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Nautical Hemisphere
Signed by Charles Whitwell
Late 16th century; English
Brass; 280 mm in diameter

A circular brass plate is blank underneath but has a bevelled slot for mounting on a stand and four pivoted catches for removing the outer semicircles of the top side. The signature in on the underside: 'C. Whitwell fecit.' There is a shackle and suspension ring for using the instrument in the vertical plane.

The upper side has an outer altitude scale in degrees 90 to 0 to 90 to 0 to 90, divided to 10, subdivided to 5 and to 1, numbered by 10. Within this is a zodiacal scale, using the same degree divisions renumbered 0 to 30 by 10 for each scale of the named signs. Within this is a calendar scale, with the months names in English, each with a scale of days divided to 10, subdivided to 5 and to 1, numbered as appropriate by 10, the first point of Aries being at 10 1/2 March. Within this are scales of hours, 0 to 12 twice, divided and numbered to 1 hour, and the 32 points of the compass, 16 marked with initials.

On this plate, pivoted at its centre, is a brass disc, with a planisphere of named stars and a zodiacal scale with the signs named and each having a scale of 30 degrees, and a lunar scale on the edge. At the centre pivots an alidade and index, its length being the radius of the disc, while two hinged semicircles are carried by diametric extensions. One, representing right ascension, has a scale of hours, 0 to 12 with Roman numerals, and is also divided 0 to 8 to 0; the other, intersecting semicircle, representing declination, has a scale of degrees 90 to 0 to 90, while at the centre is a wider band for a scale of solar declination, divided according to the sun's position in the zodiac. This latter semicircle also has a pivoted alidade equal to its radius. The intersection slot has the remains of a broken spring for holding the semicircles friction-tight.

Surrounding these pivoted parts are two intersecting semicircles that are fixed to the base plate, except that one of them is hinged. The one not hinged has a degree scale 0 to 90 to 0, divided to 10, subdivided to 5 and to 1, numbered by 10, and also has a scale 0 to 8 to 0. The hinged semicircle has degree scales on both sides, 90 to 0 to 90, divided to 10, subdivided to 5 and to 1, numbered by 10. The intersection slot is fitted with a spring to hold the hinged semicircle friction-tight.

The four slots in the base plate are marked with dots corresponding to dots at the ends of the removable semicircles.

Jim Bennett

Istituto e Museo di Storia della Scienza, Firenze
Inventory nos 1099 and 1122

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