TERRESTRIAL MAGNETISM.] METEOROLOGY 165 points where the magnetic intensity is least is called the dynamic equator. It coincides very nearly with the dip equator. 30. Secular Variation. The earth then as a magnet must be supposed to have two sets of centres of force. "We shall next attempt to show that these centres cannot be regarded as constant both in position and intensity. It should be premised that, while there is no well-established evidence to show that either the pole of verticity or the centre of force to the north of America has perceptibly changed its place, there is, on the other hand, very strong evidence to show that we have a change of place on the part of the Siberian focus and also on the part of its analogue in the southern hemisphere. Table I. (p. 166), given by Gilpin (Phil. Trans., 1806), 1 exhibits the change in the position of the needle in Great Britain from FIG. 31. Lines of Equal Magnetic Dip, 1882. the earliest observations up to the beginning of the present century. 31. Between the dates recorded in this table the needle has been pointing more and more to the west, which implies either a relative increase in the power of the American as compared to the Siberian focus, or a motion of the Siberian focus from west to east. On the first supposition the lines to the eastward of the Siberian focus for instance, the line of no variation depending on a balance between it and the American focus should be drawn in towards it, or they should travel westwards ; but if the latter supposition is true, or this focus has been moving eastwards while retaining its force, the lines to the east of it should be found moving eastwards also. There is strong evidence that the latter is the case, and that in the northern hemisphere there has been a long-continued progression FIG. 32. Lines of Equal Horizontal Force, 1882. to the eastwards of the system of magnetic lines on both sides of the Siberian focus. In the southern hemisphere also we have proof that the analogous focus has been travelling, not from west to east, but from east to west. 32. There is some reason to believe that the eastward motion of the Siberian focus has been recently reversed, and that it is now going from east to west. Table II. shows the declination observed at Btishey Heath (Herts) during 1817-20, and at Kew from 1858 to 1882. It would appear from Table II. that the maximum westerly declination was reached in 1818, and that the needle has since that date been travelling eastwards. A similar change has taken place
1 Taken from Walker s Afagnetiim.