PART II. POLAR MAGNETIC PHKNOMENA AND TERRELLA EXPERIMENTS. CHAP. II. 439 



92. By means of the long series of perturbations that we have now gone through, we have 

 succeeded in obtaining a more or less clear idea of the magnetic storms, and have classified them 

 according to their appearance and course. As, however, the material employed was large, it may be 

 advisable to go once more briefly over the principal results at which we have arrived in the preced- 

 ing pages. 



The perturbing forces are calculated from the deviations from the normal daily course followed by 

 the magnetic elements on calm days, as represented in Article 14. On the charts, the horizontal 

 components are shown by current-arrows, of which the length is proportional to the size of horizontal 

 component of the perturbing force, and whose direction gives the direction of an electric current over 

 the- place, which would produce a magnetic force similarly directed. These current-arrows, however, are 

 only a geometrical representation of the perturbing forces, and indicate nothing whatever as to the 

 existence of such currents. 



In a number 01 places, moreover, the vertical component of the perturbing force is given by a 

 line at right angles to the current-arrow, on the left of it left of a person, standing on the earth and 

 facing the direction of the current-arrow if P, is positive, that is to say if the force is directed towards 

 the earth, and on the right of it if the force is directed upwards. 



The storms that are first described are those which exhibited the simplest conditions, while later 

 on, the more complicated perturbations are taken. 



We succeeded, in this way, in first separating the so-called equatorial perturbations from the polar. 

 Each of these types of perturbation have their characteristic area of perturbation, which is clearly 

 apparent from the charts, as also from the comparison of curves which we made for each perturbation 

 studied. 



We have considered that the perturbations should be divided in all into five different types, 



1. The positive equatorial storms, 



2. The negative equatorial storms, 



3. The positive polar storms, 



4. The negative polar storms, and 



5. The cyclo-median storms. 



Of these it is especial!}' the positive and negative polar storms, and the positive equatorial storms, that 

 are most frequently met with. 



The chief peculiarities of the positive equatorial storm are as follows: 



Everywhere in low and medium latitudes, positive perturbing forces are met with in the horizontal 

 intensity, while at the same time in declination no deflections, or only very small ones, are found. In 

 the vertical intensity, only small perturbing forces occur. 



If we consider the conditions in rather lower latitudes, we find the strongest perturbing forces in 

 the equatorial regions, while the perturbing forces decrease in strength with increasing distance from the 

 magnetic equator. 



