it must be stated that these diagrams are not, and could not be, drawn simply from observations. They are drawn from a theory (to be explained hereafter) founded upon all the observations which could be collected before the preparation of the charts (published in 1840). In general they represent very accurately the facts of observation but in later years some sensible but not important inaccuracies have been discovered in the southern hemisphere.

In Figures 20 and 21, it is to be remarked that the magnetic meridians might have been drawn through any arbitrary points of the geographical equator; they are in fact drawn through the points of east longitude 8°, 18°, 28°, &c.: 8° and 188° being the points at which the Magnetic Equator or line of no dip crosses the geographical equator. The magnetic meridians cannot generally be great circles of the sphere, because the two Magnetic Poles through which all must pass are not exactly opposite: they have moreover other irregularities of form, which do not depend on the character of the stereographic projection, but are equally conspicuous when the curves are traced on a globe. There is no trace of more than one pole either in the north or south.

The form of the lines of equal dip is remarkable. Commencing with the line of no dip or Magnetic Equator, it is easily seen that it is not a great circle: its greatest northerly distance from the geographical equator occurs at about 55° east longitude, or 47° from its node (instead of 90°), and its greatest southerly

distance at about 318° east longitude, or 50° from the same node. The inclination of the Magnetic Equator to the geographical equator is greater near the west coast of Africa than in any other part. Proceeding to the neighbouring lines, it will be seen that the increase of dip is nearly double of the increase of latitude; and upon this circumstance was founded the conjectural law, tan. Dip = 2 tan. distance from magnetic equator; to which we shall advert in the next Section. Nearer to the Magnetic Poles the curves are oval, or rather pear-shaped; but the major axes of the northern curves, and those of the southern curves, are not in the same direction. The North Magnetic Pole (in longitude 265°) and the South Magnetic Pole (in longitude 152°) are not opposite each other. These remarks show that the Earth's magnetism cannot be represented as the power of one magnet, and that the distribution of magnetism about the Earth is unsymmetrical.

The curves of equal Horizontal Force, Figures 28 and 29, are still more strange. The greatest values are 3733 at the geographical equator in longitude 259°, and 3.673 in 14° north latitude, longitude 103°. Proceeding along an equatoreal belt, one minimum of 3.039 is reached in longitude 345o, north latitude 3o, and another 3-408, in longitude 156°, south latitude 13o. Proceeding in either direction, north or south, from this equatoreal belt, the Horizontal Force gradually diminishes to 0 at each pole. The north pole is in the north of Baffin's Bay: the south in South Victoria.

But the forms of the southern curves only seem to indicate the existence of two poles of magnetic force. This indication differs remarkably from that which is founded upon the system of curves to be mentioned

next.

The curves of equal Total Magnetic Force present us with the singular phenomenon of two poles of maximum force in the north, and only one in the south. The numerical values of the forces at the former are, 6.160 west of Hudson's Bay, and 5·911 in Siberia: that of the latter 7.898 in South Victoria. Proceeding from these towards the equatoreal belt, the equatoreal maxima 3-686 and 3.649 are reached in longitude 252°, south latitude 7o, and in longitude 110°, north latitude 6o, and the equatoreal minima 2·828 and 3·248, near St. Helena in longitude 355o, latitude 16° south, and in longitude 179°, latitude 6o north. There is a rude approach to the law, that the Total Force at the Magnetic Poles is double that at the Magnetic Equator.

The theoretical connexion of these facts will be treated in the next Section.

At the end of Article 41 it was remarked that, at Greenwich, the Dip and Total Force are diminishing. Interpreting these by the remarks above, it would seem that the Magnetic Equator is approaching to Greenwich, or the North Magnetic Pole is receding from Greenwich. And remarking also the westerly change in direction of north magnetic meridian, from the sixteenth century to the year 1824, and its subsequent easterly motion (Article 24), it would seem that

the north magnetic pole has rotated round the terrestrial pole in a small circle from east to west, and having passed the point where its westerly azimuth as viewed from Greenwich is maximum, it is still continuing its course in that circle. It seems probable that in the fifteenth or sixteenth century it was situated between North Cape and Spitzbergen: it is now north-west of Hudson's Bay.

Valuable information on these changes, from the earliest period to the years about 1830, will be found in the work "Terrestrial and Cosmical Magnetism, the Adams Prize Essay for 1865, by Edward Walker, M.A." (Deightons, Cambridge). There is also some accurate information applying to later years, but not possessing all the completeness which might have been obtained from published records.

SECTION VI.

THEORIES ON THE PHYSICAL CAUSE OR REPRESENTATION OF TERRESTRIAL MAGNETISM.

43. Reasons for believing that Terrestrial Magnetism is not produced, in any important degree, by magnetic forces external to the earth.

If there were an external cause for magnetism, it seems scarcely conceivable that some large part of it would not act in planes parallel to the geographical equator: and, if so, its effects at any one place would undergo very great changes in the earth's diurnal revolution; every part of the earth being presented, in the course of a day, in different aspects towards forces so acting. Now the fact is that the diurnal changes are very small, perhaps at Greenwich part of the whole horizontal force. It would seem therefore certain that external bodies or space do not produce any sensible part of the magnetism in the planes to which the earth's axis is normal. And this carries with it a very strong improbability that they produce any sensible magnetic forces in the direction of the earth's axis also.