By means, therefore, of
a table indicating those changes, under different latitudes and longitudes,
and of what are called variation charts, the uncertainty arising from them
is in a great measure done away. Another source of error however existed,
which does not seem to have been noticed till the period of Captain Cook's
voyages: it was then found, "that the variation of the needle differed very
sensibly on the same spot, with the different directions of the ship's
head." Captain Flinders attributed this to the iron in the ship, and made a
number of observations on the subject; these have been subsequently added
to and corrected, so that at present the quantity of variation from this
cause can be ascertained, and of course a proper allowance made for it. It
does not appear that any material improvement has been made in the
construction and use of the log,--that useful and necessary appendage to
the compass,--since it was invented about the end of the sixteenth century.
These are the most important improvements in nautical knowledge and
science, which renders navigation at present so much more safe and
expeditious than it formerly was; there are, however, other circumstances
which tend to the same object; the more full, accurate, and minute
knowledge of the prevalent winds at different times of the year, and in
various parts of the ocean; the means of foretelling changes of weather;
and, principally, a knowledge of the direction and force of the currents
must be regarded as of essential advantage to the seaman.
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