Abstract: A mechanical device for the movement and selective locking of a chair and for controlling a gas piston. The mechanical device comprises a first arm and a second arm spaced apart from the first arm which are connectable to the chair. A pivoting pin extends between and is connected to the first arm and the second arm. A boxlike element including a first and second aperture is pivotally connected to the pivoting pin such that the arms may selectively pivot with respect to the boxlike element. A molding box is connected to the pivoting pin and a resilient member engages the molding box and the boxlike element. A shaft extends through the first and second apertures in the boxlike element and is selectively positionable therein to selectively lock the position of the molding box relative to the boxlike element. Rotation of the shaft controls the operation of the gas piston.