A LECTURE ON MAGNETISM. 377 



we go to the equatorial regions of the earth with a suit- 

 ably-suspended needle, we shall find there the position of 

 the needle horizontal. If we sail north, one end of the 

 needle dips ; if we sail south, the opposite end 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 the 1st of June, 1831. In this manner we estab- 

 lish 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 degrees with 

 the true 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 before that time it was 

 easterly. All this proves that the earth's magnetic con- 

 stituents are gradually changing their distribution. This 

 change is very slow ; it is technically called the secular 

 change, and the observation of it has not yet extended over 

 a sufficient period of -time to enable us to guess, even ap- 

 proximately, at its laws. 



Having thus discovered, to some extent, the secret of 

 the earth's power, we can turn it to account. I hold in my 

 hand a poker formed of good soft iron ; it is now in the 

 line of dip, a tangent, in fact, to the earth's line of magnetic 

 force. 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 experiment : I hold the knob 

 uppermost, and it attracts the north end of a magnetic 



