On Rapid Periodic Variations of Terrestrial Magnetism. 



41 



As will be seen from Fig. 13, the retardation is remarkable 

 for shorter waves, sometimes amonntin«; to decidecUv more than 



Fig. 13, B. 



0-2 



0-! 



10^ 



ZO" 



ZÖ' 



40^ 



a quarter period in the case of 3(/ waves and gradually decreasing 

 in a hyperbolic cur\-e apparently tending to zero for longer waves 

 over one hour. Combining the result with that obtained in the 

 preceding article, we may trace the meridional projection of the 

 elliptic orbit described by the end of the periodic disturl)ing 

 vector of different periods. 



15. Äzimutli, of periodic disturbing fields, or the relation of 

 amplitudes of X- and Y-comj>onents. While the amplitude ratio 

 between JX and JZ depends very much on the periods, but not 

 sensibly on the hours of day, the ratio l)etween the amplitudes of 

 X- and Y-waves varies remarkably with the hours of occurrence, 

 but not sensibly with the periods. To investigate the case more 

 closely, the following procedures were adopted. 



At first, all available records were exami^ied and the numljer 

 of regular trains were counted in which the two horizontal 

 components are nearly parallel to each other, i.e. of phase 

 difference zero, and also those in which the two are nearly 

 inverted, i.e. of phase difference ^. The results are given in the 

 Table VI. and also in Fig. 14, regardless of the periods of the 

 waves. 



