Siphon uses atmospheric pressure

In the news item “Physicist Uncovers Dictionary Error” (PHYSICS TODAY, August 2010, page 28 ), Stephen Hughes takes exception to the older Oxford English Dictionary definition of a siphon as “a pipe . . . 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.” However, I think that definition is the more accurate. The effect of gravity is implied in the mention of atmospheric pressure, which would not exist without gravity. On the other hand, to state that the siphon is driven by the weight of the liquid in the longer leg assumes that the negative pressure can always draw the liquid over the top. That is a common misconception, similar to thinking that you draw water up a straw by removing air from above the liquid. In reality, atmospheric pressure is responsible. It leads to a force at the base of the straw that pushes liquid up into the straw. The point is proven by the fact that you cannot draw water up through a straw higher than about 30 feet. Likewise a siphon would not be able to draw water over a 30-foot hill. The water in the longer leg would instead separate from the water in the shorter leg, leaving a vacuum at the top, and the siphon would not work. Let’s be careful before jumping on the bandwagon of revisionists.