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Late 15th century ?; Italian
Gilt brass; 147 mm in diameter

The rim of the mater bears a 360? scale numbered by 10? clockwise starting underneath the throne, subdivided to 5? and single degrees. The rim is rivetted 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 (see below).

The throne is pierced.

The rete has several fin shaped starpointers, but no starnames are given. The ecliptic is marked with the usual Latin names of the zodiacal signs. Each sign is divided on the edge to single degrees.

The single plate and the plate engraved in the mater are marked with the circle for the equator and the tropics, azimuths for every 10?, almucantars for every 6? (numbered by 6? only on plate 1b) and lines for the unequal hours (numbered 1 to 12 except on plate 1a). On the plate in the mater east ('The plates are marked and laid out for the following climates ('CL<IM>A') with their corresponding central latitudes ('LATI<TU>DO G<RADUS>'): Mater) 1, 16?; 1a) 6, 45?; 1b) 7, 48?. It is very likely that two plates for the climates 2/3 and 4/5 respectively are missing.

The back of the instrument bears several circular scales as follows (from the outside):

1) A concentric altitude scale in the upper half two times 0? to 90? starting at the east and west points respectively, numbered by 5?, divided to single degrees.

2) A concentric scale with the names of the zodiacal signs marked with the usual Latin names ('PISCIS' instead of 'PISCES'), each sign divided to 30?, marked by 5?, subdivided to single degrees.

3) An eccentric calendar scale marked with the usual Latin names of the month (except for 'N' instead of 'M' in September to December), divided to the corresponding number of days, every fifth day marked with subdivisions to single days. The equinoxes correspond with March 11 and September 13. The bottom half of the vacant space inside these circles is taken up by a double shadow scale marked 'V{MBRA} V<ER>SA' and 'V<MBRA> ESTENSA' to the base of 12 with divisions to 2 digits (numbered by 2) and subdivisions to single digits. The top half contains a double horary quadrant for unequal hours marked 1 to 6 to 1.

The counterchanged alidade has two sighting vanes with a hole each, both are connected by a sighting tube. One arm bears the inscrption 'UMBRA INFINITA', the other markings for the unequal hours 1 to 6 to 12. The alidade can therefore be used as a sundial where the sighting vane acts as a gnomon. The rule is marked 'volvella'.

Both the letters and the numerals used on the instrument are Gothic. The engravings for the circles show obvious errors and are not very accurate. The rete at plate 1b appear to unfinished.

The names used for the months September to December and the word 'estensa' instead of 'recta' on the shadow scale suggest Italian origin.

The instrument was presented by A.W. Franks in 1867 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, no. 329.

Silke Ackermann

British Museum, London
Registration no. MLA 1867,7-5.22

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