The lodestone and the compass card

It is not known where or when it was discovered that the lodestone (a magnetized mineral composed of an iron oxide) aligns itself in a north-south direction, as does a piece of iron that has been magnetized by contact with a lodestone. Neither is it known where or when marine navigators first availed themselves of these discoveries. Plausible records indicate that the Chinese were using the magnetic compass around ad 1100, western Europeans by 1187, Arabs by 1220, and Scandinavians by 1300. The device could have originated in each of these groups, or it could have been passed from one to the others. All of them had been making long voyages, relying on steady winds to guide them and sightings of the Sun or a familiar star to inform them of any change. When the magnetic compass was introduced, it probably was used merely to check the direction of the wind when clouds obscured the sky.

The first mariner’s compass may have consisted of a magnetized needle attached to a wooden splinter or a reed floating on water in a bowl. In a later version the needle was pivoted near its centre on a pin fixed to the bottom of the bowl. By the 13th century a card bearing a painted wind rose was mounted on the needle; the navigator could then simply read his heading from the card. So familiar has this combination become that it is called the compass, although that word originally signified the division of the horizon. The suspension of the compass bowl in gimbals (originally used to keep lamps upright on tossing ships) was first mentioned in 1537.

On early compass cards the north point was emphasized by a broad spearhead and the letter T for tramontana, the name given to the north wind. About 1490 a combination of these evolved into the fleur-de-lis, still almost universally used. The east point, pointing toward the Holy Land, was marked with a cross; the ornament into which this cross developed continued on British compass cards well into the 19th century. The use of 32 points by sailors of northern Europe, usually attributed to Flemish compass makers, is mentioned by Geoffrey Chaucer in his Treatise on the Astrolabe (1391). It also has been said that the navigators of Amalfi, Italy, first expanded the number of compass points to 32, and they may have been the first to attach the card to the needle.

During the 15th century it became apparent that the compass needle did not point true north from all locations but made an angle with the local meridian. This phenomenon was originally called by seamen the northeasting of the needle but is now called the variation or declination. For a time, compass makers in northern countries mounted the needle askew on the card so that the fleur-de-lis indicated true north when the needle pointed to magnetic north. This practice died out about 1700 because it succeeded only for short voyages near the place where the compass was made; it caused confusion and difficulty on longer trips, especially in crossing the Atlantic to the American coast, where the declination was west instead of east as in Europe. The declination in a given location varies over time. For example, in northern Europe in the 16th century the magnetic north pole was east of true geographic north; in subsequent centuries it has drifted to the west.

Despite its acknowledged value, the magnetic compass long remained a fragile, troublesome, and unreliable instrument, subject to mysterious disturbances. The introduction of iron and then steel for hulls and engines in the 19th century caused further concern because it was well known that nearby ironwork would deflect the compass needle. In 1837 the British Admiralty set up a committee to seek rational methods of ensuring the accuracy of compasses installed on iron ships. In 1840 the committee introduced a new design that proved so successful that it was promptly adopted by all the principal navies of the world. Further refinements, aimed at reducing the effects of engine vibration and the shock of gunfire, continued throughout the century.

The liquid magnetic compass

The liquid magnetic compass, now almost universally used, is commonly accompanied by an azimuth instrument for taking bearings of distant objects. The compass consists of a set of steel needles with a compass card, attached to a float, in a bowl of water and alcohol. In modern instruments, the magnetic element is often in the form of a ring magnet, fitted within the float. The card is usually of mica or plastic with photographically printed graduations; metal cards with perforated graduations also are used. Cards are usually graduated clockwise from 0° at north to 359°, with the eight principal points indicated.

A jewel is fitted at the centre of the float to bear on an iridium-tipped pivot attached to the bowl of the compass. The liquid in which the directional system is placed serves two purposes: to reduce the weight on the pivot point, and thereby to minimize friction; and to damp out oscillations from the ship’s motion. The bowl is closed on the top and bottom by glass, the bottom glass permitting illumination from below, and is mounted in gimbals. A flexible diaphragm or bellows attached to the bowl accommodates the change in volume of the liquid caused by temperature changes. The ship’s heading is read with the aid of the lubber’s line, which is oriented toward the forward part of the compass to indicate the direction of the ship’s centre line.

When the ship alters course, liquid at the side of the bowl tends to displace slightly, deflecting the card and causing what is known as swirl error. To minimize swirl error, the card is often made considerably smaller in diameter than the bowl. The directional system is made sufficiently bottom-heavy (pendulous) to counteract the downward pull of the vertical component of the Earth’s magnetic field, which would otherwise cause the system to tilt.

The simplest, and probably earliest, azimuth instrument consists of two sights on opposite sides of the compass bowl connected by a thread. The assembly can be rotated to permit sighting on the distant object. Because it is impossible to sight through the instrument and look at the compass card simultaneously, a prism (mirror) is positioned to reflect an image of the card, which is given a second set of graduations with reversed figures. Modern azimuth instruments embody a number of refinements, but the principle remains unchanged.

The binnacle, formerly called the bittacle, is the receptacle in which the compass is mounted. Originally constructed in the form of a cupboard, it is now usually a cylindrical pedestal with provision for illuminating the compass card, usually from below. It contains various correctors to reduce the deviations of the compass caused by the magnetism of the ship. These usually consist of properly placed magnets, a pair of soft iron spheres (or small strips close to the compass), and a vertical soft iron bar called the Flinders bar, which originated in recommendations made by the English navigator Matthew Flinders.

Binnacles are sometimes constructed so that an image of part of the compass card can be projected or reflected through a tube onto a viewing screen on the deck below. This arrangement can make it unnecessary to provide a second compass for the helmsman and may allow the binnacle to be placed in a position less susceptible to magnetic disturbances.

William Edward May John Lawrance Howard

Marine charts

The portolano

During the course of 15 centuries or more, the coastal pilot book of Classical times evolved into the portolano, or portolan chart, the harbour-finding manual of the Middle Ages. An early portolano for the whole Mediterranean Sea, Lo compasso da navigare (1296), gives directions in terms of half points—that is, halves of the angles defined by the 32-point compass. From such works, accumulated over generations and collected during the 13th century into a single volume for the entire Mediterranean, the first marine charts were drawn. On these charts, most of which were compiled in Genoa, Venice, and Majorca, north was at the top, rather than east, as was the practice on most land maps of the time. They carried a scale of distances and a colour-coded pattern of rhumb lines, or loxodromes (with lines of the same colour crossing the Earth’s meridians at a constant angle, so that following each rhumb line maintains a constant bearing). To set a course between two ports, the pilot would join the corresponding points on the chart with a straight line, find the rhumb line most nearly parallel to it, and trace the rhumb line back to its parent wind rose, from which he obtained the required heading. As long as the ship’s location was to be found by dead reckoning (keeping a running record of the distances and directions traveled), the Mediterranean chart was entirely adequate. Questions of latitude, longitude, compass variation, and curvature of the Earth’s surface could be safely ignored.

The Mercator chart

When the Portuguese, under the leadership of Prince Henry the Navigator, ventured farther south along the west coast of Africa, they encountered navigational difficulties by assuming that the charts used in the Mediterranean could simply be extended. Over long distances the rhumb lines could not be taken as straight, and the charts bore no relation to the new methods of checking the dead reckoning that Portuguese astronomers and mathematicians had devised. These methods required a chart on which positions were expressed as latitudes and longitudes rather than bearings and distances. Such a chart had to embody a practical method of representing the curved meridians and parallels on a flat surface. Even for an area as large as the Mediterranean, this can be done without grossly falsifying either distances or directions, but for larger regions some distortions are inevitable, and a choice has to be made between alternative mapping techniques. On certain types of charts, distances can be shown accurately, but directions cannot; on other types, directions are reliably presented, but the scale of distance varies greatly between different parts of the chart. The navigator accepts the second type because the risk of lengthening the voyage is preferable to that of missing the target.

In 1569 the Flemish cartographer Gerardus Mercator published a world map that he had composed using a “projection suitable for navigation,” the details of which he did not disclose. (The Mercator and other projections are treated in the article map.) On a Mercator chart the meridians of longitude are represented by equally spaced vertical lines, and the parallels of latitude are represented by horizontal lines that are closer together near the Equator than near the poles. The uneven spacing of the parallels compensates for the increasing exaggeration of the east-west distance between adjacent meridians at higher latitudes; this distance decreases on the Earth but remains the same on the chart. In 1599 the English mathematician Edward Wright supplied a rational explanation of Mercator’s projection and provided tables by which the distorted distances could be corrected.

Latitude measurements

Portuguese seamen determined latitude by observing the elevation angle of the polestar—that is, the angle between its direction and the horizontal. They knew from astronomical studies that the star does not lie exactly on the extension of the Earth’s axis, so that it appears to move daily in a small circle around the celestial pole, but the necessary correction (as much as 31/2° in the 15th century) could be applied by noting the position of the nearby star Kochab. When the navigators got close to the Equator, these stars fell below the horizon; there it became necessary to rely on observing the altitude of the noonday Sun and calculating latitude with the aid of an almanac.

The first instruments used at sea for elevation angle measurements seem to have been the quadrant and the astrolabe, long known to astronomers. For both devices the reference direction was actually the vertical, rather than the horizontal, but conversion of the readings was an elementary matter. The mariner’s astrolabe, however, was less widely used than its 16th-century successor, the cross-staff, a simple device consisting of a staff about 3 feet (1 metre) long fitted with a sliding crosspiece (see photograph). The navigator, holding the staff to one eye, would move the crosspiece until its lower end coincided with the horizon and its upper end with the polestar (see figure). The desired elevation could then be read from the intersection of the crosspiece with the staff, on which a scale was marked in degrees. The cross-staff remained in use until the 18th century despite several drawbacks, the most serious being that it required the observer to look directly into the Sun. Coloured shades were fitted to the crosspiece, but the decisive improvement was made in 1594 by the English navigator John Davis. His instrument, called the backstaff because it was used with the observer’s back to the Sun, remained common even after 1731 when the octant (an early form of the modern sextant) was demonstrated independently by John Hadley of England (see photograph) and Thomas Godfrey of Philadelphia. In the octant and the sextant, two mirrors—one fixed, the other movable—bring the image of the Sun into coincidence with the horizon. In the hands of the practiced observer, the modern sextant can be used to measure elevation angles with an accuracy of 10 seconds of arc—that is, close enough to determine a ship’s north-south position within a few hundred metres.

Sextant. Celestial navigation at sea. Sailor using sextant. Travel and navigation.
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