TERRESTRIAL MAGNETIC POLES. 95 



Now in progressing along one of the magnetic meri- 

 dians in Figure 20, the observer who follows the direc- 

 tion of the horizontal needle is in fact continually pro- 

 ceeding in the plane of dip. And, if he finds the dip 

 continually increasing (that is, if he advances towards 

 the smaller circles of Figure 20), he will at last arrive 

 at the place of vertical dip. Or, conversely, if he starts 

 from the place of vertical dip, and continues in the 

 course defined by any one of the directions of the hori- 

 zontal needle into which he will immediately fall, he 

 will pass away from the place of vertical dip in the 

 plane of dip, and will therefore, for a time at least, have 

 dip continually diminishing. From all this it appears 

 that the pole of no dip must be the same as the pole 

 common to every magnetic meridian ; that is, the pole 

 to which all magnetic meridians converge. 



The pole to which the lines of equal horizontal force 

 are related, that is, the point where horizontal force 

 vanishes, is evidently the pole of vertical dip. 



Thus the Magnetic Pole is a common pole for the 

 convergence of magnetic meridians, for the verticality of 

 dip, and for the evanescence of horizontal force. 



But the pole of greatest total force is entirely differ- 

 ent in its properties from these. It has not necessarily 

 any connexion with them. There may be any number 

 of points where the total force is maximum (in compari- 

 son with the points that surround them, to a considerable 

 distance). The number of such points in the north may 

 be different from that in the south. 



We will now proceed with the diagrams. And first 



