40 NATIONAL ACADEMY OF SCIENCES 



longitude 97 west. The position of the south magnetic pole, 

 calculated from observations made in its vicinity by Captain 

 Scott, of glorious memory, in his expedition of 1901-04, is 72 50' 

 south latitude, 153 45' east longitude. Thus the two magnetic 

 poles are not only displaced about 20 from the geographical 

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

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

 in paths near the geographical poles in a period not yet definitely 

 known. In addition to these peculiarities, it must be added that 

 the dip-needle shows the existence of local magnetic poles, one of 

 which has recently been found by Dr. Bauer at Treadwell Point, 

 Alaska. At such a place the direction of the needle undergoes 

 rapid change as if 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 varia- 

 tion 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 fre- 

 quent and accurate surveys of the earth's magnetic field is illus- 

 trated by the fact that the Carnegie has found errors of five or 

 six degrees in the variation charts of the Pacific and Indian 

 oceans. 



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

 ciently illustrated by its topographical features, marked devia- 

 tions from the uniform magnetic properties of a magnetized 

 steel sphere are not at all surprising. The phenomenon of the 

 secular variation is one of the peculiarities toward the solution 

 of which both theory and experiment should be directed. 



Passing over other remarkable phenomena of terrestrial mag- 

 netism, we come to magnetic storms and auroras, which are 

 almost certainly of solar origin. 



