ELEMENTARY MAGNETISM. 365 



magnetic pole the needle sets vertical. The south 

 magnetic pole has not yet been found, but Sir James 

 Eoss discovered the north magnetic pole on June 1, 

 1831. In this manner we establish a complete paral- 

 lelism 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 meri- 

 dian; the declination is westerly. In the year 1660, 

 however, the declination was nothing, while before that 

 time it was easterly. All this proves that the earth's 

 magnetic constituents are gradually changing their dis- 

 tribution. 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 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 con- 

 straining 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 experiment : 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 ex- 

 periment and satisfy yourselves that the fluids shift their 

 position according to the manner in which the poker is 

 presented tp the earth. It has already been stated that 



