254 POPULAR LECTURES AND ADDRESSES. 



Antarctic explorations have shown the magnetic 

 poles to be about 20 from the true poles. 



Shortly before Gilbert's time it had become 

 known in Europe that there the needle did not 

 point to true north, but several degrees to the 

 east of true north, and not to the same number 

 of degrees from the north in different places. The 

 deviation of the needle from the true or astro- 

 nomical north and south line was then called, and 

 is called by sailors to the present day, the 

 " variation " of the needle. Gilbert erroneously 

 explained the different magnetic variations in 

 different places by magnetic action of hills and 

 headlands, and was thus led to the false conclusion 

 that there would be no variation at great distances 

 from the land or in the central parts of a great 

 continent. We now know that the variation of the 

 needle depends in the main on the fact that the 

 magnetic axis of the earth deviates about twenty 

 degrees from the axis of rotation, and that the 

 amounts of the variation in different parts of the 

 world are somewhat nearly as they would be if the 

 distribution of terrestrial magnetism were regular 



