Note 30 — Part II

The earth itself is a magnet according to Gilbert, who considered that the inflections of the lines of equal declination and inclination depend upon the distribution of mass, the configuration of continents, or the form and extent of the deep, intervening ocean basins. It is difficult to connect the periodic variations which characterize the three principal forms of magnetic phenomena (the isoclinic, the isogonic and the isodynamic lines) with this rigid system of the distribution of force and mass, unless we represent to ourselves the attractive force of the material particles modified by similar periodic changes of temperature in the interior of the terrestrial planet.... Of these lines, the isogonic are the most important in their immediate application to navigation, whilst we find from the most recent views that the isodynamic, especially those which indicate the horizontal force, are the most valuable elements in the theory of terrestrial magnetism (Humboldt, “Cosmos,” 1859–1860, Vol. I. pp. 180–181, 185; Vol. II. p. 334, wherein references are made to Gauss, “Resultate der Beob. des Magn. Vereins,” 1838, s. 21; Sabine, “Report on the Variations of the Magnetic Intensity,” p. 63).