Borda circle

The repeating circle consists of a circular varnished brass frame, with a detached wooden handle. The index arm carries the index glass and the telescope arm carries the horizon glass. The double-ended tangent screw and clamping screw are located on the back of the index arm. A second double-ended tangent screw and clamping screw are at the ocular end of the telescope arm. The instrument has five detached shades, two red, two orange, and one green, and four detached horizon shades, one green, two red, and one orange. Index- and horizon-glass adjustment is made by square-headed screws and a detached key.

Attached to the circle is a magnifier on a 60mm swivelling arm. There are also two threaded telescope brackets with perpendicular adjustment made by a rising-piece and two milled knobs. The telescope is 142 mm in length with an inverted image. An extra drawtube is 91 mm long with an inverted image and two parallel cross wires. Another extra drawtube is 84 mm long with an erect image. Accompanying the circle is a sight-tube is 82 mm in length, an extra drawtube for the magnifier, which is 21 mm long, two red shaded eyepieces, three additional detached shades, which are all missing, two rectangular brackets and an adjusting key.

The instrument has a varnished brass limb with an inlaid silver scale from 0° to 720° by 20 arcminutes. Brass finder arc from 155° to 0° to 145° by 1°, with two brass stops. The repeating circle has three silver verniers measuring to 1 arcminute, with zeros at the right.

The repeating circle is contained in a square fitted mahogany box.

Object Details

ID: NAV0072
Collection: Astronomical and navigational instruments
Type: Repeating circle
Display location: Not on display
Creator: Jecker; Jecker, Francois Antoine
Date made: circa 1820
Credit: National Maritime Museum, Greenwich, London, Adams Collection
Measurements: Overall: 78 mm x 305 mm x 290 mm
Parts: Borda circle
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