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Diptych Dial
Signed by Georg Karl
Dated 1598; German
Gilt brass; 104 x 88 x 12 mm

The instrument has a rectangular box with a single cover, retained by a spring catch. The sides of the box are engraved with leaf decoration.

The outer face of the cover has a decorated band of wreath engraving set in relief around its outer edge. The plate carries a horizontal pin gnomon sundial titled 'HORIZONTALE HOROLOGIVM' for latitude 48? (gnomon missing). There are lines for common hours VI to XII to VI, Italian hours 11 to 23 and Babylonian hours 1 to 13. Declination lines are also provided to indicate the sun's position in the zodiac, each line indicating the sun's entry into a sign. The signs are represented by their symbols, from Cancer to Sagittarius on one side ('ZEICHEN DER SON IM ABN') and Capricorn to Gemini on the other ('ZEICHEN DER SON IM ZU').

The cover also carries a lunar volvelle surrounded by the inscription 'AVF POL GRAT 48 ANNO M D XCVIII'. The hour scale is on an elevated circular plate and the hours run I to XII, I to XII, with each hour divided to 1/4. Within the hour scale is the scale for the age of the moon 0 to 28 [291/2]; from 10 to 26, only the even days are numbered. The moon scale is only revealed by the cut-away sections of the silvered volvelle on either side of its pointer. The volvelle has a scale of hours 3 to 12 to 9, with each hour divided to 1/4, and the caption 'INDEX DIERVM AETATIS LVNAE'. There is a circular aperture which reveals a diagram of the phases of the moon and an aspectarium with polygons and symbols for conjunction, trine, quadrature and sextile.

The reverse of the cover has a vertical string gnomon sundial for 48?, with hours VI to XII to VI, each divided to 1/2 and 1/4. Within the hour scale is the signature '[star] GEORG [star] 1598 [star] KARL [star]' and the inscription '[star] ICH KOMPAST [star] KAN [star] NICHT [star] RECHT WEISSEN WAN ICH [star] BIN [star] NAHENT PEI [star] EISSEN'.

Within the inscription is a volvelle for converting between ordinary and Italian hours. A fixed scale of hours I to XII, I to XII with each hour divided to 1/4 surrounds a silvered volvelle with a scale of hours 1 to 24, each hour again divided to 1/4. The volvelle is captioned 'HABETVR HORA ITALICA SI XXIIII PONAS SVPER HORAM OCCASVS'. The centre of the volvelle is decorated with an engraved eight-pointed star.

In the space above the dial is a table of eight German towns all of whose latitudes are given as 48?, with the heading 'ETLICHE FIRNEME STET SO AVF 48'.

The plate of the main box has two horizontal sundials. A string gnomon dial has a scale of hours IV to XII to VIII, each hour divided to 1/2 and 1/4, and surrounding the inscription 'HORIZONTALE [star] DISSER KOMPAST IST AVF [star] ELEVATONIS POL 48'.

There is also a horizontal pin gnomon sundial with lines for common hours VII to IIX [sic] to V, for Italian hours 12 to 23, and for Babylonian hours 2 to 12. As with the similar dial on the cover there are also declination lines to give the sun's position in the zodiac, each line being identified by the symbol for the appropriate sign (though Taurus was mistakenly first stamped in the place of Aries). The gnomon is missing.

In the centre of the plate is the compass surrounded by scrollwork, foliage and fruit engraving. The bowl is 23 mm in diameter and rather than a brass wire ring, the compass glass is held in place by a clasp. The silvered base plate has a direction cross with the four cardinal points identified by their Latin abbreviations. A crude arrow has later been scratched to record the variation of the compass.

In a corner of the plate there is a plumb bob with a silvered and engraved support sprung to the vertical.

The base of the instrument carries a table giving, for 16 days in the year, the length of the day and night and the time of sunrise and sunset. The days are chosen so that the length of the day alters by one hour increments.

Stephen Johnston

Museum of the History of Science, Oxford
Inventory number 47563

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