756 



B1RKELAND. THE NORWEGIAN AURORA POLARIS EXPEDITION, 19021903. 



In my earlier work, "Expedition Norvegienne 1899 1900" (p. 7), I drew attention to a number o 

 very regular sinusoidal oscillations that were observed at the Haldde Observatory on March 19 and 20 

 1900. I here reproduce on an enlarged scale the previously published curves showing this condition (fig. 2-^1 

 On the i8th May, 1910, we had the opportunity of observing exactly similar rapid, regular oscil- 

 lations simultaneously in two sets of magnetic apparatus, which were placed at a distance of about 300 

 metres from one another. They proved to be accompanied by exactly similar oscillations in the earth- 

 currents, and the two appear to be exactly synchronous, although an eventual small phase-alteration 

 could scarcely be demonstrated. I here reproduce those curves in which these oscillations are notio 



It will be seen that the oscillations occur in two epoch 



At the end of these epochs there is also rapid registerin 



with one set of magnetometers. It is here therefo 



that the period of oscillation can best be determine 



For this we find the following values: 



119 sec. n8 sec. 



122 113 



1 28 121 



I 24 I 09 



Mean value: 119.3 sec - 

 With regard to the cause of these oscillatii 

 we will only refer the reader to Art. 122. What wt 

 would especially call attention to here is that tli> - 

 oscillations occur simultaneously and probably t : 

 273. synchronously in earth-current and magnetism. 



//- 



hyfij"**~>^^\/<f\f~ -vwwv/v*"^^^ ' ""**'*'* 



f 1 



r 



Earth currents and magnetic elements 17 18 May, 1910. 

 Fig. 374. 



