Mechanical equinoctial dial for latitudes 20°-90° North. The dial has a square brass base supported by four turned levelling screws. On the base-plate there is a latitude scale along the meridian line with a slot down the centre allowing a small brass rest to move. The rest can be fixed in place by means of a plate on the underside. Hinged to an extension on the North end of the base-plate are the hour-circle and plumb bob support which is mounted vertically against the edge of the base-plate extension. It is pierced for the admission of a green string plumb line (broken) and a conical brass bob.

The hour-circle has a pivoted strut attached to its underside which is used to set the latitude of the instrument. Pivoted to the centre of the hour-plate is an index arm with a pointer at one end and a circle at the other. The circle, which is a minute-dial, has a pointer pivoted to the centre on an arm, which is attached to a gear wheel beneath the minute-dial. Screwed to the index arm is an arced declination scale, with date and zodiacal scales, which also functions as a support for the pivoted alidade. The alidade carries two shaped pinhole sights. The end which moves over the declination scale has a rectangular hole cut away with pointers on opposite edges to mark the declination. This dial is a typical example of the type of mechanical equinoctial dial most commonly produced in the 18th century.

For more information regarding this dial please refer to the OUP & NMM catalogue, 'Sundials at Greenwich'.