Local Magnetic Disturbance of the Compass on board Ship. - The term 'local magnetic disturbance' has reference only to the effects on the
compass of magnetic masses external to the ship in which it is placed. Observation shows that such disturbance of the compass in a ship afloat is experienced
only in a few places on the globe.
Magnetic laws do not permit of the supposition that it is the visible land which causes such disturbance, because the effect of a
magnetic force diminishes in such rapid proportion to the distance from it increases that it would require a local centre of magnetic force of
an amount absolutely unknown to affect a compass half a mile distant.
Such deflections of the compass are due to magnetic minerals in the bed of the sea under the ship, and when the water is shallow, and the
force strong, the compass may be temporarily deflected when passing over such a spot, but the area of disturbance will be small, unless there
are many centres near together.
It is very desireable that whenever a ship passes over an area of local magnetic disturbance, the position should be fixed, and the facts
reported as far as they can be ascertained.