ELEMENTARY MAGNETISM. 365 



magnetic pole the needle sets vertical. The south mag- 

 net 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 direction of the magnetic needle in London, which 

 is called the magnetic meridian, encloses an angle of 

 24 with the astronomical meridian, this angle being 

 called the Declination of the needle for London. The 

 north pole of the needle now lies to the west of the 

 true meridian; the declination is westerly. In the year 

 1660, however, the declination was nothing, while be- 

 fore that time it was easterly. All this proves that 

 the earth's magnetic constituents are gradually chang- 

 ing their distribution. This change is very slow: it is 

 therefore called the secular change, and the observation 

 of it has not yet extended over a sufficient period to 

 enable us to guess, even approximately, at its laws. 



Having thus discovered, to some extent, the secret 

 of the earth's magnetic power, we can turn it to ac- 

 count. 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, making the lower end of the poker a north 

 pole, and the upper end a south pole. Mark the experi- 

 ment: When the knob is uppermost, it attracts the 

 north end of a magnetic needle; when undermost it 

 attracts the south end of a magnetic needle. With 

 such a poker repeat this experiment and satisfy your- 

 selves that the fluids shift their position according to 

 the manner in which the poker is presented to the 



