18 



Art. 9. -T. Terada: 



e) The beginning of these waves is sometimes gradual, but 

 frequently abrupt, starting quite suddenly after a period of 

 dormant calmness (Fig. 5). The trains with the abrupt beginning 



Fig. 5, B (J original size). 



Remark the regularity with which similar disturbances are repeated. 



td> Z9. 1913 



Feb 2g J9I3 



are frequently accompanied by an abrupt increase of the average 

 value of the X-component whicli gradually attains a maximum 

 and then falls slowly (see Figs. 5 and 6). The sudden increase of 

 the X-component is usually accompanied by the simultaneous 

 sudden change of the average value of tlie Y-component, which 

 is at the outset generally toward E before midnight and toward W 

 after midnight. It seems as if an electric current were suddenly 

 started Avith rapidly increasing, and fluctuating intensity, with 

 its direction NW to SE, or SW to NE according to the hour of 

 occurrence. Disturbances of this kind seem to have been studied 

 by Birkeland who succeeded in tracing a system of whirling 

 currents (tourbillons de courants) extending over the N-hemi- 

 sphere, fixed relative to the sun. The slow change of the 

 horizontal components gradually attains a rotatory character, the 

 sense of which seems to generally confirm the result obtained by 

 R.B. Sangster,^-* i.e. mostly counterclockwise (N-W-S) before, and 



1) R. B. Sangster, Proc. Roy. Soc, 84 A. 1911. 



