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Inventory no. 54619 - Former Display Label


Gilt and silvered copper, signed "Erasmus Habermel fec:"; undated.

Within the circular scale of 360° is a scale of transversals which, used in conjunction with the scale on the radial arm, enables angles to be measured to two minutes of arc. Within the scale of transversals are a series of scales for constructing regular polygons having 3, 4, 5, 7, 11 or 13 sides, or a number of sides corresponding to multiples of some of these numbers.

Instruments for constructing regular polygons were of use to military surveyors in the design of the characteristic fortifications of the period. To construct a polygon with this instrument, the radial arm is rotated until it lies along the radial line on the scale where all the scales of polygons begin. A line is then drawn along the tangential rule. Without moving the protractor the radial arm is then moved to the next division on the scale marked with the number corresponding to the number of sides of the polygon it is desired to construct. Another line is then drawn against the tangential rule. This operation is continued until the polygon is complete.

In the centre of the protractor is a magnetic compass, the plate of which is divided into four quadrants, each of 90°.

Underneath the base-plate of the protractor project four points which prevent the instrument from slipping.

Purchased with a grant from the Pilgrim Trust

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