PART I. ON MAGNETIC STORMS. CHAP. I. 



57 



We have therefore in general employed different scales for the arctic regions and for the rest of 

 the earth. On the Mercator chart, the scale given is the one employed for the more southerly stations. 

 The scale for the four Norwegian stations is only a fraction generally Vs of that given on the chart. 



In order to indicate this, we have written beside the arrow the fraction by which the scale marked 

 on the chart must be multiplied in order to find out the scale employed for the place. When, for instance, 

 the fraction Vs is found on the chart, this signifies that each length-unit of the arrow is equivalent to a 

 force 5 times as great as that which would be directly indicated by the scale given on the chart. 



On the polar chart, on the other hand, the conditions are reversed. There we have given the 

 scale that is employed for the polar stations, that is to say for the places where the perturbation is 

 strongest; and the scale for the more southerly stations is given in the same manner by a multiplier. 



In order to make the charts easy of comprehension and give a direct idea of the course of the 

 perturbation, the same scale has as far as possible been kept for the whole of a perturbation. On the 

 other hand, the scale will not be the same for all perturbations, as it must be chosen so as to give the 

 arrow on an average a suitable length. 



As the vertical intensities are of the greatest importance for a complete determination of the character 

 of the perturbation, they are also placed upon the charts, in order that both their magnitude and direction 

 may be taken thence. They are represented by lines drawn out from the place at right angles to the 

 current-arrows, and are marked on the same scale as the latter. Their direction is determined in the 

 following manner. If we imagine ourselves to be standing on the place in question, and looking out in 

 the direction of the current-arrows, the vertical arrow is placed on the left if P e is turned downwards, 

 on the right if it is turned upwards. Or we might express it as follows: Let P, be turned 90 with the 

 hands of a clock, the observer facing the direction of the current-arrow. 



It appears from Ampere's law, that when the perturbation at a place is due to a horizontal current- 

 system above the earth, the vertical arrow will point out towards the places where the current has its 

 greatest density. 



This law has a special application to the arctic stations. 



As the current-arrow, however, very often does not give the direction for a horizontal current, but 

 is only a representative of the perturbing force, the vertical arrow loses this significance; but it gives, 

 at any rate, P, in magnitude and direction. 



For the purpose of distinguishing the vertical arrow from the current-arrow, the latter is made a 

 little thicker and with an arrow-point. 



It is only from a very few stations, however, that there are registerings of variations in vertical 

 intensity. As a rule, arrows will be marked for the following: 



The Norwegian stations Kaafjord, Dyrafjord, Axeleen, and Matotchkin Schar; 



and also 



Christchurch, Tiflis, 



Munich, Val Joyeux, 



Pawlowsk, Wilhelmshaven, 



Pola, Zi-ka-wei, 

 Potsdam, 



and sometimes for Irkutsk and Jekaterinburg. In general, no oscillation will be noticed in the V. I. curve 

 for Zi-ka-wei, partly on account of the small sensibility. Upon the whole, moreover, P, will be small, 

 often imperceptible, in southern latitudes. 



Birkeland, The Norwegian Aurora Polaris Expedition, 19021903. 



