10 AKSEL S. STEEN. M.-N. Kl. 



the general outlines, but also intend to try and master the characteristic 

 details of the diurnal variation, we shall be obliged, I think, to proceed 

 very much as the bacteriologist does when he wants to study a new 

 bacteria; we must tr}^ to get a pure culture of the phenomenon. 



What I mean by this will appear from the following remarks. We 

 will start with the above-mentioned theory propounded by Schuster, 

 and now generally accepted in the leading magnetic circles, that the 

 diurnal variation of terrestrial magnetism is principally due to the influence 

 of a system of electrical currents outside the globe, issuing normally from 

 the sun. If we can now further assume that in a given case, during a 

 period of at least 24 hours, this current-system has remained unaffected 

 by other chance impulses issuing from the sun, and that for that space 

 of time Mre could have at our disposal magnetograms from uniformly con- 

 structed and regulated registering instruments, set up at a sufficient number 

 of points evenly distributed over the earth, we should be able, by the 

 aid of the stricdy simultaneous observations, to obtain some idea of the 

 relation of the current-system to the magnetic field of the rotating earth; 

 we should be able, for instance, to map the projection of the current- 

 system upon the earth's surface, and determine its altitude above that 

 surface. It would of course be necessary to treat a whole series of such 

 calm periods, at suitable intervals distributed over several years, before 

 the investigations could be regarded as complete. 



The mode of procedure here described must, however, as will be 

 easily understood, in the mean time be regarded only as a chimera, 

 which is not capable of practical realisation. I nevertheless believe 

 that the investigations of the diurnal variation of terrestrial magnetism 

 ought as far as possible to be carried out on these ideal lines, as the 

 simple attempt I have been able to make with the rather primitive ob- 

 servation-material at my disposal seems distinctly to show. 



When I say that we ought in the first place to turn our attention 

 to the conditions at the equinoxes, and not, as von Bezold maintains, by 

 preference deal with observations from the time about the solstices, this 

 is because it seems to me reasonable to assume that the phenomenon 

 will appear with the greatest regularity when day and night are of equal 

 length at all points of the earth's surface, a view in which I have been 

 strengthened by the following investigation, of which the results are 

 graphically presented in Fig. i. 



I have employed the calm days in each month out of the obser- 

 vations of the polar year from Bossekop and Sodankyla, and constructed 

 the mean vector-diagram for each of those stations with the combinations 



