ons, as, for example,
in the case of a ship, repaired and hammered in dock, steaming in an
opposite direction at sea. This latter cause of deviation is called
sub-permanent magnetism. The horizontal directive force on the needle on
board is nearly always less than on land, sometimes much less, whilst in
armour-plated ships it ranges from .8 to .2 when the directive force on
land = 1.0. If the ship be inclined to starboard or to port additional
deviation will be observed, reaching a maximum on north and south
points, decreasing to zero on the east and west points. Each ship has
its own magnetic character, but there are certain conditions which are
common to vessels of the same type.

Instead of observing the deviation solely for the purposes of correcting
the indications of the compass when disturbed by the iron of the ship,
the practice is to subject all deviations to mathematical analysis with
a view to their mechanical correction. The whole of the deviations when
the ship is upright may be expressed nearly by five co-efficients, A, B,
C, D, E. Of these A is a deviation constant in amount for every
direction of the ship's head. B has reference to horizontal forces
acting in a longitudinal direction in the ship, and caused partly by the
permanent magnetism of hard iron, partly by vertical induction in
vertical soft iron either before or abaft the compass. C has reference
to forces acting in a transverse direction, and caused by hard iron. D
is due to transient induction in horizontal soft iron, the direction of
which passes continuously under or over the compass. E is due to
transient induction in horizontal soft iron unsymmetrically placed with
regard to the compass. When data of this character have been obtained
the compass deviations may be mechanically corrected to within
1 deg.--always adhering to the principal that "like cures like." Thus the
part of B caused by the permanent magnetism of hard iron must be
corrected by permanent magnets horizontally placed in a fore and aft
direction; the other part caused by vertical soft iron by means of bars
of vertical soft iron, called Flinders bars, before or abaft the
compass. C is compensated by permanent magnets athwart-ships and
horizontal; D by masses of soft iron on both sides of the compass, and
generally in the form of cast-iron spheres, with their centres in the
same horizontal plane as the needles; E is usually too small to require
correction; A is fortunately rare