siphon

Tube in the form of an inverted U with unequal arms. When it is filled with liquid and the shorter arm is placed in a tank or reservoir, liquid flows out of the longer arm provided that its exit is below the level of the surface of the liquid in the tank.

Theoretically a siphon can work in a vacuum, using a liquid that does not boil away and that has a cohesion (tensile strength) great enough to keep the column of liquid in the shorter arm unbroken. In practice, using ordinary, impure water, which has a low cohesion, the siphon relies on the atmospheric pressure on the reservoir surface to raise liquid to the height of the apex of the tube, replacing that falling from the long arm. This means that a siphon cannot raise water above about 76 cm/30 in, the height of a column of water that balances atmospheric pressure.