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Mahogany pelorus case

Date: 1890-1910
Overall: 132 x 251 x 251 mm, 2.18 kg
Medium: Wood, leather, brass, felt
Credit Line: ANMM Collection
Classification:Tools and equipment
Object Name: Pelorus case
Object No: 00036875
Place Manufactured:London

User Terms

    This mahogany box stores a pelorus (00036874) made by Negretti and Zambra, one of the larger and better-known English makers of optical and navigational instruments. A pelorus – or ‘dumb compass’ – is used to calculate magnetic bearings in order to check the deviation of a ship’s standard compass. The pelorus could also be used as a sighting device to obtain bearings on stars and other objects, which could then be used to calculate the course of a ship.
    SignificanceThis case is associated with a Negretti and Zambra pelorus. A pelorus was an essential instrument for the safe navigation of a vessel out of sight of land and would have been used for ocean voyaging.
    HistoryThe pelorus is a navigational device that was allegedly named after Hannibal’s chief pilot and navigator. In its most basic form it is a graduated ring by which bearing of stars or other objects can be related to compass bearing and to the course of a ship. It has two main purposes in navigation, firstly, to calculate the magnetic deviation in a ship’s compass, and secondly, to be used in coastal navigation to establish running fixes.

    The ship’s compass is probably the single most important piece of navigational equipment on board a vessel. It is a vessel’s primary tool of navigation and allows the vessel to be steered and a course determined when the vessel is out of site of land or other reference marks.

    However magnetic compasses are influence by ‘variations’ in the Earth’s magnetic field and by magnetic ‘deviations’ caused by the presence of magnetic material in the vessels hull or equipment. An area’s magnetic ‘variation’ is noted on marine charts and navigators take this ‘variation’ into account when calculating course. Magnetic ‘deviation’ varies from vessel to vessel and compass to compass, and is usually calculated by a compass-adjuster. A compass, which has not yet been adjusted, or a compass where the magnetic deviation is not known is next to useless as a navigational instrument and may in fact cause the vessel to go off course and get into difficulty.

    Compass adjustment is both an art and a science. In principle the process is simple, exact and repeatable, however there are many ways to go about the process of eliminating compass error, and to know the exact magnitude and direction of any error that cannot be removed.

    A compass-adjustor will survey a vessel and ensure that the entire vessel’s compasses are aligned with the centreline of the vessel. They will then check to see if the compass is being influenced by any material in its vicinity such as tool, electrical circuits, metal fittings etc and that the compass is functioning properly. The compass-adjuster will then calculate the remaining ‘deviation’ in the vessel’s compass by using a gyrocompass, a shadow pin compass, a pelorus, or run known ranges from a given point.

    In the case of the pelorus, the instrument is carefully aligned with the centreline of the vessel, the azimuth of the sun is calculated and the pelorus set to the azimuth. The lubber line of the pelorus is then set to a series of cardinal heading. The vessel is steered as precisely as possible on those headings while the compass adjuster notes the difference between the actual course steered and that indicated by the pelorus. Recorded errors are adjusted by the positioning of magnets in and around the compass. Those errors that cannot be removed are recorded on the compass’s ‘deviation’ card.

    Additional Titles


    Web title: Mahogany pelorus case

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