



a.^ 





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^ 



t ' 



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•1 



V. 



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:t . 



T=--^i^H 



r\r 





le 



fV 



til . 



^ 'T 



T^nnr].' 



u 



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r Oi 



to 



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Sect. II.] 



TEREESTKIAL MAGNETIi^M. 



27 



Application of the Mesult^. 



13, The observations when thus received require that 

 the several corrections arising from the influence of the 

 I m^ the variations of temperaturej the changes in the 

 magnetic force of the magnets, and from various other 

 sources, should be sought out, computed and applied 

 and the true or corrected results finally derived. Thes 



5 



-e 



form the materials from which it is intended to construct 



maps, showing tlie variations 



o 



magne 



o 



and 



of the magne 



clination 



and declination, corresponding to the present 



epocii 



^\ over the whole surface of the y;lobe. 



& 



The 



varia- 



tions of the three elements are shown on these maps, by 

 lines comiecting, for example, in the maps of the magnetic 

 force, those points, where the intensity is observed to be 

 the same ; — in the maps of the inclination, those points 

 where the inclination is observed to be the same ;— and in 

 the maps of the declination, those points vvhere the de- 

 clination is observed to be the same. These lines are 

 known by the names of Isodynamic, isoclinal, and Iso- 

 genic lines. The Isogenic lines, which form the maps of 

 the declination (or variation charts), have a direct practi- 

 cal importance and value in navigation, %vhich in a notice 

 addressed to naval officers needs not to be dwelt on. In 

 theoretical respects, the Isodynamic and Isoclinal lines 



are not less essential ; the three form the basis of a svste- 

 matic view of terrestrial magnetism, as it manifests itself 

 to us on the surface of the globe. 



The mode in which the results are made to contribute 



_^ r 



to the formation of these maps is the following 



c2 



Th- 



^^±rf-> 



