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The Jeffersonian cyclopedia;

a comprehensive collection of the views of Thomas Jefferson classified and arranged in alphabetical order under nine thousand titles relating to government, politics, law, education, political economy, finance, science, art, literature, religious freedom, morals, etc.;

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4472. LATITUDE AND LONGITUDE, Magnetic needle.—[continued].
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4472. LATITUDE AND LONGITUDE, Magnetic needle.—[continued].

As far as we can conjecture
your idea here[Paris], we imagine you
make a table of variations of the needle, for all
the different meridians. To apply this table
to use, in the voyage between America and
Europe. Suppose the variation to increase a
degree in every one hundred and sixty miles.
Two difficulties occur: 1, a ready and accurate
method of finding the variation of the place;
2, an instrument so perfect as that (though the
degree on it shall represent one hundred and
sixty miles) it shall give the parts of the degree
so minutely as to answer the purpose of the
navigator. * * * I make no question you have
provided against the doubts entertained here,
and I shall be happy that our country may have
the honor of furnishing the old world what it
has so long sought in vain.—
To John Church man. Washington ed. ii, 236.
(P. 1787)