510 



the northern magnetic hemisphere, the times of the 

 semi-annual inequality movement are systematically 

 earlier than in the opposite half-year, when the sun is 

 in the southern signs, and when the direction agrees 

 with that of the regular solar-diurnal movement of the 

 southern hemisphere : just as the times of the regular 

 solar-diurnal movement are throughout the year earlier 

 in the northern than in the southern hemisphere. In 

 this minor detail, as well as in the broader features, we 

 have similar semi-annual and hemispherical agreements 

 and differences. The distinctive character of the two 

 half-years is seen in its simplest form at magnetically- 

 equatorial stations, where the regular solar-diurnal varia- 

 tion drops out altogether, and the semi-annual inequality 

 constitutes the sole diurnal variation (during the hours 

 of the day). It is well shown (in the forenoon extremes 

 particularly) in the Phil. Trans. 1847, Art. VI. Plates 

 3 and 4, where the analogy with the hemispherical dif- 

 ference of the regular solar-diurnal variation is also 

 pointed out. It seems, as far as can be yet judged, to 

 be a general and systematic natural feature, and one too 

 important to be safely overlooked by those who are 

 seeking for physical explanations of the minor magnetic 

 variations. 



3. Diurnal Variation caused by the Disturbances. 



IT has been stated in Note II., that amongst the perio- 

 dical laws by which the mean effects of the disturbances 

 are governed, there is one which has for its period a 

 mean solar day. We have still, therefore, to consider 



4 



