Perpetuated by dictionaries for nearly a century, it's surely the most persistent scientific howler in the history of the English language. Siphons – those ingenious plastic tubes we use to fill or drain everything from aquariums to petrol tanks – move liquid by "the force of atmospheric pressure".
Except, how could a siphon possibly work by a difference in pressure when atmospheric pressure is the same for the liquid at both ends of the tube? Bleeding obvious when you think about it. Even I can figure that out 25 years after I scraped through A level physics.
And yet according to the Guardian science desk's own coffee-stained Collins, a siphon is "a tube placed with one end at a certain level in a vessel of liquid and the other end outside the vessel below this level, so that atmospheric pressure forces the liquid through the tube and out of the vessel".
The prestigious Oxford English Dictionary and numerous online dictionaries say much the same. Apparently the OED has been getting it wrong since 1911. Surely in all that time somebody must have noticed?
Finally somebody has: Dr Stephen Hughes, a physics lecturer at Queensland University of Technology in Brisbane. Dr Hughes stumbled on the error after seeing an enormous siphon at work in South Australia transferring the equivalent of 4,000 Olympic swimming pools from the Murray river into Lake Bonney. Dr Hughes says the siphon transferred 10 billion litres of water over two months without a pump.
Inspired by this feat, he decided to write an article about the phyics of siphoning for use by science teachers, only to discover that every dictionary he consulted claimed it was atmospheric pressure, not gravity, that pushed liquid through a siphon tube.
"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," Dr Hughes said.
The most up-to-date version of the OED defines a siphon as:
"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."
As any petrol thief knows, to get the liquid over the "hump" of the tube you have to suck the other end or, more pedantically, lower the pressure in your lungs to beneath atmospheric pressure by expanding them. Once the liquid has passed the highest point in the tube, the continuous chain of cohesive bonds between the liquid molecules in the tube, and the force of gravity, do the rest.
Dr Hughes emailed the OED's editors and got this reply from spokesperson Margot Charlton:
"The OED entry for siphon dates from 1911 and was written by editors who were not scientists ... Our files suggest that no one has queried the definition before. We are revising that entire dictionary text now, and I have copied your helpful comments to the revision file, to ensure they are taken into account when the entry is rewritten."
In their defence, Ms Charlton pointed out that the 2005 edition of the Oxford Dictionary of English correctly attributed a siphon's operation to gravity.
Dr Hughes has just published a paper on how siphons work and is appealing for readers to tell him if the same error is perpetuated in the dictionaries of other languages, and whether school textbooks also get it wrong.
Is there anything else the editors of the OED should know about?