Magnetic Azimuth Sundial, Ivory Diptych of Bloud Type

Magnetic Azimuth Sundial, Ivory Diptych of Bloud Type

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This magnetic azimuth dial is of the Bloud type (after Charles Bloud of Dieppe, fl. 1650). The dial is a rectangular ivory diptych. It has a round metal volvelle on the rear face. The volvelle is marked around the edge with the months (in French) and days; there is a seven-column grid of numbers at the center which serves as a perpetual calendar. Rows 1 and 2 give the numbers of months (March = 1), rows 3-7 contain the numbers 1 to 31 with 4 zeroes completing the final row. Volvelles on Bloud dials are supposed to move, but this one appears to be soldered and nailed in place. The inside of the diptych contains a compass bowl with a metal oval hour scale calibrated from VII to XII to V by I, a compass needle which is not resting on its pin, and a printed paper compass bowl insert. The paper insert has latitudes for eighteen cities, mostly French, cardinal directions in French, and an outer scale calibrated from 0 to 90 four times, by 10 degrees. There is an hour scale calibrated from 5 to 12 to 7 by units in the ivory around the compass bowl. The opposite interior face contains a metal volvelle calibrated from I to XII twice by units, with the months and days around the dial's outside edge. A smaller metal dial affixed on top of the volvelle is calibrated from 1 to 30 and is decorated with a floral motif. This dial has a pointer which indicates the date. The front face contains an engraved circular equinoctial dial calibrated from 1 to 12 twice by units with a central polar dial calibrated 8 to 12 to 4 twice. The pin gnomon and string are missing. This dial probably dates from the second half of the 1600s.
Currently not on view
Object Name
Sundial, Horizontal
Physical Description
ivory (overall material)
metal (overall material)
overall, closed: 1.5 cm x 7.2 cm x 8.1 cm; 19/32 in x 2 27/32 in x 3 3/16 in
ID Number
accession number
catalog number
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Medicine and Science: Mathematics
Science & Mathematics
Data Source
National Museum of American History
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