[footnote] *Humboldt, 'Ueber die secul??re Ver??nderung der Magnetischen
Inclination' (On the secular Change in the Magnetic Inclination), in Pogg.
'Annal.', bd. sv., s. 322.
We have described the distribution of magnetism on the surface of our planet
according to the two forms of 'declination' and 'inclination'; it now,
therefore, remains for us to speak of the 'intensity of the force' which is
graphically expressed by isodynamic curves (or lines of equal intensity).
The investigation and measurement of this force by the oscillations of a
vertical or horizontal needle have only excited a general and lively
interest in its telluric relations since the beginning of the nineteenth
century. The application of delicate optical and chronometrical instruments
has rendered the measurement of this horizontal power susceptible of a
degree of accuracy far surpassing that attained in any other magnetic
determinations. 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.*
[footnote] *Gauss, 'Resultate der Beob.
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