786. The magnetic needle can therefore assume a different direction on every part of the earth, according as its relative position is varied between two principal masses of metals, and even as is indicated by the movement of the compass with the change of geographical longitudes, it is removed from one metallic mass, approaches another, and by this becomes more powerfully attracted. Upon the whole, however, the direction must tend toward the poles. Such are the phenomena of _declination_ of the magnetic needle; and we can now comprehend why this is present.
787. But there must be also spots upon the earth, where the needle points straight to the north, probably, when it is between two metallic masses, or stands at a certain angle to them. These are the _lines without declination_, of which there are many as is well known, but which have not as yet been reduced to any law. Nor can they ever be so, because we shall never become acquainted with the metallic beds.
788. As the solid land, so far as it projects out of the water, forms a horse-shoe figure, of which the two Americas represent one leg, Europe with Asia and Africa the other leg, the sea being interposed between them; so must the lines without declination fall principally in the ocean, between the earth's crura. Moreover as both these crura of the earth are unequal in size, so must the one influence the needle more than the other, and in this respect also there can be therefore no regularity in the lines without declination. Thus everything co-operates, to the effect of rendering the direction of the magnetic needle unequal; such as the distribution of metalliferous mountains, of masses of iron, of the earth's hemispheres, of the earth's crura, and inversely the earth has probably obtained this horse-shoe form through magnetism. The cavities of the earth probably do not fall in there, because the ground, namely the mountains, is supported by metallic plates. On this account the earth probably maintains an oblique position in her course. In accordance with this the earth's axis had first changed, when the metals were generated. According to this also, the sea had rushed in first, when the metallic veins were present. Lastly, in accordance with this, the metals would be generated, when the whole earth was still covered, and the veins filled with water. Unto such conclusions the philosophy of nature can alone conduct us. The cause of a horse-shoe magnet acting more powerfully than any other shaped instrument, resides probably in the form of the earth's crura.