Cbeorg and nature of dBa0netism. 33 



gradual deviation going on, but as it follows 

 a certain law mariners are able to tell just 

 what the deviation should be at a certain time. 

 The magnetic pole revolves around the polar 

 axis of the earth once in about 320 years. A 

 thermal current (one produced by heat) of 

 electricity seems to flow around the earth 

 caused by the irregularities of temperature at 

 the earth's surface, as the sun makes his daily 

 round. These earth currents vary at times, 

 and other phenomena are the occasion. This 

 will be discussed when we come to electric 

 storms. 



The value of the earth's magnetism is seen 

 most in the science of navigation. A mag- 

 netic needle is only a slender permanent mag- 

 net suspended very delicately, and when not 

 under local influence it points north and south 

 on the magnetic axis. The law of its action 

 may be explained as follows: Take a straight 

 bar magnet of fairly good power and suspend 

 a magnetic needle over it. The needle will 

 arrange- iNolf parallel to tlio bar magnet. The 

 north polo of the needle will point toward the 

 south polo of the b:ir magnet.. Tn the presence 

 of tin magnet the needle is not affected by the 

 earth, hut yields to a superior force. If, how- 

 'lie bar magnet is taken out of the way 

 of the needle it will immediately arrange it- 

 self north :nid -nuth. Of eiirso if the earth's 

 magnetic axis changes the needle will vary 



