iv The Earth a Spinning Ball 63 



of the Earth exercise a powerful attraction on a suspended 

 magnet ( 348). 



99. Periodical Magnetic Changes. In 1576, when the 

 declination of the magnetic needle was first measured in 

 London, the north-seeking pole pointed 1 1 east of north, 

 but the easterly declination gradually diminished until in 

 1652 the needle pointed due north, and, the change still 

 continuing, in 1815 it pointed 24^ west of north. Since 

 then the declination has gradually diminished, being only 

 17 W. at London in 1891, and decreasing about 9' per 

 annum. The dip is subject to a similar slow change. These 

 changes were formerly accounted for by supposing that the 

 magnetic poles changed their position on the Earth's surface. 

 Recent observations indicate that this is not the case ; they 

 rather suggest that the alteration of declination and dip 

 may be produced by geological changes taking place in the 

 Earth's crust. Commander Creak, as the result of the 

 " Challenger " observations, states that the change is most 

 rapid at several points in a line drawn from the North Cape 

 along the Atlantic to Cape Horn, and that the British 

 Islands are situated in the region where the rate of change 

 is greatest of all. 2 Regular changes of shorter period also 

 occur, the needle daily swinging perhaps 5' or 6' to E. 

 and W. of its average position and back again ; and there 

 is a yearly periodicity as well. Irregular variations of much 

 greater extent, sometimes amounting to one or two degrees, 

 are called magnetic storms, and are closely connected with 

 the appearance of the aurora ( 174). Auroras and 

 magnetic storms are most frequent at intervals of about 1 1 

 years, corresponding to the periods of greatest frequency of 

 sun-spots. It is remarkable that whenever a great uprush 

 of heated gas takes place in the Sun, producing solar 

 prominences ( 1 1 6), there is a simultaneous disturbance 

 of all the delicately - hung magnetic needles on the 

 Earth. Thus it appears that while the Earth's magnetism 

 resides in the massive rocks of its crust, and is probably 

 produced and maintained by the Earth's rotation, the 

 Sun's energy exercises a regulating or disturbing influence 

 upon it. 



