EAETH AND SUN AS MAGNETS HALE, 147 



Dr. Bauer in his extensive surveys represents the best now in use, 

 and also because I wish to contrast the widely different means em- 

 ployed by the Carnegie Institution for the investigation of solar and 

 terrestrial magnetic phenomena. The support of the dip needle is 

 hung in gimbals, so that observations may be taken when the ship's 

 deck is inclined. The smallest possible amount of metal enters into 

 the construction of this vessel, and where its use could not be avoided, 

 bronze was employed instead of iron or steel. She is thus admirably 

 adapted for magnetic work, as is shown by the observations secured 

 on voyages already totaling more than 100,000 miles. Her work 

 is supplemented by that of land parties, bearing instruments to re- 

 mote regions where magnetic observations have never before been 

 made. 



The dip needle clearly shows that the earth is a magnet, for it 

 behaves in nearly the same way as the little needle used in our ex- 

 periment with the magnetized sphere. But the magnetic poles of 

 the earth do not coincide with the geographical poles. The north 

 magnetic pole, discovered by Ross and last visited by Amundsen 

 in 1903, lies near Baffins Bay, in latitude 70° north, longitude 

 97° west. The position of the south magnetic pole, calculated from 

 observations made in its vicinity by Capt. Scott, of glorious memory, 

 in his expedition of 1901-1904, is 72° 50' south latitude, 153° 45' 

 east longitude. Thus the two magnetic poles are not only displaced 

 about 30° from the geographical poles; they do not even lie on the 

 s'ame diameter of the earth. Moreover, they are not fixed in posi- 

 tion, but appear to be rotating about the geographical poles in a 

 period of about 900 years. In addition to these peculiarities, it 

 must be added that the dip needle shows the existence of local mag- 

 netic 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 geogi'aphicai 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 direc- 

 tion very materially. Thus a variation chart, which indicates the 

 deviation of the compass needle from geographical north, affords an 

 excellent illustration of the irregailarities of terrestrial magnetism. 

 The necessity for frequent and accurate surveys of the earth's mag- 

 netic field is illustrated by the fact that the Carnegie has found errors 

 of 5° or 6° in the variation 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 fi'om the 



