190 



GENERAL SCIENCE 



some force causes the molecules to arrange themselves in 

 a certain order. The force which we have used in our 

 experiments is simply the magnetic force in other mag- 

 nets. After the molecules in a bar have been arranged 

 so that the bar itself acts as a magnet, any force which 

 disarranges the molecules will cause a loss of magnetism 

 in the bar (Figure 156). 



Magnetic Condition of the Earth. The earth is a great 

 magnet with its S pole somewhere near the geographical 



north pole and its N pole near 

 the geographical south pole. Dr. 

 William Gilbert first suggested 

 that the earth was a magnet and 

 offered it as an explanation of 

 the behavior of the compass. All 

 of us have read of the troubles 

 of Columbus on his first voyage, 

 and especially those due to the 

 alarm of his sailors when they 

 discovered that the compass no 

 longer pointed north. The reason 

 for the variation of the compass 

 is found in the fact that the 

 magnetic poles and the geo- 



FIG. 158. A Dipping Needle. & , . , , . . , 



graphical poles do not coincide. 



The number of degrees which the needle varies from 

 the true north and south line is called its declination. 

 There are not many places where the needle points 

 exactly north or where the declination is zero. Lines 

 passing through points of zero declination are called 

 agonic lines (Figure 157). Lines which pass through 

 points having the same declination are called isogonic 

 lines. 



