are determined is therefore somewhat diminished. In the 

 next place there are no observations from Dyrafjord; and they would have been of the greatest impor- 

 tance here, as that station would probably have been situated not far from the storm-centre, the effects 

 of which seem traceable in the district to the south of it. The current-arrows at Matotchkin Schar and 

 Axeleen seem to indicate that this is the effect of a positive polar storm. The very small perturbing 

 force at Kaafjord may possibly indicate that that station was situated in the vicinity of the point of 

 divergence; and the positive P, that we find is in accordance with this. It is impossible to say with 

 any certainty what precipitation there might be on the night-side of the earth. The only northern sta- 

 tion in this district from which we have observations, is Sitka; and there the conditions of the horizon- 

 tal intensity also indicate that we are near the field of precipitation of a negative polar storm, as we 

 find negative values of P k . There is moreover a comparatively wide deflection in the declination, so 

 that the current-arrow is not directed north-west along the auroral zone, but almost due north. This 

 circumstance perhaps indicates that the storm-centre was situated a little to the west of the place. There 

 is no distinctly-marked area of convergence in southern latitudes, and as the system can only be com- 

 paratively weak this is natural enough, as we are very badly off for stations in that part of the world. 



The second polar storm -- Chart III exhibits fields, the form and nature of which are of the 

 greatest interest. A glance at the chart shows us two distinct characteristic areas, an area of conver- 



