A well-balanced, machined aluminium disc moving smoothly about its axis and secured in a vertical plane on a tripod support. The disc rotates on a ball and features a number of holes symmetrically set in concentric rings of defined sizes. Four pins with scale pans may be put into the holes at any angle. Plumb line and horizontal graded mirror scale are included. A levelling screw, two side pulleys, and four scale pans are included.