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Surveying Instrument
16th century; Italian
Brass; 180 mm in radius

A plate comprising the eighth of a circular disc has an alidade pivoted at its apex and a fixed sight on each straight edge. The limb has an inner degree scale 0 to 40, divided to 5, subdivided to 1, numbered by 5, and on the outer side of the scale a diagonal scale with lines running in alternate directions, divided to 20 minutes by concentric arcs and to 4 minutes by dots. The plate has a right-angled isosceles triangle with its hypotenuse on one edge of the octant, and the two smaller sides with scales 0 to 100, divided by 5, subdivided to 1 with dots, numbered by 5. From the 5 divisions parallel lines are drawn across the triangle in a sinecal arrangement, and the dotted subdivision is continued on these parallels. The alidade has slit and pinhole sights and a small handle, and a scale 0 to 140 with the same unit as the scales on the triangle, divided to 5, numbered by 5.

The reverse of the octant plate has a cylindrical bar fitting into a sleeve mount in which it rotates and can be clamped by a screw. The sleeve is set on a compass mount above a cylinder, thus forming a universal mount.

Jim Bennett

Istituto e Museo di Storia della Scienza, Firenze
Inventory no. 20

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