TERRESTRIAL MAGNETISM. 113 



in reference to magnetism, had indeed recognised in the 

 autumn of 1827 that " the inclination is greater at 9 A.M. 

 than at 6 P.M. ; while the intensity of the magnetic force, as 

 measured by the vibrations of a horizontal needle, attains its 

 minimum at the former, and its maximum at the latter, of 

 these two epochs/' ( 134 ) The whole diurnal march of the 

 inclination has now been solidly established by means of 

 many thousand regularly continued hourly observations at 

 the British magnetic observatories since 1840, and by their 

 laborious discussion. This is the place for bringing together 

 the obtained facts as the foundations of a general theory of 

 terrestrial magnetism. Before doing so, it is desirable to 

 remark that, in considering the periodic fluctuations of the 

 three elements of terrestrial magnetism in their entire 

 character, we ought, with Sabine, to distinguish in the 

 " turning hours/' or hours of maxima or minima, between 

 two greater, and therefore important, extremes ; and other 

 intervening minor (though for the most part not less 

 regular) fluctuations. The recurring movements of the 

 inclination and declination needles present to us, then, as 

 do the variations of the intensity of the total force, principal 

 and secondary maxima and minima ; in the most usual cases 

 both principal and secondary, forming a double progression 

 with four turning hours, but less commonly a single maximum 

 and minimum only, forming a simple progression with but 

 two turning hours. The march of the total force in Van 

 Diemen Island, for example, is of the latter description, while 

 the inclination at the same station follows a double progres- 

 sion ; and at Toronto, in Canada, a place whose position in 

 the northern hemisphere corresponds almost exactly to that of 

 Hobarton in the southern hemisphere, the diurnal march of 

 VOL. IT. 



