The instrument consists of a lid with a nocturnal, a folding equinoctial dial and a compass.
The nocturnal has a zodiacal calendar whose outer scale of months has abbreviations for the month names and is divided to 10 and 2 days. The inner zodiac scale is partly obliterated by the volvelle but gives the symbol for each sign and is divided to 10?. The volvelle has a scale of hours 1 to 12, 1 to 12, with the night hours (6 to 12 to 6) having teeth, and with midnight having a longer pointer used in setting the date. The index arm is marked 'Vrsae minor' and also serves as a catch for closing the instrument.
The inner face of the lid carries a table of the latitudes of 21 places.
The hour ring of the equinoctial dial is marked 1 to 12, 1 to 12 and there is a simple cross gnomon. Set into the hour ring is a folding strut used to set the ring for latitude. In use, its end is positioned on a latitude scale running 30 to 60?, divided to 10 and 2, and numbered by 10. The latitude scale is incorporated into the circular brass ring which retains the compass glass.
The compass occupies the full diameter of the instrument. The bowl is divided into 32 wind directions and the four principal winds are named in Italian. Magnetic variation is marked with a line which mimics the form of the blued needle.
Around the outer circumference of the compass bowl is the inscription 'VIGILATE QVIA NESCITIS DIEMNEQ: HORAM'.
The underside of the compass bowl is plain except for a series of decorative engraved circles.
The instrument has a blind-tooled, leather-covered case.
Provenance: Lewis Evans Collection I. 131.