Epact Scientific Instruments of Medieval and Renaissance Europe

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Theodolite
Signed by Augustine Ryther
Dated 1590; English
Brass; 235 mm in diameter

The altazimuth theodolite has a base socket with a clamping screw, rising to a short plate with two holes where wing-headed screws secure the circular horizontal plate, cut away to form a square inscribed within the circle, while inscribed within the square is a disc with a central hole for the vertical pivot and two threaded holes for securing screws. The outer circle has a scale of degrees 0 to 360, divided to 10, subdivided to 5 and to 1, numbered by 10. The square has four shadow squares, each with scales 0 to 60 to 0, divided to 5, subdivided to 1, numbered by 10, with inner scales 0 to 12 to 0, divided to 3, numbered by 3.

Above the horizontal plate pivots a counterchanged alidade with folding slit and window sights, and attached to the alidade is a circular glazed compass box with a needle and coloured compass rose, with initialled cardinal points and a circular scale 0 to 120, divided to 10, subdivided to 5, to 1 and to 1/2, numbered by 5.

Carried above the alidade by a brass ring surrounding the compass box, is the support for a vertical semicircle, with an inscribed double shadow-square, and above the semicircle an alidade with pinhole and window sights and slit sights. The semicircle pivots in the vertical plane with the alidade, and has a clamping screw, the scales on both sides being read by fiducial edges in the stand. A short plumb-line is missing. Each side of the semicircle has a degree scale 90 to [0] to 90, divided to 10, subdivided to 1, numbered by 10; each side of the double square has scales 0 to 12 to 0 to 12 to 0, divided to 3, subdivided to 1, numbered by 3. The vertical semicircle, etc, can readily be removed and the instrument used as a simple, rather than an altazimuth, theodolite. Signed: '[star] ARyther ? fecit ? 1590 [star]'.

Jim Bennett

Istituto e Museo di Storia della Scienza, Firenze
Inventory no. 240

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