2Q4 PRELIMINARY. 



306. DISTRIBUTION OF TERRESTRIAL MAGNETISM. The ele- 

 ments of terrestrial magnetism, strength, declination, and inclination 

 are not the same at the different points of the earth. These elements 

 vary as a function of the geographical co-ordinates according to very 

 complicated laws ; but if we are content with a first approximation, 

 the variations may be formulated in a very simple manner. 



The magnetic meridian at any one place cuts the surface of the 

 globe along a great circle; all points of this great circle have the 

 same plane for magnetic meridian. 



All magnetic meridians intersect along the same diameter ; this 

 diameter is the magnetic axis of the earth ; the points where it cuts 

 the surface have been named, though incorrectly, magnetic poles. 

 The magnetic axis makes an angle of about 15 with the axis of 

 rotation of the earth. 



It is evident that the declination varies from one point to 

 another on the same magnetic meridian. The only exception 

 is the meridian which, passing both through the magnetic axis 

 and the terrestrial axis, is identical with the geographical meridian ; 

 for all corresponding points the declination is null. On one side 

 of this great circle, the north pole turns to the west and the 

 declination is west; on the other, it turns to the east and the 

 declination is east. 



The great circle perpendicular to the magnetic axis is called the 

 magnetic equator. In all points of the magnetic equator, the earth's 

 force is horizontal and the inclination zero. On either side the 

 inclination increases to the magnetic poles where it is 90: in the 

 northern hemisphere, the north pole dips downwards ; and in the 

 southern hemisphere, the south pole. 



307. HYPOTHESIS OF A TERRESTRIAL MAGNET. Biot tried if 

 it were possible to represent the magnetic condition of the globe, 

 and the variation of the magnetic elements on its surface, by the 

 hypothesis of a central magnet in the direction of the magnetic axis ; 

 he found that the results of calculation agreed the better with the 

 observations the smaller was the distance of the poles of this 

 imaginary magnet. 



If we thus replace the earth by a magnet which is infinitely small 

 as compared with the radius that is to say, by two equal masses 

 of opposite signs which are very near each other, we know (153) 

 that at the latitude A, counting from the magnetic equator, the 

 inclination is given by the equation 



tan 1 = 2 tan A. 



