“The full OED is currently undergoing its first major revision and update for one hundred years. At present just over one quarter of the text has been revised and published at www.oed.com.
The OED entry for siphon is at present unrevised: the existing definition for the main sense states that a siphon works ‘by means of atmospheric pressure, which forces the liquid up the shorter leg and over the bend in the pipe’. Since this definition was published, in 1911, this explanation has been queried, most recently by Dr Stephen Hughes. Dr Hughes states that it is not atmospheric pressure but the cohesion of the water that makes a siphon work: the water in the down arm in effect pulls the water in the up arm.
As a comprehensive historical dictionary, OED seeks to present and evaluate the widest evidence of usage. The way the action of the siphon is explained has varied, and the picture is more complex than reflected either in OED‘s definition or in that of Dr. Hughes. Indeed, there is continuing debate among scientists as to which view is correct. A common explanation is that atmospheric pressure acts on the liquid in the upper container to push it into the tube as gravity pulls downwards the liquid in the lower length of tubing (so that a vacuum would otherwise form in the space vacated). Recently the rise of the liquid in the ‘up’ tube has been attributed to the cohesiveness or tensile strength of the moving liquid rather than to atmospheric pressure: the falling liquid in the ‘down’ arm in effect drags or pulls that in the ‘up’ arm. We would expect to reflect this debate in the fully updated entry for siphon, due to be published later this year.”