This is a typical example of an 18th-century English nocturnal, an instrument designed for telling the time at night. It is made of boxwood with a heart shape cut out of the handle, which also has the words 'FOR BOTH BEARS' stamped into it. On the back is a circular table for the rule of the Pole Star (used when measuring the observer's latitude from the altitude of the Pole Star), which surrounds an 8-point compass rose. Date and hour scales appear on the front of the main body. On top of this is a rotatable disc with an hour scale, a lunar age scale and two shaped pointers, one marked 'GB', the other marked 'LB' (indicating the Great Bear and Little Bear). A rotatable pointer sits on top.

The inscription on the back of the handle reads 'R C 1730'. It is believed, however, that this inscription was made more recently, although the instrument itself does date from the 18th century.