92 TERRESTRIAL MAGNETISM. 



Two magnetic poles, one in each hemisphere, at un- 

 equal distances from the earth's poles of rotation : these 

 are points on the earth's surface at which the magnetic 

 inclination is 90, and at which, therefore, the horizontal 

 force vanishes. 



The magnetic equator: i. e. the curve or line encompass- 

 ing the earth, on which the inclination of the needle is 0. 



Lines of equal declination, and on which the decima- 

 tion of the needle is (isogonic lines, and lines of no 

 declination) . 



Lines of equal inclination (isoclinal lines) . 



Eour points of greatest intensity of the earth's 

 magnetic force ; two, of unequal strength, in each hemi- 

 sphere. 



Lines of equal terrestrial magnetic force (isodynamic 

 lines) . 



The undulating line which connects those points at 

 which the force is weakest in each meridian, and which 

 has, on that account, been termed a " dynamic equator," 

 or " equator of force." ( 88 ) Tt does not coincide either 

 with the geographical or the magnetical equator. 



The boundaries of the zone (generally of very weak 

 magnetic force), in which the horary (or diurnal) varia- 

 tions of the needle at certain hours of the day conform, 

 during one part of the year to the diurnal variation of 

 the phsenomena in the northern, and during the other 

 part of the year to those of the southern, magnetic 

 hemisphere ;( 89 ) taking part, therefore, alternately in the 

 variations of both hemispheres. 



In this enumeration I have reserved the word "pole" 



