

CHAPTER IX 

 TERRESTRIAL MAGNETISM 



The earth is a great magnet, and, like the small magnets with 

 which we are familiar, has two poles. One of these poles is called 

 the North Magnetic Pole, and the other the South Magnetic Pole. 

 One end of the compass needle points toward one of these poles, 

 and the other toward the other. If we were to follow the directions 

 pointed by the compass needle, we would be led to the North 

 Magnetic Pole in the one case, and to the South Magnetic Pole in 

 the other. The magnetic meridians (Fig. 199) connect the magnetic 

 poles. 



The magnetic poles are far from the geographic poles, and they 

 are not exactly opposite each other. Their positions appear to 

 shift a little from year to year, but the change is not known to be 

 great. The north magnetic pole is in latitude about 70 N. and in 

 longitude 97 or 98 W., while the south magnetic pole is in lati- 

 tude 72 25' S. and longitude 155 16' E., according to the recent 

 determination of the Shackleton Expedition. 



Since one end of the magnetic needle points to the North 

 Magnetic Pole, it follows that the compass does not indicate true 

 north and south in many places. At points north of the North 

 Magnetic Pole, the " north" end of the needle points in a southerly 

 direction. At points to the east of the same pole, it points west- 

 ward, and at points west, eastward. The departure of the needle 

 from the true north and south is magnetic declination. A line con- 

 necting places of no declination is an agonic line, and lines connect- 

 ing places of equal declination are isogonic lines. 



Fig. 200 shows an agonic line in the United States running 

 from Lake Superior to South Carolina. On this line the magnetic 

 needle points due north and south. At all places east of this line 

 the needle points west of true north, and such places have west 



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