Siphon

si'fon, a device for carrying liquids over an obstacle from one level to a lower level. The siphon in common use has the form of a bent tube with arms of unequal length. The shorter is the receiving arm. The intervening height over which a liquid may be siphoned is the height to which atmospheric pressure would raise the liquid. It corresponds to the barometric column, and is for water the same as the height to which water may be raised by the ordinary lift pump at sea level. Water may be siphoned over an obstacle thirty feet high. The air must be exhausted from the siphon in order that the water may rise as a consequence of atmospheric pressure. This may be done by means of suction by the mouth or by a pump; or the siphon may be filled with the liquid which is to be removed, and placed in position while still filled. In either case, the water in the long arm tends to run out and create a vacuum at the top of the bend. Atmospheric pressure on the supply surface forces water up the short arm to keep the pipe full. There is no limit as to size. An air-tight city aqueduct works on the same principle as a slender tube.