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Diptych Dial
Dated 1595; Flemish
Ivory and brass; 67 x 50 x 17 mm

This Flemish diptych dial is decorated with leaves and scrolls; the inscriptions and decorations are filled with red and black colour.

Two brass clasps allow the dial to be closed. A pivoted plate on the left edge of the lower leaf covers a hole which presumably once contained a pin gnomon.

The outer surface of the upper leaf contains a brass nocturnal with 'VRSA MAIOR' written on the foldable arm. An index points to the hour when the arm is aligned with the appropriate stars. The movable disc with siderial hours ('HORA VRSA MAIOR'), numbered anticlockwise 4 to 12 to 8, and the common hours ('HORA SOLIS'), numbered clockwise 4 to 12 to 8, has small teeth on the outside to count the hours at night. The disc can also be used as an equatorial pin gnomon dial (pin missing) when set to the appropriate latitude with the help of a latitude scale (0? to 70?, numbered by 10? and divided to 5?) on the outer edge of the leaf. Around the disc is a calendar scale with the abbreviated names of the months, numbered with the corresponding number of days and divided to 10 and single days.

The inner surface of the upper leaf contains a lunar volvelle, consisting of a central brass disc with a 24 hour scale, numbered anticlockwise twice 1 to 12, and two surrounding scales: another 24 hour scale (numbered anticlockwise twice 1 to 12) and a scale indicating the age of the moon 1 to 29. The brass disc has an aperture for the phases of the moon and is further engraved with planetary aspects. The disc can also be used as an equatorial dial when set to the appropriate latitude as described above.

Outside the dial are the inscriptions 'CIRCVLVS AEQVINOCTIALIS' and 'GRADVS AD ELEVATIO NEM POLI' and the date '1595'.

The inner surface of the lower leaf has a horizontal string gnomon dial (string missing), the hours numbered 5 to 12 to 8. The inset compass with magnetic deviation of circa 5? E has a printed compass card signed 'I. Hall London' which might well be a replacement.

The outer surface of the lower leaf contains a very worn inscription with a latitude list of 15 cities, of which the first four are 'ANTWERBEN DORDRECHT ENCKHVUSEN GHENT' .

The instrument was formerly in the Ilbert collection and was purchased in 1958. It 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. 39, no. 89. For Flemish diptych dials see A. J. Turner, "Ivory Diptych Sundials in Flanders", in S. Lloyd, Ivory Diptych Sundials 1570-1750, Catalogue of the Collection of Historical Scientific Instruments, Harvard University (Cambridge, Mass., 1992), pp. 145 ff.

Silke Ackermann

British Museum, London
Registration no. MLA CAI-2414

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