siphon, tube through which a liquid is lifted over an elevation by the pressure of the atmosphere and is then emptied at a lower level. To start the siphon, it must first be filled with the liquid before it is placed into position. The elevation over which a siphon will lift a liquid is limited by the atmospheric pressure. At sea level, water may be lifted c.34 ft (10.4 m). The siphon must discharge at a level lower than that of the liquid at the intake.