Siphon Wars: Pressurist weighs into Gravitite boffin

We've just had a missive from a US reader regarding that most pressing scientific question of the moment: Just how do siphons work?

Those of you with a scientific bent will recall the recent Oxford English Dictionary outrage, in which one Dr Stephen Hughes of Queensland University of Technology laid bare a 99-year-old gaffe in the lexicographical tome's definition of siphon:

A pipe or tube of glass, metal or other material, bent so that one leg is longer than the other, and used for drawing off liquids by means of atmospheric pressure, which forces the liquid up the shorter leg and over the bend in the pipe.

Not so, insisted Hughes, who explained that gravity was in fact what makes the siphon tick. He also revealed that pretty well every source he checked repeated this grave error, observing: "An extensive check of online and offline dictionaries did not reveal a single dictionary that correctly referred to gravity being the operative force in a siphon."

The OED's Margot Charlton said: "The OED entry for siphon dates from 1911 and was written by editors who were not scientists."

But hold on a minute, insists reader Robert Weaver:

Hello from Colorado, USA -

Your article describing the grumblings of a physics professor who claims that atmospheric pressure is not the operative force in a siphon (posted in Physics, 10th May 2010) lacks one essential detail--the "professor" is wrong. Gravity is indeed involved in a siphon's activity: It draws water downward in the outflow leg, lowering the pressure inside the leg. However, without pressure at the mouth of the inflow leg to drive the liquid upward, siphoning will not occur. Without atmospheric pressure, siphons (and drinking straws, by the way) won't work, even where there's gravity.

Air "weighs" about 14.7 pounds per square inch of area on which it rests, including the surface of a liquid; this pressurizes the liquid to this amount. When you "suck" on a straw, you are reducing the pressure at the mouth end of the straw, and the pressure at the other end drives the liquid into your mouth. The downward side of a siphon "sucks" the same way.

Maybe the academic in question is into first causes: It is true that atmospheric pressure is the result of gravity "pulling" air downward and compressing it, but saying that atmospheric pressure isn't relevant to siphoning is idiotic (i.e., the "professor" sucks).

Put more simply, try sucking on a straw or siphoning in a vacuum--it won't work. Ask a physicist.

Yours truly, Robert Weaver Colorado Springs, Colorado, USA

P.S. Siphons more than 32 feet high don't work at all--not enough atmospheric pressure. P.P.S. Perform a service to English-speakers everywhere! Inform the OED folks that they might want to think twice before revising their definition of a siphon.

Crikey. It looks like pistols at dawn to settle this one once and for all, unless we take as definitive Wikipedia's pronouncement on the matter... ®