Semi-circle-Two brass semi-circles 4.6" (11.6cm) dia. , pivoted together so that one rotates under the other, graduated 0-180, 0-180. The upper semi-circle carries a fiducial edge for reading the scale of the lower, the lower semi-circle carries a brass clip with a fiducial edge for reading the scale of the upper.
Arma-Two brass arms 9.5" (24.1cm) long, which are formed from the bases of the semi-circles, are jointed to two further arms of the same length, so that when the semi-circles are moved to the 90 deg position the arms form a square.
Compass-Dia. 1.5" (3.8cm) mounted at centre of upper semi-circle. Graduated to 8 points.