Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/174

Rh 164 METEOROLOGY [TERRESTRIAL MAGNETISM. foci was first conjectured from the behaviour of the lines of variation or declination. Now it will be noticed by looking at the variation map (fig. 30) that all the lines of equal magnetic variation appear to converge to a point in the extreme north of the American continent. This point is not, however, coincident with the chief focus of force, which lies decidedly to its south ; but it is no doubt coincident with the point denoting a dip of 90, the locality of which may be inferred from the map of magnetic dip (fig. 31), and it is likewise no doubt coincident with the position of a zero of horizontal force which may be inferred from the map of horizontal force (fig. 32). Thus we have a point to the extreme north of America which has the following properties : (1) the various lines of declination converge to it ; (2) the needle points vertically down wards at it ; and (3) the horizontal force vanishes at it. At this FIG. 29. The Earth s Magnetism as shown by the Distribution of Lines of Equal Total Force, in Absolute Measure (British miles), with the Position of the Magnetic Poles and Equator, approximately for 1875. point therefore the horizontally balanced needle, having no hori zontal force acting upon it, will point in any direction. This point is, strictly speaking, the pole of verticity, but, inas much as there is only one such point in each hemisphere, these may for convenience sake be termed the magnetic poles, so that we speak of two centres or foci of maximum force and one pole in each hemi sphere. Eist Variation In the northern hemisphere Sir Frederick Evans 1 assumes the stronger or American focus to be in 52 N. and 90 W., and the weaker or Siberian focus in 70 N. and 115 E. In the southern hemisphere he assumes the position of the stronger focus to be 65 S. and 140 E., and of the weaker focus probably 50 S. and 130 E., these being thus not far separated from each other or from the magnetic pole. The nearness together of the southern foci is prob- Wusl Vu-iaUOO FIG. 30. Lines of Equal Magnetic Variation, 1882. ably the reason why the total force is greater at the southern than it is at the northern foci. The magnetic pole (of verticity) in the northern hemisphere was reached by Sir James Ross in 1831. The position of vertical dip was observed by him to be 70 5 N. and 96 43 W. The magnetic pole (of verticity) in the southern hemisphere was nearly attained by the same navigator in a voyage made in 1839-43. Its position is probably 73J S. and 147^ E. The line of no dip is called the magnetic or dip equator its position is given in figs. 29 and 31. The line connecting all the 1 Elementary Manual for the Deviation of tlie Compass in Iron Ships.