Ou Rapid Periodic. Variations of Terrestrial Magnetism. 



47 



The frequencies of the two groups in different hours were 

 calculated, as shown in Table X. and also in Fig. 17. 



Table X. 



For each group, the frequency shows apparentl}^ a seinidiurnal 

 period. Comparing the result w4th Fig. 16, it may be noticed 

 that the clockwise rotation predominates in those hours where 

 the direction of the disturbing field is at the maximum deviation 

 from the meridian, while the counterclockwise rotation falls 

 most frequently in tlie intermediate hours. It must be also 

 remarked that the sense of rotation during night hours is 

 generally opposite to that determined by Sangster^^ for distur- 

 bances of decidedly longer durations, and therefore also opposite 

 to the sense in which the abrupt disturbing field described in §5 

 e) tends. In Sangster's case the sense of rotation showed a 

 diurnal period, being of the same sense throughout twelve hours, 

 while in the case of the short waves here in question, it shows 

 a semidiurnal period as given above. 



PART IV. 

 Discussion of the Results. 



IS. The results of the present investigations so far described 

 seem to tlirow some light, however faint, on the actual origin 



1) Sangster, loc. cit. 



