Combined ruler and horizontal dial for latitude 52° North. This dial is made of ivory and comprises two arms with a flat metal circular hinge. It has a six-inch rule, numbered and subdivided to eighths, tenths, twelfths and sixteenths. When the rule is folded together it is ready for use as a sundial, which is marked on the other side. The hour scale is numbered clockwise and its hour-lines radiate from the base of the gnomon. This is silver and is hinged so that it can fold flat between the two arms when not in use.

The bottom left corner carries a compass with a hinged silver lid. The compass has a 32-point rose with eight points marked by English initials and a fleur-de-lys for North. The compass card is free to rotate and thus acts as the needle. Inside the lid of the compass are instructions for use: 'Hold the Rule level with its Head towards North (as indicated by the Compass) and raise the gnomon which will show by its Shadow on the Figures the true time of the day'. It is stamped with a registration mark on its bottom right: 'REGISTD JUNE 28 1853'. A similar dial and folding rule is in the Whipple Museum (accession no. Wh: 0668). It is marked with the same registration date, but made of boxwood rather than ivory. Another ivory sundial was sold at Sotheby's in 1996.

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