742 TERRESTRIAL MAGNETISM. 



If a steel needle like that in fig. 375, which before 

 magnetisation is balanced on the pivot in a horizontal 

 position, be magnetised, the north pole will be found 

 to incline or ' dip ' downwards. The inclination wotild be 

 still more considerable than it appears in this experi- 

 ment, if the needle, instead of being supported in the 

 middle of the conical cavity which lies above its centre 

 of gravity, were supported at the centre of gravity 

 itself. Such a needle would be in neutral equilibrium, 

 and therefore the action of terrestrial magnetic force 

 upon it would not be interfered with by the force of 

 gravity. A magnetised needle which is suspended by 

 a horizontal axis passing exactly through the centre 

 of gravity of the needle dips in England about 68. 

 This angle, which the needle makes with a horizontal 

 plane, is called the magnetic inclination. In the northern 

 hemisphere of the globe a magnetic needle is nearer to 

 the northern magnetic pole of the earth than to the 

 southern ; hence the directive action of the northern 

 magnetic pole is greater than that of the southern, and 

 the north pole of the needle dips. It follows that 

 in the southern hemisphere the south pole of the 

 needle dips ; that the inclination is greater the nearer 

 a needle is to either of the magnetic poles of the 

 earth ; and further, that between the northern and 

 southern magnetic poles there must be a line round 

 the globe, at any point of which both poles of the 

 needle are equally acted upon by the magnetic poles 

 of the earth, and where the needle is therefore hori- 

 zontal. This line is called the ' magnetic equator.' 

 The magnetic equator does not coincide with the ter- 

 restrial equator; it is, however, pretty near to it, and 

 crosses it in several points. 



