Description:A set of small balls of equal mass models the air. The balls are set into motion by vibration of the walls with the device level. A bar is positioned in the device to divide the volume into two parts, with all of the balls on one side. A rough observation of the rate at which balls hit the wall is then made. The bar is removed, keeping the motion the same. Note that fewer balls hit the same section of wall in the same time, indicating that when the volume increased the pressure decreased.