"KOTES. Ivii 



conclusion which the sagacity of Halley enalled him to draw more than 

 150 years ago (Phil. Trans. 1683, No. 148, p. 208), that the complexity 

 and seeming irregularity of the declination observed in different parts of the 

 earth were due to forces which must produce two points of greatest at- 

 traction in each hemisphere. 



The interval which has elapsed since the phenomena of the magnetic force 

 have been an object of attention, has been too short to admit of any direct 

 inference being drawn in regard to the effect of secular change on the 

 isodynarnic lines : but an attentive consideration of features which are ob- 

 viously connected with each other in the three systems of elementary 

 lines, confirms what indeed could scarcely have been doubted that altera- 

 tions which are known to have taken place in the configuration and posi- 

 tion of the isogonic and isoclinal lines in the last two and half centuries, 

 have been accompanied by corresponding changes in the lines of equal force. 

 The influence of secular change appears chiefly to affect the portions of 

 the isodynamic lines which are most nearly connected with the two minor 

 foci : from considerations which need not be particularised here, these foci 

 appear to have moved in opposite directions the Siberian from west to 

 east, and the minor focus in the south from east to west. The remarkable 

 closed systems of isogonic lines in Siberia and in the south Pacific, to which 

 M. de Humboldt has referred in page 171 of the text, and in note 152, appear 

 to have undergone a movement of translation in the same directions as the 

 minor foci. The ./Etiology of this science is of so remarkable a character, 

 that it seems not improbable that the secular changes, wliich now appear 

 to us the most mysterious branch of the phseuomena presented to our notice, 

 may eventually prove a means of conducting to a solution of the problem 

 of highest interest that of the physical causes of terrestrial magnetism. 



Although it is scarcely safe to anticipate that any portion of phenomena 

 may be less deserving of attention than another, yet, as M. de Humboldt 

 has remarked that meteorology must seek its foundations and its advance 

 first within the tropics, because the phsenomena may there be viewed under an 

 aspect of less complexitythan. elsewhere, so the converse may be remarked 

 in respect to magnetism; inasmuch as the characteristics of the magnetic 

 system are less distinctly marked within the tropics than in the higher 

 latitudes, and the influences of the two hemispheres are so blended in the 

 inflexions of the magnetic lines within the tropics, that to understand them 

 it is necessary to have continually present in the mind the phenomena of both 



