This instrument is made up of a circular plate divided into four quadrants. Two of the quadrants contain a grid used for trigonometric operations, such as the calculation of sines and cosines. The third quadrant bears the shadow square, for elevation measurements, including the measurement of terrestrial and stellar heights. The fourth quadrant could be used for calculating longitude. At the centre of the instrument a graduated alidade is free to rotate. Around the circumference there is a degree scale. A cylindrical section on the back of the instrument was used to place the instrument on a stand.