The rim of the mater bears a 24-hour scale numbered twice I to XII clockwise starting at the throne, divided to half hours and subdivided to 6 minutes. The rim is soldered to the backplate. The inside of the rim has a recess at 12 o'clock underneath the throne to accommodate the tongues of the plates. The inside of the mater bears markings for a plate for latitude 42? (see below).
The throne is barrel shaped with an inset swivel pin.
The rete has worm-shaped starpointers, with the names of 20 given as follows: 'finis flux<us>', 'algenib, aldebara<n>', 'alhayoth', 'rigil', 'alge<n>se', 'alhabor', 'algomeisa', 'alfart', 'cor', 'cauda', 'spica', 'alra', 'cor [without pointer]', 'yed', 'alhaue', 'wega', 'althair', 'cauda', 'alferas'.
The ecliptic is marked with the usual Latin names for the zodiacal signs. Each sign is divided into 30? with divisions to 2?.
The mater and the two plates are marked with circles for the equator and the tropics, azimuths for every 5?, almucantars for every 2?, and the lines for the unequal hours (numbered 1 to 12). The following latitudes are served: Mater 42?; 1a) 45?; 1b) 47?; 2a) 49?; 2b) 51?. The rete does not line up with the rim suggesting that several plates are missing.
The back of the instrument has several concentric circular scales as follows (from the outside):
1) A 360? altitude scale marked four times 0? to 90? starting on the east-west line in both directions, numbered by 15?, divided to 5? and subdivided to single degrees.
2) A zodiacal scale marked with the usual Latin names. Each sign is divided to 30? and divided to single degrees.
3) A Julian calendar scale marked with the usual Latin names of the months, divided to the corresponding number of days, every tenth day marked, with divisions to 5 days and subdivisions to single days. The equinoxes correspond to March 10 3/4 and September 13 3/4.
The bottom half of the vacant space inside these circles is taken up by a shadow scale to the base of 12 on the left (numbered by 4, divided to single digits and subdivided to half digits), and an horary quadrant for unequal hours, numbered 1 to 6 to 12 on the right.
The one armed rule and the counterchanged alidade are void of markings.
The markings are all engraved with Gothic numerals and letters respectively.
The instrument was purchased from A. Joseph in 1857 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. 114 ff., no. 332.