MAGNETISM 



The dip circle is the instrument used for measuring the 

 vertical component of magnetic force, and consequently 

 it is a very important instrument in the polar regions. It 

 consists of a magnetised needle swinging on a horizontal 

 axis, and the readings are taken in degrees from the 

 vertical. 



The magnetic poles, or ends of the magnetic axis of 

 the earth, do not bear any necessary relation to the 

 geographical poles, which are the extremities of the rota- 

 tion axis of the earth. They are not diametral, but are 

 unsymmetrically placed. In this connection one authority 

 says : "In natural magnets the points at which attraction 

 takes place, otherwise called poles, are generally unsym- 

 metrically placed and depend entirely on the internal 

 structure of the magnet as well as on the irregularities 

 of its surface." 



The magnetic poles are not fixed spots but are con- 

 stantly travelling onward, executing an unknown path 

 and apparently completing a cycle in a period of many 

 hundreds of years. Besides this onward movement of a 

 few miles per year, there is a lesser daily oscillation. The 

 North Magnetic Pole was reached in 1831 by Sir James 

 Clark Ross, who afterwards visited the Antarctic in the 

 hope of securing the double event, but he was successful 

 only in locating the South Magnetic Pole by observations 

 made on his ship at a distance. In the interval between 

 1841, when these observations were made, and 1902, when 

 the Discovery expedition again located the South Mag- 

 netic Pole, it had moved about two hundred geographical 

 miles to the eastward. 



Observations of magnetic declination and dip were 

 taken at intervals along the route to the South Magnetic 

 Pole. Those taken on the coast, when compared with 

 values determined by the Discovery expedition, indicate 

 that the magnetic pole has, in the interval, moved in a 



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