Borda circle

The repeating circle consists of a circular polished brass frame, with a wooden handle. The index arm carries the index glass and the telescope arm, with the finder arc, is semi-circular and carries the horizon glass. The tangent screws and clamping screws are positioned on the back of both index arms. The instrument has four index shades, three red and one green, and three horizon shades, two red and one green. Index-glass adjustment is made by a screw and on the horizon glasses by capstan screws.

Attached to the circle is a magnifier on a hinged 115mm swivelling arm serving both verniers with a frosted glass shade. There is also a threaded telescope bracket with perpendicular adjustment made by a rising-piece and a milled knob. The telescopes, unidentified parts and the box are missing.

The instrument has a polished brass limb with an inlaid silver scale from 0° to 720° by 20'. It also has a polished brass finder arc from 180° to 0° to 160° by 1°, with two brass stops. The repeating circle has two silver verniers measuring to 20 arcseconds, with the zeros at the right.

Object Details

ID: NAV0083
Collection: Astronomical and navigational instruments
Type: Repeating circle
Display location: Not on display
Creator: Dolberg, Adolf
Date made: circa 1860; circa 1880
Credit: National Maritime Museum, Greenwich, London
Measurements: Overall: 125 mm x 230 mm x 245 mm
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