THE EARTH AND SUN AS MAGNETS 



109 



ical poles: they do not even lie on the same diameter of the earth. 

 Moreover, they are not fixed in position, but appear to be rotating about 

 the geographical poles in a period of about 900 years. Tn addition to 

 these peculiarities, it must be added that the dip-needle shows the ex- 

 istence of local magnetic poles, one of which has recently been found by 

 Dr. Bauer's party at Treadwell Point, Alaska. At such a place the 

 direction of the needle undergoes rapid change as it is moved about the 

 local pole. 



The dip-needle, as we have seen, is free to move in a vertical plane. 

 The compass needle moves in a horizontal plane. In general, it tends 

 to point toward the magnetic pole, and as this does not correspond with 

 the geographical pole, there are not many places on the earth's surface 

 where the needle indicates true north and south. Local peculiarities, 

 such as deposits of iron ore, also affect its direction very materially. 

 Thus a variation chart, which indicates the deviation of the compass 

 needle from geographical north, affords an excellent illustration of the 

 irregularities of terrestrial magnetism. The necessity for frequent and 

 accurate surveys of the earth's magnetic field is illustrated by the fact 

 that the Carnegie has found errors of five or six degrees in tlie varia- 

 tion charts of the Pacific and Indian oceans. 



In view of the earth's heterogeneous structure, which is sufficiently 

 illustrated by its topographical features, marked deviations from the 

 uniform magnetic properties of a magnetized steel sphere are not at 

 all surprising. The phenomenon of the secular variation, or the rota- 

 tion of the magnetic poles about the geographical poles, is one of the 

 peculiarities toward the solution of which both theory and experiment 

 should be directed. 



Passing over other remarkable phenomena of terrestiial magnetism. 



Fig. 4. Direct I'iiutogFwVph of I'akt nr the Son, April 30, 1&08. 



