The rim is riveted to the mater. The mater has a degree scale 0 to 360, numbered clockwise by 5 and divided to 1. The mater itself is empty. There is small pin at the bottom close to the rim which keeps the plates in place. The equal hours are marked twice 1 to 12, the 9 is the wrong way round both times. The numerals are engraved rather than punched. The Latin names of the winds are inscribed on the outside edge as follows: 'VELNOTVS', 'LIBONOTVS', 'AVSTROAFRICVS', 'APHRIC<VS>', 'LIBO', 'FAVONI<VS> VEL ZEPHYRVS', 'CHORVS', 'CIRCIVS', 'SEPE<N>TRIO', 'BOREAS', 'A<N>A4VILO [sic]', 'VVLTVRNVS', 'SVBSOLANVS', 'EVRVS', 'AVROAVSTER', 'EVRONOTVS', 'AVSTER'.
The three lobed throne is decorated with foliate scrolls. The suspension ring appears to be a replacement.
The back bears has a degree scale clockwise 0 to 90 to 0 to 90 to 0, numbered by 10 and divided to 5 and 1. This subdivision is also used for the zodiacal scale, marked with Latin names of the signs. Each sign has a 30? scale, numbered by 10. Further to the inside is an eccentric calendrical anticlockwise scale starting at the first point of Aries and the corresponding number of days, numbered by 10 and divided to 1. The equinoxes are at 11 March and 11 September. December is abbreviated 'DECEN'.
The lower half of the vacant space is taken up by a pair of shadow scales to the base 12, numbered by 3 and divided to 1. Underneath these scales is the signature 'BASTIEN LE SENEY FACIT'. The top half is taken up by a double hour quadrant, the hour lines are marked 1 to 7 and 5 to 12 respectively, the 4, 5, and 9 are the wrong way round. Inside is the coat of arms of Henry VIII, but the quartering is incorrect: France (the fleurs-de-lis) should be in the first quarter (top left). Additionally, the lions in the first quarter (top left) face in the wrong direction. The letters 'H' and 'R' flank the shield and two roses are inscribed at the bottom.
There are two plates, both are marked on one side only, the other side being left blank. One is twice as thick as the other. Both have a small hole along the meridian line close to the tropic of Capricorn matching the pin in the mater. The plates are marked and laid out for the following latitudes:
1a) '52 G 30 M', with crude markings for the almucantars every 5?, numbered by 5 from 5 to 65, and for the azimuths every 10?. There are markings for the unequal hours, numbered 1 to 12, and the twilight curve.
2a) '53 G', with crude markings for the almucantars every 5?, numbered by 5 from 5 to 55, and the azimuths for roughly every 5?, but irregularly spaced. There are markings for the unequal hours, numbered 1 to 12, and the twilight curve.
On both plates there is an unusual wide gap between the tropic of Capricorn and the edge.
The rete with an unusually thick ecliptic ring, engraved with depictions of the zodiacal signs, is divided 12 times into 30?, numbered by 10. There are 20 star pointers, but only 8 stars are labelled as follows: 'CRVS AQUA', 'VENTER CETI', 'MARIS CET', 'CANIS MINOR', 'COR LEONIS EST ICI [?]', 'SPICA VIR', 'VOCIFER', 'CAPVT HERCVLIS'.
The counter-changed alidade with a bevelled edge has two sighting vanes. One arm of this alidade is a replacement. But the inscription 'QUI' is still visible close to the centre hole, carried on the other arm as 'MAL Y PENCE', the remaining part of Honi Soit Qui Mal Y Pense, the motto of the order of the garter.
The rule bears the inscription 'ROY HENRY PAR LA' / 'GRASE DE DEIV' along the arms.
Both the alidade and the rule have stepped pointers at the end. The pin with a lion's head is fixed to the rule. The wedge is a replacement.
The instrument was donated in 1878 by Augustus W. H. Meyrick 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. 115 ff., no. 335. See also C. Jagger, Royal Clocks: The British Monarchy and its Timekeepers 1300-1900, (London, 1983), pp. 8-10.