des Magn. Vereins', 1838, 21;
Sabine, 'Report on the Variations of the Magnetic Intensity', p. 63.
One of the earliest facts yielded by observation is, that the intensity of
the total force increases from the equator toward the pole.*
[footnote] *The following is the history of the discovery of the law that
the intensity of the force increases (in general) with the magnetic
latitude. When I was anxious to attach myself, in 1798, to the expedition
of Captain Bandin, who intended to circumnavigate the globe, I was requested
by Borda, who took a warm interest in the success of my project, to examine
the oscillations of a vertical needle in the magnetic meridian in different
latitudes in each hemisphere, in order to determine whether the intensity of
the force was the same, or whether it varied in different places. During my
travels in the tropical regions of America, I paid much attention to this
subject. I observed that the same needle, which in the space of ten minutes
made 245 oscillations in Paris, 246 in the Havana, and 242 in Mexico,
performed only 216 oscillations during the same period at St. Carlos del Rio
Negro (1 degree 53' north lat. and 80 degrees 40' west long. from Paris), on
the magnetic equator, i.e., the line in which the inclination =0; in Peru (7
degrees 1' south lat.
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