A siphon is a long tube that drains a liquid. The container the liquid is drained from must be higher than the container it is being emptied into. The flow of this liquid is caused by a difference in hydrostatic pressure, which allows the liquid to be drained without being pumped.
Hydrostatic pressure is the pressure that a liquid exerts resulting from its potential energy and by ignoring its kinetic energy. This is used to separate true pressure from osmotic pressure, the difference in the pressure of a column of fluid that has been caused by the effect of gravity.
To start a siphon one must first fill it, or prime it. Once the siphon is primed, atmospheric pressure acts on both ends. The longer leg, the lower end, has greater weight, thus causing gravity to drain the liquid and maintain the pressure. At this point, nothing else needs to be done. Gravity will continue pulling the liquid through the siphon until something breaks it, causing cavitation, or until the outlet equals the level of the container.
