TERRESTRIAL MAGNETISM, ETC. 253 



the point of the earth's surface where the dipping- 

 needle rests with its magnetic axis vertical and its 

 true south pole downwards ; the south magnetic 

 pole is the point where the dipping-needle rests with 

 its axis vertical and its true north pole downwards. 

 The line of no dip, or that line round the earth at 

 every point of which the dipping-needle is hori- 

 zontal, is called the magnetic equator. At either 

 pole a horizontal needle, supported so as to be free 

 to turn round a vertical axis, shows no directive 

 tendency; thus the mariner's compass altogether 

 fails at the magnetic poles, and for hundreds of 

 miles round them shows but very feeble directional 

 tendency. 



Gilbert fell into one grand error by a dereliction 

 from his own principles of philosophy. He as- 

 sumed, without proof from observation, that the 

 earth's magnetic poles must concide with the " poles 

 of the world," as he calls those points which we 

 nowadays call the true astronomical poles, to dis- 

 tinguish them from the magnetic poles, being, in 

 fact, the points in which the earth's surface is cut 

 by its axis of rotation. Modern Arctic and 



