however, by observing the time of vibration of a dippingneedle in the magnetic meridian: or by observing the extent to which the the needle is displaced by a given weight attached to a thread which is wrapped round the axis of the needle. The theory of these is so simple that there is no need to delay on them.

The ring-shaped lines in Figures 20 and 21 represent

(B is the primary and A the secondary northern pole of greatest magnetic intensity.)

the lines of equal dip over the surface of the earth: and the lines in Figures 35 and 36 represent the lines of

equal total magnetic force. The numbers upon the latter system of lines shew the value of the total force

(A is the southern pole of greatest magnetic intensity, and B the primary pole of small intensity.)

in Metric measure. To present more vividly to the eye the general facts of dip and total force over the earth, Figure 37 is drawn, exhibiting the directions of dip and the magnitude of total force along a meridian of the earth. The magnitude of force is shewn rudely by the lengths of the symbolical needles at the different points of the meridian. The map is on the orthographic pro

jection. It will be remarked that there is a little inaccuracy near the south pole, arising from the circum

stance that it is impossible to include the north and the south magnetic poles in the same geographical meridian.

The Dip and the Total Terrestrial Magnetic Force at any place, like the elements of which we have treated in Articles 24 and 34, are slowly changing. In 1843 the dip at Greenwich was about 69° 1′; it has diminished, with a rate continually accelerating, till in 1868 it

was 67° 56'.

in 1848 the

Adopting as elements of calculation that dip and horizontal force were 68° 47′ and 3.76, and, in 1866, 68° 1′ and 3·85: the total force was, in 1848, 10:39, and, in 1866, 10-28 (in English units), or 4.791 and 4.740 (in Metrical units).

42. Reference to the points of principal interest in Figures 20, 21, 28, 29, 35, 36: secular change in the place of North Magnetic Pole.

Before entering upon the consideration of the diagrams, we will allude to some general points regarding the connexion of the magnetic meridians with the curves of equal dip and of equal horizontal force.

Adopting, as the most convenient definition of Magnetic Pole," (when not qualified by any other words), "the point where the dip is vertical," there is no reason in nature why there should not be more than one magnetic pole in the north (or in the south). If, at one of these proximate places, the red pole dips, and at the other the blue pole dips, there must be between them a place of no dip (in the same manner as, in Figure 37, there is a place of no dip between the north and south poles of vertical dip with opposite poles of the needle). But, if the red pole dips at both, there is some complication introduced into the forms of the equal-dip curves. We will however commend these to the examination of the speculative student: remarking that we have no reason to think that there is more than one Magnetic Pole or place of vertical dip either in the north or in the south.

Now in progressing along one of the magnetic meridians in Figure 20, the observer who follows the direction of the horizontal needle is in fact continually proceeding in the plane of dip. And, if he finds the dip continually increasing (that is, if he advances towards the smaller circles of Figure 20), he will at last arrive at the place of vertical dip. Or, conversely, if he starts from the place of vertical dip, and continues in the course defined by any one of the directions of the horizontal needle into which he will immediately fall, he will pass away from the place of vertical dip in the plane of dip, and will therefore, for a time at least, have dip continually diminishing. From all this it appears that the pole of no dip must be the same as the pole common to every magnetic meridian; that is, the pole to which all magnetic meridians converge.

The pole to which the lines of equal horizontal force are related, that is, the point where horizontal force vanishes, is evidently the pole of vertical dip.

Thus the Magnetic Pole is a common pole for the convergence of magnetic meridians, for the verticality of dip, and for the evanescence of horizontal force.

But the pole of greatest total force is entirely different in its properties from these. It has not necessarily any connexion with them. There may be any number of points where the total force is maximum (in comparison with the points that surround them, to a considerable distance). The number of such points in the north may be different from that in the south.

We will now proceed with the diagrams. And first