Physicists still argue about exactly why a siphon lets water run uphill
Anyone who has emptied a fish tank with a hose knows the trick: water climbs up and over the rim with no pump at all. Egyptians were doing it by 1500 BC. Yet how a siphon actually works has sparked a surprisingly stubborn scientific argument, and both sides may be partly right.
The classic explanation leans on air pressure. Gravity tugging on the longer, falling leg of the tube lowers the pressure at the siphon's crest. Atmospheric pressure on the reservoir then shoves liquid up into that low-pressure zone, much as it does in a drinking straw. A common objection is that air presses equally on both ends, so it should cancel. It does not fully, because the taller column on the outlet side eats away more of that push before it reaches the top.
The rival cohesion theory says the falling liquid pulls the rising liquid along, like a chain slipping over a pulley. Siphons have been shown to run in a vacuum and to lift higher than air pressure alone allows, which favours cohesion. But everyday siphons keep flowing when a bubble completely breaks the liquid column, and one demonstration ran for more than 28 minutes this way. Some even start with the top of the tube full of air. Liquid that is not touching cannot pull itself along, so in normal conditions air pressure is doing the work. Cohesion only matters when liquid clings tightly to clean tube walls, the same kind of tension that lifts water through the xylem of plants.
Height matters, not weight. A fat intake tube holding far more liquid than a thin outlet tube still drains, because what counts is the vertical drop from each reservoir surface to the crest.
The history is long. Pythagoras's Justice cup on Samos, from the 6th century BC, used a hidden siphon, and Hero of Alexandria wrote about siphons at length. Baghdad's Banu Musa brothers devised a double-concentric version in the 9th century. In the 17th century Galileo blamed nature's supposed horror of a vacuum for their height limit, an idea Torricelli and Pascal later overturned.
Source: Siphon
