to 12 degrees south lat. I regard the discovery of the law of
the decrement of magnetic force from the pole to the equator as the most
important result of my American voyage." Although not absolutely certain,
it is very probable that Condorcet read Lamanon's letter of July, 1787, at a
meeting of the Paris Academy of Sciences; and such a simple reading I regard
as a sufficient act of publication. ('Annuaire du Bureau des Longitudes',
1842, p. 463.) The first recognition of the law belongs, therefore, beyond
all question, to the comparison of La Perouse; but, long disregarded or
forgotten, the knowledge of the law that the intensity of the magnetic force
of the Earth varied with the latitude, did not, I conceive, acquire an
existence in science until the publication of my observations from 1798 to
1804. The object and the length of this note will not be indifferent to
those who are familiar with the connection with it, and who, from their own
experience, are aware that we are apt to attach some value to that which has
cost us the uninterrupted labor of five years, under the pressure of a
tropical climate, and of perilous mountain expeditions.
p 186
The knowledge which we possess of the quantity of this increase, and of all
the numerical relations of the law of intensity
p 187
affecting the whole Earth, is especially due, since 1819, to the unwearied
activity of Edward Sabine, who, after having observed the oscillations of
the same needles at the American north pole, in Greenland, at Spitzbergen,
and on the coasts of Guinea and Brazil, has continued to collect and arrange
all the facts capable of explaining the direction of the isodynamic system
in zones for a small part of South America.
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