Azimuth compass, consisting of a magnetic compass in a brass bowl, hung in gimbals that are weighted at the bottom with lead. The compass card (diameter 6.5 inches) is made of paper and is graduated to 32 points and quadrantally in degrees, with a metal ring, also graduated quadrantally in degrees, attached to its rim. The north point is indicated by a crown and east by a fleur-de-lis. The card is inscribed 'RALPH WALKER INVENT.'. A single steel needle is mounted edgewise on the bottom of the card. There are also two pieces of wax either side of the south point in order to balance the card. The cap is made of brass. A locking piece operated from the side of the bowl can be used to hold the card steady when it is not in use.

The compass is mounted on a two-part circular wooden stand. A metal label on the stand bears the inscription, 'THIS PATENT COMPASS / FOR STELLAR AND / SOLAR COMPASS CORRECTION / WAS THE PROPERTY OF / ADMIRAL SIR JOHN DUCKWORTH, BART., K.C.B. / OF WEAR, TOPSHAM / BORN 1748. DIED 1817. / THE INVENTOR, RALPH WALKER, / WAS BORN / DIED'.

On top of the compass is a brass sun-attachment . This is effectively a sundial mounted in horizontal trunnions. It has a semi-circular dial at the centre, marked in hours VI-XII-VI (and VI-0-VI above) to 2 minutes. Mounted perpendicular to this is a brass piece on which a small lens can move by means of a screw piece. The lens moves against a scale marked in months and days on one side, and in zodiacal signs and days as well as solar declination (the scale for which is marked AE at the centre) on the other. The sun-attachment bears the inscription 'Invented & Made by R. Walker London', and is able to rotate on its monting, with a degree scale (marked 90-0-90) with vernier to 15 minutes and locking screw on one side.

The compass was designed by Ralph Walker, a resident of Jamaica. It was intended as a solution to the longitude problem and, Walker having sailed to England on the 'Providence', the idea was put to the Board of Longitude in 1793. The idea was to use magnetic variation (which was known to vary according to geographical position) as a means of determining longitude. Walker's proposal consisted of a compass with a sun-attachment for indicating the true direction, with which the magnetic compass's reading could be compared to find the variation. He described the method in 'A Treatise on Magnetism with a Description and Explanation of a Meridional and Azimuth Compass for Ascertaining the Quantity of Variation' (1794).

When used, the lens is set according to the date and then focuses the sunlight onto the concave dial, allowing true north to be found without any knowledge of the time. Walker also made improvements to the design of the magnetic compass, notably fitting the needles edgewise instead of flat and making the pivot point level with the gimbal axes rather than below them.

Walker's compasses were sent for trial on the 'Invincible', 'Glory' and 'Lynx' in 1794, but proved unsuitable for determining longitude. Nevertheless, they received considerable praise as compasses and were adopted for the Royal Navy the following year, but proved expensive and officers found them difficult to use, although the Royal Academy at Portsmouth offered instruction in their use. They were therefore only supplied to ships in which the Captain or Master could prove that they understood how to use them. In 1819, the Navy decided to stop buying the compasses.

Ralph Walker (1749-1824) was born in Tullibody, Clackmannanshire, Scotland, and brought up on his father's farm. Having learnt navigation, he spent the period from 1765 to 1783 at sea before settling in Jamaica as a planter. Having returned to England in 1793, Walker later became a civil engineer, carrying out various works on the docks around the Isle of Dogs, Blackwall and Rotherhithe, and later becoming Engineer for the East London Water Works Company. The NMM has a portrait print of Walker (PAD3061).