IX 

 WORKING WONDERS WITH A TOP 



IF you were asked to define a compass, you would 

 probably say that it is essentially a magnetized 

 needle, so balanced that it can point to the North Pole. 

 If asked to be a little more accurate you would per- 

 haps explain that the magnetized needle does not 

 point to the true pole, but toward a "magnetic pole" 

 several hundred miles removed from the earth's axis. 



If still greater accuracy were required, you might 

 add that the magnetic needle varies the exact direc- 

 tion of its pointing with different longitudes of the 

 earth's surface; that it suffers deflection from various 

 cosmic disturbances, including sun spots; and that it 

 is very notably influenced by the proximity of any 

 magnetic metal, so that the compass on an iron or 

 steel ship requires a variety of adjustments and cor- 

 rections. But you might affirm, and until very re- 

 cently you would have been quite correct in affirming, 

 that notwithstanding these defects the magnetic com- 

 pass is the sole reliance of the mariner in steering 

 his ship in cloudy or foggy weather, and that lacking 

 this instrument, no navigator would dare to head the 

 prow of his vessel very far out of sight of land. 



Such being the traditional attributes of the com- 

 pass, it is rather startling to learn that there has 



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