SECT. XXXIV. MAGNETIC ELEMENTS. 343 



from all others in having four poles of maximum magnetic force 

 of different intensities, the two in the northern hemisphere 

 having a secular motion in a contrary direction from the two in 

 the southern. They are not even symmetrically placed ; hence 

 the magnetic intensity varies so much in the different points on 

 the earth's surface, that the dynamic equator, or line passing 

 through all the points of least intensity, is a very irregular curve 

 surrounding the globe, but by no means coinciding with the ter- 

 restrial equator. In consequence of the mean action of these 

 four forces, the north end of a magnetised needle, arranged so as 

 to revolve in a vertical plane, dips or inclines beneath the 

 horizon in the northern hemisphere, and the south end in the 

 southern. The two hemispheres are separated by a line en- 

 circling the earth, called the magnetic equator, or line of no dip, 

 in which the dipping or inclination needle is horizontal. On 

 each side of this line the inclination increases till at last the 

 needle becomes perpendicular to the horizon in two points, or 

 rather small spaces, in each hemisphere, known as the magnetic 

 poles, which are quite different from the poles of the earth's 

 rotation. The mean action of the four poles of magnetic inten- 

 sity causes the mariner's compass, or a magnetic needle suspended 

 so as to revolve in a horizontal plane, to remain at rest when 

 pointing to the two magnetic poles. It is then in the magnetic 

 meridian of the place of observation, which is thus determined 

 by the mean action of all the four magnetic forces. 



These mean values of the three magnetic elements, namely, 

 the declination, inclination or dip, and magnetic intensity, are 

 well known to be subject to secular, annual, and diurnal varia- 

 tions. The secular only become sensible after some years, but 

 the annual and diurnal variations have a double progression 

 that is to say, two maximum and two minimum values in 

 their respective periods of a year and twenty-four hours ; for 

 example, the declination needle makes two deviations to the west 

 and two to the east in the course of twenty-four hours, and that 

 with great regularity. Now General Sabine discovered that the 

 double progression arises from two combined or superposed varia- 

 tions having different hours of maxima and minima, and that 

 they are due to two distinctly different causes the one being 

 the difference in the sun's position relatively to the place of 

 observation at the different seasons of the year, and hours of the 



