Terrestrial magnetism. — "Do the forces causing the diurnal 

 variation of terrestrial magnetism possess a potential?" By 

 Miss Annie van Vleuten. (Communicated by Dr. J. P. van 

 DER Stok). 



(Gommunicaled in the meeting of June 30, 1917). 



1. In their analysis of the diurnal field of variation, Schuster ^) 

 and Fritsche -) assumed that this tield possesses a potential, the 

 necessity of this hypothesis not being proved a priori. For a test by 

 means of integration along a closed curve on the surface sufficient 

 data are still lacking, but another method enables us to ijiquire how 

 far the horizontal forces can be deduced from one and the same 

 function. 



According to the Gaussian theory the potential of the terrestrial 

 magnetic field is determined, when we know : 



either the Northcomponent over the surface, or the Eastcomponent 

 over the surface and the values of the force, directed North along 

 a curve joining the two poles. 



As in this case the forces are those causing the diurnal variation, 

 hence completely periodical, the Eastcomponent on the surface alone 

 is sufficient to determine the potential. 



When for a number of points on the surface A A" and A F are 

 given, we can deduce from AA^ a function jtx representing the 

 potential, if there exists such a function ; in the same manner we can 

 deduce from AFa function :n[y and compare these two expressions. 



2. From the diurnal variation at Pavlovsk (59°41 N. 30°29 E.); 

 Sitka (57°3' N. 135°20' W.); Irkoutsk {52°16' N. 104°J9' E.); De 

 Bilt (52°6' N. 5°11' E.); Cheltenham (38°44' N. 76°51' W.); Zi-ka 

 Wei (3in9' N. 12i°2' E.); Honolulu (21°J9' N. 158°4' W.); 7iöw?% 

 (18°54' N. 72°49' E.); Buitenzorg (6°35' S. 106°47' E.) and Samoa 

 (13°48' S. J7r46' W.), all for the period 19U6— 1908, for each of 

 the components a function has been deduced in this manner.') 



1) The Diurnal Variation of Terr. Magn. Phil. Trans. Vol. 180 (1889) A. 



2) Die Tagliche Periode der Erdm. Elemente St. Petersburg 1902. 



^) This research is treated more at length in a thesis for the doctorate. 



