When people speak of "deviation of the compass" they mean the difference of a ship's compass from the magnetic meridian, caused by the near presence of iron In iron ships the amount of deviation depends upon the direction, with regard to the magnetic meridian, in which the ship lay when being built. It is least when the ship has been built with her head south. Armor-plated ships should be plated with their head in a different direction from that in which they lay when built.

The mode now generally employed to correct deviation is by introducing on board ship masses of iron and magnets to neutralize the action of the ship's magnetism so far as possible.

Compasses are sometimes carried on masts in iron vessels as a means of removing them from the disturbing influence of the iron of the hull. In this position they serve as standards of comparison for the binnacle compass.

Wooden ships are also affected, though in a far less degree, by the direction in which they lie when building.