isogonic

Their combined action may therefore be graphically represented by three systems of lines, the 'isodynamic, isoclinic', and 'isogonic' (or those of equal force, equal inclination, and equal declination).

COSMOS: A Sketch of the Physical Description of the Universe, Vol. 1 Alexander von Humboldt 1814

Admitting the existence of two principal solid masses whose general direction is from south to north, and that these masses are more susceptible of permeation by the ethereal fluid than the waters in which they are suspended, we have a general solution of the position of the magnetic poles, and of the isogonic, isoclinic, and isodynamic lines.

Outlines of a Mechanical Theory of Storms Containing the True Law of Lunar Influence T. Bassnett

But besides these variations which we have mentioned, there are changes steadily going on, by which the isodynamic, isogonic and isoclinic lines are permanently displaced on the surface of our planet.

Outlines of a Mechanical Theory of Storms Containing the True Law of Lunar Influence T. Bassnett

He assumes the declination of the magnetic needle at any give point on the surface of the Earth to be invariable (variatio uniuscujusque loci constans est), and refers the curvatures of the isogonic lines to the configuration of continents and the relative positions of sea basins, which possess a weaker magnetic force than the solid masses rising above the ocean.

COSMOS: A Sketch of the Physical Description of the Universe, Vol. 1 Alexander von Humboldt 1814

The isogonic lines are the more important in their immediate application to navigation, while we find from the most recent views that isodynamic lines, especially those which indicate the horizontal force, are the most valuable elements in the theory of terrestrial magnetism.

COSMOS: A Sketch of the Physical Description of the Universe, Vol. 1 Alexander von Humboldt 1814

In this portion of the earth, in northern Asia, between the mountains of Werchojansk, Jakutsk, and the northern Korea, the isogonic lines form a remarkable closed system.

COSMOS: A Sketch of the Physical Description of the Universe, Vol. 1 Alexander von Humboldt 1814

In like manner, we observe that the isogonic curves, when they pass in their secular motion from the surface of the sea to a continent or an island of considerable extent, continue for a long time in the same position, and become inflected as they advance.

COSMOS: A Sketch of the Physical Description of the Universe, Vol. 1 Alexander von Humboldt 1814

(isodynamic), equal inclination (isoclinic), and equal deviation (isogonic).

COSMOS: A Sketch of the Physical Description of the Universe, Vol. 1 Alexander von Humboldt 1814

[footnote] * To determine whether the two oval systems of isogonic lines, so singularly included each within itself, will continue to advance for centuries in the same inclosed form, or will unfold and expand themselves, is a question of the highest interest in the problem of the physical causes of terrestrial magnetism.

COSMOS: A Sketch of the Physical Description of the Universe, Vol. 1 Alexander von Humboldt 1814