  The partition is capable of being inserted into the box, dividing it into two separate volumes, and is also capable of sliding, frictionlessly, along the box to the left or to the right. When the partition is inserted in the box, collisions with the molecule exert a pressure on the partition. If the partition moves in the direction of the pressure, it may be coupled to a pulley and the force used to lift a weight. If the partition moves against the pressure, this requires the coupled pulley to lower a weight, working against the pressure exerted by the molecule.