Another error, called heeling error, is caused by the change in the
disposition of the material of the ship with reference to the compass.
It is brought about when the ship rolls. For example, a ship heading
on a northerly course would, if rolled to port, place all magnetic
material of the ship to the eastward of the compass. This pulls the
north end of the compass to the eastward. The action and effect would
be just opposite to this on a roll to the starboard. The result is that
the needle is caused to oscillate in either direction. The helmsman
in his attempt to keep “on” will cause the ship to traverse a sinuous
course.
The card and needle of the magnetic compass are placed in a bowl
filled with a liquid. The purpose in so doing is to make the action
of the card somewhat sluggish, so that it will not follow very slight
magnetic distractions or ship movements. Every time the course of the
ship is changed the sluggish action, due to adhesion between the bowl,
liquid and card, pulls the compass off the meridian. Official test has
shown that from three to four minutes are required for the compass to
overcome this “lag.” The “lag” is somewhat less in the dry card compass.