Compass Card.
The card in a compass; it is circular in shape, and its centre coincides
with the axis of rotation of the magnetic needle; on it are marked the
points of the compass, at the ends generally of star points. (See
Compass, Points of the.) It may be fixed, and the needle may be poised
above it, or it may be attached to the needle and rotate with it.
Compass, Declination.
An instrument by which the magnetic declination of any place may be
determined. It is virtually a transit instrument and compass combined,
the telescope surmounting the latter. In the instrument shown in the
cut, L is a telescope mounted by its axis, X, in raised journals with
vernier, K, and arc x, for reading its vertical angle, with level n. The
azimuth circle, Q, R, is fixed. A vernier, V is carried by the box, A,
E, and both turn with the telescope. A very light lozenge-shaped
magnetic needle, a, b, is pivoted in the exact centre of the graduated
circles, Q R, and M. The true meridian is determined by any convenient
astronomical method, and the telescope is used for the purpose. The
variation of the needle from the meridian thus determined gives the
magnetic declination.