e needle is so mounted that it only moves
freely in the horizontal plane, and therefore the horizontal component
of the earth's force alone directs it. The direction assumed by the
needle is not generally towards the geographical north, but diverges
towards the east or west of it, making a horizontal angle with the true
meridian, called the magnetic variation or declination; amongst mariners
this angle is known as the variation of the compass. In the usual
navigable waters of the world the variation alters from 30 deg. to the
east to 45 deg. to the west of the geographical meridian, being westerly
in the Atlantic and Indian oceans, easterly in the Pacific. The vertical
plane passing through the longitudinal axis of such a needle is known as
the magnetic meridian. Following the first chart of lines of equal
variation compiled by Edmund Halley in 1700, charts of similar type have
been published from time to time embodying recent observations and
corrected for the secular change, thus providing seamen with values of
the variation accurate to about 30' of arc. Possessing these data, it is
easy to ascertain by observation the effects of the iron in a ship in
disturbing the compass, and it will be found for the most part in every
vessel that the needle is deflected from the magnetic meridian by a
horizontal angle called the deviation of the compass; in some directions
of the ship's head adding to the known variation of the place, in other
directions subtracting from it. Local magnetic disturbance of the needle
due to magnetic rocks is observed on land in all parts of the world, and
in certain places extends to the land under the sea, affecting the
compasses on board the ships passing over it. The general direction of
these disturbances in the northern hemisphere is an attraction of the
north-seeking end of the needle; in the southern hemisphere, its
repulsion. The approaches to Cossack, North Australia; Cape St Francis,
Labrador; the coasts of Madagascar and Iceland, are remarkable for such
disturbance of the compass.

[Illustration: FIG. 2.--Admiralty Compass (Frame and Needles).]

[Illustration: FIG. 3.--Thomson's (Lord Kelvin's) Compass (Frame and
Needles).]

[Illustration: FIG. 4.--Section of Thomson's Compass Bowl. C, aluminium
cap with sapphire centre; N, N', needles; P, pivot stem with pivot.]

The compass as we know it is the result of the necessities of
navigation, which have increased from century to century. It