342 LINES OF EQUAL INCLINATION. [SECT. xxx. 



and the latter in a vertical plane. It is well known that 

 the north end of the declination needle, or magnet, always 

 points to the north, and the south end to the south ; and 

 that it only remains at rest when in that position, being 

 then in the magnetic meridian of the place of observation. 



The north end of the dipping needle dips or bends beneath 

 the horizon in the northern hemisphere, and the south end 

 dips below the horizon in the southern. The two hemi- 

 spheres are separated by a line encircling the earth, called 

 the magnetic equator, or line of no dip, in every point of 

 which the dipping needle remains horizontal. The magnetic 

 equator crosses the terrestrial equator in several places, ex- 

 tending alternately on each side, but never receding farther 

 from it than about twelve degrees. It deviates but little 

 from the terrestrial equator in that part of the Pacific 

 Ocean where there are few islands ; but the deviation in- 

 creases where the islands become more frequent, and it is 

 greatest, both to the south and north, in traversing the 

 African and American continents. 



The needle dips more and more in going, north and south, 

 from the magnetic equator, and at last becomes perpen- 

 dicular to the horizon in two points, or rather two small 

 linear spaces, known as the magnetic poles, which are quite 

 distinct from the poles of the earth's rotation. That in the 

 northern hemisphere, determined by Captain Ross in the 

 year 1831, is in 70 north latitude, and 97 west longitude ; 

 while the south magnetic pole, since determined by Sir 

 James Ross, lies in the interior of Victoria Land, in 70 

 south latitude, and 102 east longitude. 



Lines of equal dip or inclination are such as may be ima- 

 gined to pass through all those points on the globe where 

 the dipping needle makes the same angle with the horizon. 

 The dip is subject to a secular variation. According to 

 Colonel Sabine it has been decreasing in the northern 

 latitudes, for the last fifty years, at the rate of three 

 minutes annually, which is probably occasioned by a 



