22 



TERRESTR I AL MAGNETISM. 



[Sect. II. 



Direction of the EartJts Magnetic Force. 



8. The direction of tlie Eui'th's magnetic force under- 

 goes every possible variation at diiFerent parts of tlie 

 Earth's surface. For ihe purpose of determining am] re- 

 presenting this direction, it has long been customary to 

 refer it to two planes — the horizontal and the vertical 

 planes— and to take the geographical north as the zero 

 of the horizontal plane^ and the horizontal Hne us the 



zero of the vertical plane, 



m 



( 



.* 

 "^ 





as it is more usually called by naval men) is the angular 

 difterenccj measured on the horizontal plane, between the 

 direction of the north end of a magnet or needle and the 

 geographical north point ; and the inclination (or dip^ as 

 it is frequently called) is the angular difference, measured 

 on the vertical plane, between the direction of the same 

 north end of a magnet or needle and the horizontal zero 

 point. (The north end of a magnet here spoken of is that 

 end which in Europe points towards the north, and dips 

 below the horizon.) Tlie declination is called West, if 

 the direction of the north end of the magnet or needle is 



to the west 



SfcoOTanhical 



reckoned 



from 0" to 180% passing from North through West to 

 South. In like manner, the declination is called East, if 

 the direction of the north end of the needle is to the east 

 of the geographical north, and is reckoned from 0^ to 



180% passing from North through East to South, The 



positive and neoative signs are also sometimes applied 



instead of the terms West and East, in which case + sig- 

 nifies West, and — East Declination. 



The Inclination is counted positive, or has the sign 





\ 



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