Description

A 17th century Double Dial for the Pocket, French, c. second half 17th century, the eight-sided all-brass dial plate measuring 2″ x 2-11/16″ (5 x 7 cm). The plate is engraved with a circular chapter ring divided every hour from 4am to 8pm, with a circular Zodiacal calendar scale, and with a semi-elliptical hour scale divided again from 4am to 8pm. The circular chapter is engraved with radial hour lines, and set with a hinged gnomon; the Zodiacal circle is crossed by a straight meridian line of 14 holes for inserting a vertical pin gnomon at the appropriate date position; and the field within the elliptical chapter is filled with engraved floral petals aligned with the hour lines. There is also a small inset glazed magnetic compass with good early needle and directionals labeled “Nor, Sud, Est, Oest,” and bearing an engraved fleur-de-lys north mark. The underside is plain, with two feet, compass box, and gnomon spring. Condition is fine with a separate brass pin gnomon and the original pebbled leather covered wood case lined in white and reddish satin.

The double analemmatic dial was described by J.L. Sieur de Vaulezard in 1640. Its big advantage was its independence of knowledge of magnetic north, and thus independence of the magnetic compass. Knowing the date, one places the vertical pin gnomon in the appropriate position, sets the dial on a horizontal surface, and rotates it until both sundials read the same time, which is the correct apparent solar time. This instrument is also equipped with a small compass, for convenience and in fact as a means for determining the magnetic declination, the local difference between astronomical north and magnetic north. The double dial is particularly interesting, and has been constructed in various forms (see for example Thomas Tuttell’s version, Tesseract Cat. 50 Item 20). The present miniature variant, with its insertable pin gnomon, and with its pocket case reminiscent of those of Butterfield-type dials, is otherwise unknown to us.