On Kapid Poriodie Variations of Terrestrial Magnetism. 



17 



one or the other component only. Even if a train of waves may 

 be traced in both components, the variation of the disturbing field 

 is generally of irregular rotatory character. 



c) Generally, no corresponding regular trains of short waves 

 with such remarkable duration can be seen during the evening 

 hours, nor is the remarkal)le parallelism between X- and Y- 

 components so conspicuous as in the morning. 



cV) During the later hours of the evening, the short waves 

 with periods of less than one minute generally disappear, while 

 very regular waves with 2-4 minutes periods appear instead, 

 frequently forming beautiful trains of nearly simple harmonic 

 waves. Another remarkable characteristic of the evening waves 

 is that the Y-component waves are generally inverted with respect 

 to the X-waves, i.e. the fluctuation of the horizontal magnetic 

 field is such as could be caused by the periodic variation of a 

 current running from NW to SE. Generally speaking, liowever, 

 the horizontal components show more or less rotatory character, 

 X- and Y-components showing frequently decided phase dif- 

 ference. This latter point wnll be fully investigated later. 



For these longer waves, the Z-component is more con- 

 spicuous than for the shorter waves frequent in day time, and 

 runs remarkabl}^ similar to the X-component, though lagging 

 behind it by a considerable fraction of the period (see Plates 

 I- IV). 



Fig. 5, A (reduced to \ original size). 



Note the rotatory character of the horizontal component in the upper curves. 



Ton 24 1913 



Tun. 2 6 . 



