Syphon, an inverted U-shaped tube used to draw liquids over the containing walls of reservoirs. It acts upon the principle that the flow will be in the direction of that leg which contains the greatest vertical height of liquid. The instrument will act only when the bend at the top is not higher above the level of the water in the reservoir than the atmospheric pressure at the locality is capable of sustaining a column of the liquid acted upon, which for water could only be a little over 33 ft. at the sea level, and at a height of 15,700 ft. less than 20 ft. (See Pump, vol. xiv., p. 82.) If the liquid is mercury, the bend of the syphon could not be more than 29 or 30 in. above the level in the reservoir at the sea level. In practice the outer or discharge leg is usually longer than the one immersed in the liquid; but the only requirement is that its orifice shall be lower than the level of the liquid. It is convenient to have a suction pipe attached to the outer leg for producing exhaustion, the lower orifice in that leg being closed at the time.