SECT, xxx.] TERRESTRIAL MAGNETISM. 3-41 



SECTION XXX. 



Terrestrial Magnetism Magnetic Poles Magnetic Equator Lines of equal 

 Inclination The Intensity Foci of Magnetic Intensity Dynamic Equator 

 Lines of equal Intensity Form of these Lines Declination Lines of no 

 Variation Lines of equal Variation Secular Variation of the System 

 Horary Variation Line partaking of the Horary Changes of both Hemi- 

 spheresMagnetic Storms Origin of the Mariner's Compass Natural 

 Magnets Artificial Magnets Polarity Induction Intensity Hypotheses 

 of two Magnetic Fluids Distribution of the Magnetic Fluid Electrifying 

 and Magnetising of Light Diamagnetism. 



IN order to explain the other methods of exciting electricity, 

 and the recent discoveries in that science, it is necessary to 

 be acquainted with the general theory of magnetism, and 

 also with the magnetism of the earth, the director of the 

 mariner's compass his guide through the ocean. 



The distribution of terrestrial magnetism is very compli- 

 cated ; and the observations simultaneously made at the 

 various magnetic establishments recently formed in both 

 hemispheres have changed many of the opinions formerly 

 received with regard to that science. 



Terrestrial magnetism is an unseen power, whose influence 

 extends over every part of the earth, depending on the heat 

 and motion of the sun, on galvanic currents circulating 

 through the earth's surface, on the unequal temperature of 

 the globe, on changes of temperature which elicit magnetic 

 and electric currents, and on its own changes. What part 

 any of these causes have, individually, on the general result, 

 or the reasons of its perpetual changes, are altogether un- 

 known ; but its complicated and threefold phenomena are 

 brought into evidence, and measured by the declination 

 needle, or mariner's compass, and the dipping needle. The 

 first is suspended so as to move only in a horizontal plane, 



