62 The Realm of Nature CHAP. 



the meridians of longitude converge and all meet at the 

 poles. 



98. Terrestrial Magnetism. The rotation of the Earth 

 is probably the cause which confers on the globe as a whole 

 the properties of a great magnet ( 79). The poles of the 

 Earth-magnet are near the poles of rotation, but do not 

 coincide with them ; the north magnetic pole lies in 70 51' 

 N. 96 46' W. and the south about 73 S. 146 E. (see map 

 Plate I.) When a small straight magnet is hung by a fine 

 thread so that it can move freely in all directions, it takes 

 up a position which in most parts of the world is nearly 

 north and south, hence its use in the mariner's compass 

 ( 438) as a ready means of finding directions. A 

 suspended magnet when free from any disturbing attraction 

 points due north and south in all places, marked in the map 

 by the curves of o or agonic lines. The angle between 

 the meridian and the direction of a suspended magnetic 

 needle is called the declination, or by sailors the variation 

 of the needle. Between the agonic lines over almost all 

 Europe, Africa, the Atlantic and Indian Oceans, the needle 

 points west of north, the lines in the magnetic chart 

 showing the number of degrees in different places. In the 

 north-west of Greenland the declination is 90, or the needle 

 points due west ; while northward of the magnetic pole it is 

 1 80, or the north-seeking pole turns due south. Over most 

 of Asia, America, the Pacific and Indian Oceans, the declina- 

 tion is to the east of north. After a freely suspended steel 

 needle, balanced so as to rest horizontally upon its pivot, is 

 magnetised one end is found to be drawn downward by the 

 magnetic attraction of the Earth. This phenomenon is 

 called the Dip of the needle. Along a certain line on the 

 Earth's surface there is no dip ; this line is termed the 

 magnetic equator and is shown in the map. North of it 

 the north-seeking pole dips more and more until at the north 

 magnetic pole it points vertically downward. South of the 

 magnetic equator the south -seeking end of a suspended 

 magnetic needle dips downward. The intensity of magnetic 

 force varies from place to place, being nearly proportional to 

 the dip. In certain regions the rocks beneath the surface 



