60 Art. 9— T. Teracla : 



waves witli periods longer than 101" which show the retardation 

 generally less than -j-, may easily l)e explained Ijy a current 



of rather diffused character suhject to fluctuations. Indeed, 

 these longer waves are generally not very regular and rarely 

 form a train with the numher of successive maxima greater than 

 three or four, — a decided contrast with the shorter waves of a 

 few minutes periods whicli form as a rule remarkably regular 

 trains and show occasionally phase retardation of JZ greater than 

 ^. If the above conjecture be justified in some measure, the 



current, of which the local fluctuation produces these longer 

 Avaves, may plausibly be identified with the circular current 

 discussed in § 18 and compared with the princijDal part of the 

 current causing the diurnal variation of the terrestrial field, since 

 in this way, the variation of the azimuth of the disturbing field 

 may also be explained satisfactorily, at least to the first ap- 

 proximation. 



The maximum current intensity may be expected probaljly 

 near the equator if the phase relation between dX and JZ is 

 never inverted throughout the N-liemisphere. It will l)e in- 

 teresting to see if the relation is actually opposite on the ^5- 

 hemisphere, v. Bemmeln found ~t^ in Batavia invarial^ly 

 insignificant. 



On the other hand, the most regular waves with periods 

 less than about 4™ show as a rule remarkable phase retardation 

 of the vertical component after the horizontal ones, generally 

 greater than — . Tliis seems apparently difficult to explain on 



the assumption of stationary waves, if we solel}" rely on the above 

 calculation. 



Next, turning for a while to the case of progressive waves, 

 we remark that one fact seems at first siglit to be in favour of 

 this assumption. As described repeatedly above, the actual ratio 

 of the amplitudes AZjAX,,, never exceeds unity and decreases 

 gradually as the frequency of the wave increases. From (12) 

 and (lo), we have indeed 



