The instrument consists of a circular plate with several volvelles, to which a handle with compass is attached.
The front is laid out for a horizontal dial with the folding gnomon set for a latitude of circa 49?. The hour scale close to the edge is numbered clockwise IV to XII to VIII, the half hours indicated by small dots.
Further to the inside is a scale for the age of the moon numbered 1 to 29 and divided to 1/2. In the centre of this scale is a volvelle which serves as a moondial. The hours are numbered IV to XII to VIII. A small index points to the age of the moon on the surrounding scale. This allows lunar time to be converted into solar time. A circular aperture shows the phases of the moon.
The dial is dated '1588' on the rim close to the 29 of the moon age scale.
The compass, fixed to the handle, has the cardinal points indicated by the first letter of the usual Latin names with a deviation of circa 10?E. A suspension apparatus is attached to the compass.
The reverse is laid out as a nocturnal with several circular scales as follows (from the outside):
1) The zodiacal signs marked with the usual Latin names (partially abbreviated), each sign has a 30? scale, numbered by 10? and divided to 2? by alternate shading.
2) A Julian calendar scale with the names of the months given as 'IAN' FEV, MAR, AVR, MAI, IEV, IEV, AVS, SEP, OCT, NOV, DEC'. Each month has 30 days, numbered by 10 and divided to 2. This appears to be confused with the division used in a zodiac scale. The equinoxes are at 10 March and 10 September.
In the centre of this scale is a volvelle marked with a 24 hour scale, numbered anticlockwise twice 1 to 12. Small teeth indicate the full hours. Two indices at 12 o'clock allow the instrument to be set to the appropriate time of the year.
An index rotating over this disc is marked 'LIGNE DE FOY'.
The instrument is contained in a wooden case with red lining.
The instrument was purchased in 1891 and is described in F. A. B. Ward, A Catalogue of European Scientific Instruments in the Department of Medieval and Later Antiquities of the British Museum (London, 1981), p. 75, no. 209 (with plate III).