Compass

Compass. The mariner's compass, an instrument used to point out the course of a ship at sea, consists of three parts, the box, the fly or card, and the needle. The box, which contains the fly and needle, is a circular open brass case, so hung or floated, or otherwise supported, that it retains approximately the same horizontal position in all motions of the ship. The fly is a circular disc, generally of paper, representing the horizon, and divided into thirty-two equal parts by lines radiating from the centre and called rhumbs or "points." The intervals are subdivided into halves and quarters, and the entire circumference is also divided into 360 parts or "degrees." The angle between "point" and "point" equals, therefore, 11-1/4 degrees. The four chief or cardinal points are north, east, south, and west, and the thirty-two in their order, beginning from north and moving round to the right, are thus known: -

The origin of the mariner's compass is lost in the mists of antiquity, and nothing certain is known of it. F. Gioja, at the beginning of the 14th century, seems to have been the first to devise some such a form of it as has been described above. The magnetic needle's variation or declination (q.v.) from the true north was discovered by Sebastian Cabot in 1497, and its inclination or dipping when it is so hung as to play vertically to a point beneath the horizon, was first remarked by Robert Norman in 1576. The different variations or declinations at various times in the same locality were originally noticed by Gillibrand in 1634, though the observation has been claimed for Gassendi. To "box" the compass is to repeat the points of the compass in their proper order.

Many modifications of the compass have been introduced, and the one now in almost universal use is the invention of Sir William Thomson. A number of strips of steel are" magnetised separately and are then either bound together or else fixed parallel to each other on a small and light framework. This compound needle is delicately pivoted at its centre, a sharp point of steel or iridium supporting a small inverted cup of agate or sapphire that is fixed to the needle. The circular card is fixed horizontally to the upper part of the needle. The compass is held in a bowl which is weighted below with castor oil, and is so supported in the bowl and the outer box as to remain horizontal when the compass-box is tilted. The presence of masses of iron or steel or of electric circuits in the neighbourhood of the needle will cause its deviation from the magnetic meridian. It is necessary to compensate for these effects by the arrangement of suitable bars of iron or magnetised steel to neutralise such external influences.