The instrument, made entirely of brass on a square wooden base (17 x 17 cm) with four levelling screws, is essentially an equatorial solar quadrant (or solar clock) made up of the following parts:
- a magnetic compass enclosed in the base which can be freely rotated with respect to a circular dial subdivided into four quadrants, each graduated from 0 to 90;
- a circular quadrant supported by a staff fixed to the base, with a joint that may be blocked and which allows the quadrant to be inclined from 0 to 90 degrees with respect to the base; the quadrant is ideally subdivided clockwise into 24 hours (each division representing 4 minutes), but in reality the subdivision is in the interval from hour III to hour XII and from hour 0 to hour IX, which corresponds to the maximum duration of the day at latitudes in the temperate zones;
- a smaller quadrant, graduated from 0 to 90 degrees (each division equal to half a degree) and hinged under the circular quadrant which bears on the upper side the words Elevatio Aequatoris, and which, thanks to a system of freely moving hinges, always places itself in a vertical position, and a small weighted wire that almost touches it allows the reading of the inclination of the plane of the circular quadrant with respect to the plane of the wooden base.
A rod (with a notch at one end and a peg at the other) is fitted above the circular quadrant; this holds a staff fitted with a second staff (alidade) at opposite ends of which there are a small converging lens and a small screen with a small circle etched at the centre; the alidade can freely rotate 360 degrees parallel to the plane of the circular quadrant and about 30 degrees perpendicularly to it.
Daumas (1953), p.335
de Clercq (1984), p. 142
Turner (1987), p. 223