ELEMENTARY MAGNETISM 385 



dips; and over the north or south terrestrial magnetic 

 pole the needle sets vertical. The south magnetic pole 

 has not yet been found, but Sir James Ross discovered 

 the north magnetic pole on June 1, 1831. In this manner 

 we establish a complete parallelism between the action of 

 the earth and that of an ordinary magnet. 



The terrestrial magnetic poles do not coincide with the 

 geographical ones; nor does the earth's magnetic equator 

 quite coincide with the geographical equator. The direc- 

 tion of the magnetic needle in London, which is called the 

 magnetic meridian, encloses an angle of 24 with the as- 

 tronomical meridian, this angle being called the Declina- 

 tion of the needle for London. The north pole of the 

 needle now lies to the west of the true meridian; the dec- 

 lination is westerly. In the year 1660, however, the decli- 

 nation was nothing, while before that time it was easterly. 

 All this proves that the earth's magnetic constituents are 

 gradually changing their distribution. This change is 

 very slow: it is therefore called the secular change, 

 and the observation cf it has not yet extended over a 

 sufficient period to enable us to guess, even approxi- 

 mately, at its laws. 



Having thus discovered, to some extent, the secret of 

 the earth's magnetic power, we can turn it to account. 

 In the line of "dip" I hold a poker formed of good soft 

 iron. The earth, acting as a magnet, is at this moment 

 constraining the two fluids of the poker to separate, mak- 

 ing the lower end of the poker a north pole, and the 

 upper end a south pole. Mark the experiment: When 

 the knob is uppermost, it attracts the north end of a mag- 

 netic needle; when undermost it attracts the south end of 

 a magnetic needle. With such a poker repeat this ex- 



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