 One set of two wires indicates lines of equal altitude 36° and 59.5° and supplements the edge of the horizon plate (altitude 0); they cross a vertical north-south wire to indicate the sun's meridian altitude at the equinoxes and solstices respectively.The second set, in addition to the north-south element, forms a complete circle and the sectors of two larger circles; all four wires intersect at the centre of the horizon plate (nadir) and a point near the top of the north-south line (zenith).The planisphere is attached to a large wheel driven by a worm gear, which is connected to a gear box containing the wheel-train.