For investigating the nature of mass and the distinction between mass and weight. Two metal trays 130 x 55 x 23 mm are joined together with two spring steel strips of 23 mm width, the distance between the trays being 215 mm. One tray is drilled to accept 3 equal cylindrical masses. The other tray can be clamped to a bench horizontally or vertically. Complete with 3 masses and a ‘G’ clamp.