[footnote] *A very slow secular progression, or a local invariability of
the magnetic declination, prevents the confusion which might arise from
terrestrial influences in the boundaries of land, when, with an utter
disregard for the correction of declination, estates are, after long
intervals, measured by the mere application of the compass. "The whole mass
of the bottomless pit of endless litigation by the invariability of the
magnetic declination in Jamica and the surrounding Archipelago during the
whole of the last century, all surveys of property there having been
conducted solely by the compass." See Robertson in the 'Philosophical
Transactions' for 1806, Part ii., p. 348, 'On the Permanency of the Compass
in Jamaica since 1660'. In the mother country (England) the magnetic
declination has varied by fully 14 degrees during the period.
In like manner, we observe that the isogonic curves, when they pass in their
secular motion from the surface of the sea to a continent or an island of
considerable extent, continue for a long time in the same position, and
become inflected as they advance.
These gradual changes in the forms assumed by the lines in their translatory
motions, and which so unequally modify the amount of eastern and western
declination, in the course of time render it difficult to trace the
transitions and analogies of forms in the graphic representations belonging
to different centuries.
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