Mechanical equinoctial dial for latitudes 0°-90° North. The brass base-plate is octagonal and supported by three turned brass legs (one of which is missing). A silvered compass is set into the obverse side of the base-plate. The cardinal points are marked in Latin and an index arm marks the magnetic variation. At the South edge of the base-plate is a spring-loaded hinged brass plumb-bob support.

Above the compass is a latitude circle within which is a copper spiral. This would have been used to set the angle of the hour-plate in conjunction with a spring, which is missing, for holding the hour-plate open. The hour-plate is hinged to the base-plate at the North edge. It has an hour scale and a lunar-age scale. Pivoted at the centre is a copper rule that carries a minute dial at one end. The index arm is pivoted by a pin, which passes through to a gear wheel on the back. Attached to the outer end of the minute-dial is a slit sight through which the Sun's rays can fall on an engraved line on a brass plate parallel to the slit. On the reverse side of the base-plate is a table of latitudes in two columns comprising places in Europe.

This instrument is an example of the early form of mechanical dial which was developed by Michael Bergauer and which can be seen in instruments by him in the British Museum (registration nos OA.374.14111 and 1888, 12-1.326), the MHS, Oxford (new inventory no. 51664) and the National Technical Museum in Prague. On the reverse side of the base-plate, it is signed 'Michael Bergauer Insprug.'

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