THE GUIDING-NEEDLE ON AN IRON SHIP. 291 



will fly into contact and cling one unto the other with the tenacity 

 of a hundred arms. And this intense affinity of opposite magnet- 

 isms is a general characteristic. 



The Earth a Magnet. — Now, to show that the earth has 

 magnetic features entirely analogous to those of the bar-magnet, 

 we will examine Fig. 2. Here the parallels of latitude and meridi- 

 ans of longitude appear as regular curves. But from a focus at N 

 radiate a series of curves which take sinuous forms and finally 



Fig. 2.— The Eakth a Magnet. P P, geographical poles ; N S, magnetic poles ; m m, magnetic 

 equator ; r v and a b, lines of equal magnetic dip ; x z and o y, lines of equal magnetic variation. 



converge toward another focus at the antipodes. These foci are 

 the magnetic poles of the earth toward which the compass-needle 

 ever points, not indeed directly, but parallel to the lines of force. 

 These poles are not coincident with the geographical poles, but, on 

 the contrary, are far removed from them. 



There are other, but minor, magnetic foci on the earth, just as 

 there are secondary poles in a bar-magnet, but so overtopped in 

 prominence by the two grand foci that they scarcely deserve 

 mention. 



The lines issuing from one pole to meet again in the other are 

 called lines of equal variation ; that is, a compass carried along 

 one of them from north to south would always point at the same 



