QUT physicist corrects Oxford English Dictionary

Dr Hughes said the Oxford English Dictionary (OED) since 1911 had incorrectly stated that atmospheric pressure was the operating force in a siphon when in fact it was gravity.

“It is gravity that moves the fluid in a siphon, with the water in the longer downward arm pulling the water up the shorter arm,” he said.
Now the illustrious Oxford English Dictionary’s editors are moving to have the definition corrected, after receiving an email from Dr Hughes.

Atmospheric pressure is actually smaller at the short end of the siphon, so it should be obvious that pressure is not the main effect. When the liquid in the long arm drops due to gravity, the cohesion of the particles pulling the remaining water along, and while this is the main effect it isn’t the only one that is present. If the pressure in the tube drops low enough, you would form low-pressure voids, but these are prevented by the atmospheric pressure on the fluid. Siphons have been shown to work in vacuum, but I would guess this is under some limited set of circumstances. Fill a tube, then turn it upside-down and put the end in a pool of the same fluid, and you will get a void at the closed end if the tube is tall enough — you’ve made a barometer. A siphon is going to run into this problem, and the height at which any given fluid will do so depends on the atmospheric pressure.