This apparatus comprises of wel balanced aluminium disc, rotating smoothly at about its axis  and  clamped on a tripod stand in vertical plane. The disc moves on ball bearing and has number of holes placed   symmetrically in concentric circles of known diameters. Four pins carrying scale pans can be inserted in to holes at any desired position . Provided with plane graduated mirror scale , plumb line , levelling screw, two side pulleys and 4 scale pans.