CHAPTER II. 

 ELEMENTARY PERTURBATIONS. 



25. It will be our endeavour, as stated in the introduction to this section, while studying the 

 perturbations, to find out their extent and course in each case. We consider it to be of the greatest 

 importance for the attainment of this object that what has taken place should be viewed as directly as 

 possible, at moments during the perturbation as numerous and close together as is practicable. This 

 then has guided us in our calculation of the perturbing force, and we considered that we arrived most 

 easily at the truth by placing the normal line actually on the magnetogram, in accordance with the pre- 

 viously mentioned rules. 



In connection with this, it should be mentioned that it would be expedient, when reading the 

 description, to have the curves before one, as there the conditions appear as directly as it is possible 

 to have them. 



With this object in view, our purpose is best served by dividing the perturbations into groups, 

 which seem to have comparatively well-defined properties. 



After the experience we have gained through the treatment of this material, it is our hope that 

 also other natural philosophers will feel convinced that we have taken the right road, a road that leads 

 to a clear comprehension of the laws of perturbations. 



It must not be imagined, however, that these groups stand as altogether separate phenomena. 



A complete acquaintance with the nature of the perturbations will assuredly lead to the assumption 

 that there is a certain genetic connection between the various groups. It is moreover our opinion that 

 this is the case, at any rate as regards the majority of the most important groups, as the physical agents 

 that consitute the currents are supposed to have in the sun their common source. 



The following treatment of perturbations will include the most important of those that occur in the 

 registerings of the thirty days( 1 ) for which we have received material from a number of observatories 

 mentioned previously - - all over the world. This choice of days is based upon observations from Kaa- 

 fjord and Potsdam. The qualities that have guided the selection have principally been strength and dis- 

 tinctness; but on the other hand, the selection was made without regard to the character of the pertur- 

 bation in other respects. As, however, the choice was based upon observations from one particular 

 region of the earth, this circumstance could not but cause the perturbations that appear especially strong 

 about the Norwegian stations, to receive a prominent place; but this, far from being a drawback, must, 

 in our opinion, be considered an advantage, as the material collected by us in our arctic expedition will 

 thereby be turned to best account. This one-sidedness, moreover, in the material is considerably reduced 

 by the circumstance that for each of the hours mentioned in the circular, we have always received regis- 

 terings for at least one day, and in the case of several of the observatories even for several days. We 



Circular of June, 1903. 



