It is composed of a brass semicircle (external diameter 15.5 cm, internal diameter 12.5 cm) closed diagonally by a strip of the same width. One face of the half-circle is divided into sexagesimal degrees and the instrument works as a semicircular goniometer, but it is not particularly accurate. In each of the two quadrants on the other face there are four concentric scales divided into 12, 24, 48 and 72 parts, the amplitude of which decreases going from the base to the middle division.
At the bottom on the right there is the number 90, at the centre 180 and at the bottom on the left 270.

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