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Inventory no. 47382 - Former Display Label

? 1700

Gilt brass and steel. Unsigned and undated.

Along the bottom (straight) edge of the protractor is a scale of 200 divisions (=260 mm.), and at either end of the scale is a complicated sight, which can be raised by sliding it up a scale equal to thirty of the divisions of the previously mentioned scale. The sights each include two pin-hole and two 'crossed wire' sights. Pivoted about the centre of the semicircle is an index arm or alidade with another scale equal to 200 divisions and similar sights to those on the base of the protractor (one of the sights is now missing). This alidade is ingeniously made to turn on a ring, and the alidade itself and the base of the protractor have been cut away, so that the centre of the semicircle (indicated by contingent points on both the alidade and the base) is not obscured. The four sights are each attached by a neat device incorporating a slot and a steel disc, turned by a key (missing), to hold the part of the sight projecting into the slot. One arm of the alidade terminates in a small circular box which can move along the semicircular arc of the protractor in the flanged rim of which it engages. Inset in the flange is a steel band with a continuous single row of indentations, which, as the circular box slides over the arc, cause an index on the box to move over a scale of sixty minutes of arc. Part of the box is cut away to reveal the scales engraved on the arc below; these scales are a scale of 180°, every 2° being numbered, and a scale for drawing polygons of up to twenty-four sides.

Billmeir Collection

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