Instruments such as this horizontal dial and declinatory were produced to assist in creating other sundials, although they can act as sundials in their own right.

Square brass base with four turned brass levelling screws. Floral decoration around screws. Degree scale divided clockwise from West to East 90° - (fleur de lys) - 90° numbered by 10°, subdivided to 5° and 1°. Inscribed Declinasion du Midy à l'Occident and Declinasion du Midy à l'Orient. Set on the base is a rotable circular disc, which carries the horizontal dial and the compass. Four concentric hour scales marked for 40°, 45°, 50° and 55° (from centre out). All divided clockwise. Reading from edge:
1. Hours numbered clockwise 4-12, 1-8, divided to 30 minutes, subdivided to 15 minutes
2. Hours numbered clockwise (IIII), V-XII, I-VII, (VIII) divided to 30 minutes, subdivided to 15 minutes.
3. Hours divided clockwise 4.30am -7.30pm numbered 5-12, 1-7, subdivided to 30 minutes.
4. Hours divided clockwise 4.30am - 7.30pm, numbered V-XII, I-VII, subdivided to 30 minutes.
Within the sccales is an engraved scene of birds, animals, plants and a seated man and a cornucopia and planetary symbols.
Shaped and decorated hinged polar gnomon folding flat when not in use, engraved with Sun, Moon and stars and curling leaf patterns. Adjustable for latitudes 38-55° numbered by 10°, subdivided to 5° and to 1°. Latitude shown by bird marker.
The compass is set at the South end of the dial. It is silvered inside. Degree scale divided 0°-90°-0°-90°, numbered by 10°, subdivided to 2°. Eight point rose with 16 points named in French (NORD, NNE, NE, ENE, EST, ESE, SE, SSE, SUD, SSO, SO, OSO, OUEST, ONO, NO, NNO). Fleur-de-lys further identifies North. Small arrow marks magnetic North at 7°W of North. Blied compass needle, surmounted by brass pivot. Glass plate over all.
This dial and its use are described in Nicolas Bion 'The construction and principal uses of Mathematical instuments' 1758 and illustrated in plate XXIII (p.244) of that book.