328 DISCOURSE ON THE STUDY 



towards a point below the horizon, and situated 

 within the earth. By tracing the variation and dip 

 over the whole surface of the globe, it has been 

 found that these phenomena take place as they 

 would do if the earth itself were a great magnet, 

 having its poles deeply situated below the surface, 

 and, what is very remarkable, possessing a slow 

 motion within it, in consequence of which neither 

 the variation nor dip remain constantly the same at 

 the same place. The laws of this motion are at 

 present unknown ; but the discovery of electro - 

 magnetism, by rendering it almost certain that the 

 earth's magnetism is merely an effect of the con- 

 tinual circulation of great quantities of electricity 

 round it, in a direction generally corresponding with 

 that of its rotation, have dissipated the greater part 

 of the mystery which hung over these phenomena ; 

 since a variety of causes, both geological and others, 

 may be imagined which may produce considerable 

 deviations in the intensity, and partial ones in the 

 direction, of such electric currents. The unequal 

 distribution of land and sea in the two hemispheres, 

 by affecting the operation of the sun's heat in pro- 

 ducing evaporation from the latter, which is probably 

 one of the great sources of terrestrial electricity, 

 may easily be conceived to modify the general tend- 

 ency of such currents, and to produce irregularities 

 in them, which may render a satisfactory account of 

 whatever still appears anomalous in the phenomena 

 of terrestrial magnetism. This branch of science 

 thus becomes connected, on a great scale, with that 

 of meteorology, one of the most complicated and 

 difficult, but at the same time interesting, subjects of 



