The instrument consists of three rings, two of which can be set perpendicular to each other, while the third one can move around freely. The instrument can be suspended from a swivelling ring fixed to a decorated arc. The arc itself is attached to the outer ring in such a way that it can move between 421/2? and 59? along the degree scale in order to set the instrument to the appropriate latitude.
The outer, meridian ring has a degree scale twice 0 to 90 and 0 to 90 on the other side, numbered by 10 and divided to 5 and 1.
The equatorial ring is pivoted to the equatorial points of the outer ring. When folded out, it is perpendicular to the meridian ring, and is marked with a 12-hour scale on both sides, numbered by 1 and divided to 20 and 4 minutes. The scales extend over 180? each. On one side is a non-linear degree scale 0 to 90 to 0, numbered by 10 and divided to 2. This scale extends over 45?.
The inside bears a calendrical scale with the names of the months indicated by their first letters, divided to 5 days and single days. The outside of the ring is engraved with the corresponding symbols of the zodiacal signs. The position of the symbol indicates the date of the entry of the sun into this particular sign. The equinoxes are at 15 March and 10 September.
A third ring is pivoted to the polar points of the meridian ring. It is fitted with an internal sliding ring carrying a pair of sights with indices. One of these indices moves over a degree scale, while the other moves over a non-linear scale with the symbols of the zodiacal signs. The sights can therefore be adjusted for the appropriate declination of the sun. The other side of this ring is marked with a double 12-hour scale, numbered by 1 and divided to 20 and 4 minutes.
The inside of the sliding ring is marked with an ecliptical scale, marked with the symbols of the zodiacal signs, and the position of 25 stars, indicated by numbers. The outside of the third ring is marked with the same numbers and the abbreviated names of these stars in the following sequence (starting at the North pole): '2 Cass Pect', '3 Gall Caud', '13 Boo sin hu', '1 Andr Cap', '15 Arctur<us>', '4 Oculus [Taurus]', '19 her ca', '23 Peg ext al', '9 Canicula', '14 Spica [Virgo]', '8 Canis mai', '25 Ceti Cada', '18 Scorpy Cor', '24 Aquary Crus', '17 Lanx mer', '5 Or sin pes', '7 Or dex hu', '22 Aquila', '10 Cor [Leo]', '12 Cauda [Leo]', '16 Coronae sep', '21 Lucida Lira', '6 hircus', '20 Cap dra', '11 Equus I<us>'.
The instrument was bequeathed by Octavius Morgan in 1888 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. 65, no. 182.