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OUR PHYSICAL WORLD 



line of force that runs through it. This and other bits of evidence 

 make scientists think that the earth is a great magnet with such 

 lines of force running from pole to pole, so making the compass 

 needle point northward. The magnetic poles, however, do not 

 quite coincide with the geographic poles, so the compass needle 

 does not point exactly to the north in most places. This devia- 

 tion must be taken into consideration in setting a ship's course. 



FIG. 82. Pattern of iron filings on a sheet of paper over a magnet 



Possibly it is currents of electricity that course around the earth 

 that make of the earth a magnet, just as we shall see it is possible 

 to make a bar of iron into a magnet by sending an electric current 

 through a wire coiled about it. But we must know something 

 of electricity to appreciate this. 



The ancients knew a little about electricity as well as about 

 magnetism. They knew that if one rubs a piece of resinous 

 substance, like amber, on cloth it will then attract light substances 

 like bits of straw or dry pith. Gilbert, an English physician, 



