300 TERRESTRIAL MAGNETISM. SECT. XXX. 



SECTION XXX. 



Terrestrial Magnetism Magnetic Poles Lines of equal and no Variation 

 The Dip The Magnetic Equator Magnetic Intensity Secular, peri- 

 odic, and transitory Variations in the Magnetic Phenomena Origin of 

 the Mariner's Compass Natural Magnets Artificial Magnets Polarity 

 Induction Intensity Hypothesis of two Magnetic Fluids Distribu- 

 tion of the Magnetic Fluid Analogy between Magnetism and Electricity. 



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 com- 

 plicated, 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. 



Its influence, arising from unknown causes in the in- 

 terior of the earth, extends over every part of its surface, 

 but seems to be independent of the form and of the 

 peculiarities of the exterior of our planet (a). Its 

 action on the magnetic needle determines the magnetic 

 poles of the earth, which do not coincide with the poles 

 of rotation. 



Mr. Hansteen of Copenhagen computed, from obser- 

 vations in various parts of the world, that there are two 

 magnetic poles in each hemisphere, while M. Gauss 

 has concluded there is only one in each (A). The 

 position of one of these poles was determined by our 

 gallant countrymen when endeavoring to accomplish the 

 north-west passage round America. It is situate in 70 

 5' 17" north latitude, and 96 46' 45" west longitude. 

 Another northern magnetic pole is known by observa- 

 tion to be in Siberia, somewhat to the north of 60 north 

 latitude and in 102 east longitude, so that the two poles 

 are 198 46' 45" asunder. In his recent voyage to the 

 Antarctic regions Sir James Ross ascertained that one 

 of the southern magnetic poles is in 70 south latitude. 



