The rim of the mater bears an equal 24-hour scale on the outside which is numbered twice from I to XII starting at the throne. The inside is graduated 0? to 360? anticlockwise starting at 6 o'clock on the right hand side, numbered by 10?, divided to 2? by alternate shading and subdivided to single degrees. The rim is rivetted to the backplate. The inside of the rim has a recess slightly left from 12 o'clock to accommodate the tongues of the plates. The inside of the mater is engraved with, and marked as, a 'Quadratum Nauticum'. The centre of the square is filled with a compass diagram with 32 directions indicated. The edges of the square are each divided into scales of 90?, numbered by 10? and subdivided to 5?. On top of the square are the inscriptions 'Longi{tudo} mi{nor} sive Occi{dentalior}' on the left and 'Longi{tudo} ma{ior} seu Orie{ntalior}' on the right; on the left hand side are the inscriptions 'Lati{tudo} mi{nor} vel austr{alior}' on the left and 'Lati{tudo} ma{ior} aut Bore{alior}' on the right, each separated by entwined lines.The instrument is signed 'Made by Humfrey Cole' and dated '1574' on the right hand side of the square. The bottom is inscribed 'AD Henr{ici} Princ{ipi} Magni Brittan{iae}' in a different hand.
The outside edge of the rim is marked clockwise with the names of 13 stars and the corresponding planetary temperaments according to Ptolemy. Some star names are abbreviated so that instead the name of the zodiacal sign the symbol is given: 'A Aquila [Mars] [Jupiter]', 'B Lanceat<or> [Jupiter] [Mars]', 'C Cor [Leo] [Mars] [Jupiter]', 'D Oculus [Taurus] [Mars]', 'E Can<is> ma<ior> [Jupiter] [Mars]', 'F Spica [Virgo] [Venus] [Mars]', 'G Cor [Scorpio] [Jupiter] [Mars]', 'H Lucida hyd{rae} [Saturn] [Venus]', 'I Arcturus [Jupiter] [Mars]', 'K Umbilic<us> Andr{omedae} [Venus]', 'L Orion [Mars] [Mercury]', ' M Crus [Aquarius] [Mercury] [Saturn]'.
The throne consists of two foliate scrolls and an inset compass under a movable lid, both the lid and the reverse of the compass are engraved with a picture of an armillary sphere.
The rete has flame-shaped star pointers. The circle for the tropic of Capricorn ends in the head and tail of a dragon. The names of 13 stars are given, mostly with their magnitude indicated in Arabic numerals, as follows: 'UMBILICUS ANDROMEDAE 3', 'OCULUS [Taurus] 1', 'SINISTER PES ORIONIS 1', 'CANIS MAIOR 1', 'LUCIDA HIDRAE 2', 'COR LEONIS 1', 'SPICA VIRGINIS 1', 'ARCTURU<S>', 'COR [Scorpio]', 'AQUILA 2', 'HUMER<US> EQUI MA{IORIS} 2', 'CRUS [Aquarius]'. The ecliptic is marked with the usual Latin names of the zodiacal signs, some of which are abbreviated. Each sign is divided into 30? numbered by 10? with subdivision into 2? indicated on the outside edge of the ecliptic.
There are four plates, one of which is completely void of markings. Two plates are marked on both sides with the circles for the equator (marked 'AEQUATOR') and the tropics (marked 'Tro{picus} Can{cri}' and 'Tropicus Capricorni'), azimuths for every 10? (numbered by 10?), the almucantars for every 10? (numbered by 10?), the lines for the unequal hours (marked in Arabic numerals from 1 to 12), and the markings for the astrological houses in the manner of Regiomontanus marked in Roman numerals I to XII. The 'Horizon obliquus' and the 'HORIZON RECTUS' are indicated and marked. An engraved twilight curve is marked 'Linea Crepusculi'. These two plates are laid out and marked for the following latitudes: 1a) 51?30'; 1b) 52?30'; 2a) 53?40'; 2b) 55?. The third plate has markings for the 'Horizontale Catholicum' and is named as such. The horizons are numbered by 10?. The 'Linea meridi{onalis}', the 'Equinoctialis' and the 'HORIZON RECTUS' are marked. On the back of this plate are markings which are usually found on the back of an astrolabe, which here take the form of a universal projection (see below). On the outside are markings for the following circular calendrical scales (from the outside): 1) the signs of the zodiac marked with the usual Latin names (some abbreviated) and the symbols anticlockwise starting at the east-west line, each sign divided into 30? numbered by 10? and subdivided to 2? indicated by alternate shading. 2) the months of the year with the usual Latin names (some abbreviated), numbered from 1 to 12. The equinoxes correspond with March 11 and September 14, the solstices with June 12 and December 12.
The bottom half of the vacant space inside these circles is taken up by a double shadow scale marked 'Umbra Versa' and 'Umbra Recta' to the base of 12 with division into 2 digits and subdivision into single digits. The top half contains an hour diagram for converting equal hours into unequal hours or vice versa; the unequal hours are marked I to XII, the equal hours 1 to 12 to 1 'HORAE ANTE ME{RIDIEI}' and 'HORAE POST MERI{DIEI}'. The equal hour markings are laid out in the form of segments of vertical hyperbolic spirals.
The back of the instrument is engraved with Gemma Frisius' universal projection with the position of 6 stars marked. These stars are only refered to by the letters A to G which correspond to the stars marked on the outside of the rim (see above). The ecliptic is marked with the symbols of the zodiacal signs. The outside is graduated four times 90? to 0? numbered by 10? and subdivided to 2? in alternate shading. The tropics are graduated 1 to 12.
Over the back moves a triple jointed brachiolus mounted on a cursor (marked 'Cursor Brachiolum') which can be slid along a horizontal rule fixed to the pin (the fixing screw being a replacement). The cursor is irregularly graduated from the centre 0? to 90? on one edge and regularly graduated from the centre 0? to 100? on the other edge. The scales are numbered by 10? with subdivisions to 2? by alternate shading. The rule is graduated along both margins , starting at the top in the centre from 0? to 90?, continuing from 90? to 270? along the whole length of the bottom and from 270? to 360? back on the top, numbered by 10? and subdivided to 2? by alternate shading. The alidade mounted on the front is pivoted on the central pin with what appears to be a unique mechanism: the pin has a circular incision into which the alidade slides. The alidade itself consits of two layers: a solid bar to which the sighting vanes are fixed and two pincer-like arms which are pivoted to the solid bar in the centre. These arms are held in position when closed by two little studs. If the arms are slightly raised and turned around, the alidade can be lifted off. The engravings on the alidade are on the movable parts and give the 'HORAE OCCA{SUS} [Sun]' and 'HO{RAE} ORTUS [Sun]' on one arm with the hours marked from 1 to 12 . On the other arm are markings for the azimuths from 0? to 30? for the 'LATI{TUDO} MERI{DIONALIS}' and 0? to 90? for the 'LATITUDO SEPTEN{TRIONALIS}', numbered by 10?.
The fitted case - probably a 19th century addition - is covered with green velvet (now partially rubbed down to the black underlay) and red velvet lining inside. All the hinges and fastenings are of silver. On the side hinge-plate which is decorated with stars is written 'Inter omnes'; on the top hinge-plate is a five-pointed star overlaid with the letter M. Three silver mounts on the lid are crudely engraved as follows: 1. 'faelicitas Illius cresca<n>t in eternum', 2. The initials 'HP', 'ICH DIEN' and three feathers, 3. 'Scientia virtusque Autoritas et' and 'virtus' and another illegible word on the edges.
The instrument was acquired in 1855 from the Bernal Collection and is described and illustrated in R. T. Gunther, "The Great Astrolabe and other Scientific Instruments of Humphrey Cole", Archaeologia, 76 (1927), pp. 273-317, esp. pp. 278-80; R. T. Gunther, The Astrolabes of the World (2 vols., Oxford, 1932), vol. 2, pp. 484-7, no. 306; and F. A. B. Ward, A Catalogue of European Scientific Instruments in the Department of Medieval and Later Antiquities of the British Museum (London, 1981), pp. 116-7, no. 336 (with plate LVI).
Silke Ackermann
