Description

This instrument is an example of an English boat compass, made during the 19th century. The instrument is made of brass, and is gimballed so that the compass remains horizontal regardless of the motion of the vessel. The black line on the interior of this compass - the so-called ‘lubber’s line’ - marks the position of the ship’s head for the helmsman.

More on the magnetic compass

The magnetic compass consists of a magnetic needle attached to a compass card, graduated into angular points, and is used for navigation. The first known dry pivoted compass is dated as early as 1269.

The compass indicates the direction of the magnetic pole but there are problems caused by the difference between magnetic and true north (true north being the useful reading). The variation between the two must be calculated in order to obtain an accurate reading. True north was sometimes simply offset from the direction of magnetic north, but this did not allow for geographical variation. For this, charts were needed. The iron used in building ships during 19th century also led to problems because the magnetism of iron affected the compasses on board.
31/08/2006
Created by: Saffron Clackson; updated by Ruth Horry on 31/08/2006