82 INTRODUCTION TO GENERAL SCIENCE 



magnetic needle. This variation is not always constant, as 

 the magnetic poles swing backward and forward. 



The cause of the earth's magnetism is not fully understood, 

 but it is probably due to the energy which the earth receives 

 from the sun. If there is any disturbance of the sun, as when 

 sun spots appear, there is always a disturbance of all the mag- 

 netic needles on the earth. Under these conditions the needles 

 act as if the magnetic poles were swinging hundreds of miles. 



References : 



1. 1002 : 437-438. A Theory of the Earth's Magnetism. 



2. 1304 : 418-419. Magnetism. 



3. 1803 : 235-237. Terrestrial Magnetism, 

 a. 1303 : 17-18. The Earth as a Magnet. 



6. 1305 : 30-33. The Earth a Huge Magnet. 



c. 1306:29-30. Magnetism and Electricity. 



d. 1307 : 274-279. The Aurora and the Earth's Magnetism. 



e. 1307 : 279. Magnetic Storms. 



/. 1309:33-39. The Earth as a Magnet and Magnetic 



Storms. 



g. 1310 : 301-305. Terrestrial Magnetism. 



h. 1311 : 274-278. The Earth's Magnetism. 



i. 1312 : 33. The Earth as a Magnet. 



j. 1809 : 367-369. Terrestrial Magnetism; 



k.- 1810:245-246. The Earth a Magnet. 



Experiment 26. The Earth's Magnetism. 



Apparatus : Bar magnet, pieces of steel, clock spring at least 

 3" long, wooden support, very fine silk thread, or untwisted 

 fiber. 



a. Support a piece of clock spring at its middle point by a 

 thread at least 12" long, and slowly bring one end of the magnet 

 near it. Do not touch the piece of clock spring with the mag- 

 net. If you happen to, ask for another piece. Approach the 



