Liquid-column gauges consist of a column of liquid in a tube whose ends are exposed to different pressures. The column will rise or fall until its weight (a force applied due to gravity) is in equilibrium with the pressure differential between the two ends of the tube (a force applied due to fluid pressure). A very simple version is a U-shaped tube half-full of liquid, one side of which is connected to the region of interest while the reference pressure (which might be the atmospheric pressure or a vacuum) is applied to the other. The difference in liquid levels represents the applied pressure. The pressure exerted by a column of fluid of height "h" and density "ρ" is given by the hydrostatic pressure equation, "P" = "hgρ". Therefore, the pressure difference between the applied pressure "Pa" and the reference pressure "P"0 in a U-tube manometer can be found by solving . In other words, the pressure on either end of the liquid (shown in blue in the figure) must be balanced (since the liquid is static), and so .