SIPHON (abase), a to or pipe by which a liquid can be decanted from one vessel to another without inverting or disturbing the vessel from which the liquid is withdrawn. This machine was probably invented in the second century B.C., by Hero of Alexandria, who, in the or' Pneumatics,' mentions its employment for the purpose of conveying water from one valley to another over the intervening ground.
The siphon is a bent tube the arms of which are of unequal length : one of the 'arms being immersed in the liquid which is to be drawn from a vessel or reservoir, and the air being removed by suction, or by means of a syringe, or by previously filling the siphon, the liquid in the vessel immediately rises in the immersed arm, in consequence of the pressure of the atmosphere on that which surrounds the tube; then passing over the bend, it flows from the open orifice at the lower extremity of the other arm. When the fluid to be raised is water, the vertical height of the bend in the tube, above the surface of the water in the vessel, must not exceed about 33 feet, because a column of water of that height would be in equilibrium with the pressure of the atmosphere, and could not by the latter be forced over the bend. If mercury were to be raised, the height of the bend in the siphon must, for a similar reason, be less than 30 inches. The external arm of the siphon must be longer than that which is immersed in the fluid, or its orifice must be on a lower level than the surface of that fluid, in order that the weight of the column of fluid in the former may exceed that in the latter, and thus a continual stream be produced.
The siphon is sometimes furnished with a suction or exhausting pipe at the side, by means of which the air can be conveniently removed and the liquid be made to rise over the bend into the longer limb. In the Wurtemberg siphon the two ends are turned up, so that when once filled it will always remain so, and act at once when one end is immersed into a liquid.
A siphon may be made to discharge water at the upper extremity by means of an air.veasel at that place. Thus, while the tube is filled
with water, if the communication between the descending branch and the lower part of the air-vessel be closed by the shutting of a valve, the water, which would have otherwise descended, rises in the vessel, where it condenses the air ; and, from the reaction of the latter, it is made to escape, as in a forcing-pump, through an open pipe whose lower extremity is under the surface of the water in the vessel. This was the invention of 3L Hachette, and is denominated the ram siphon.
In order that a fluid may issue from that branch of a siphon which is on the exterior surface of the vessel containing it, it is necessary, as has been stated above, that the extremity of the branch should be below the level of the surface of the fluid in the vessel ; but it may be observed that there is an exception to the rule when the interior diameter of that branch is very small; for example, when it is less than 1-10th of an inch. the interior diameter of the branch in the vessel being con siderably greater. For if such a fluid as water or wine be introduced into a bent tube having one branch only very small, and the open ends be uppermost, the top of the fluid in the more slender branch will, by the effect of capillary attraction, stand higher than the top of that in the other branch.. It would follow therefore, that if the bent tube were inverted, and the orifice of Its larger branch were placed under the surface of the fluid in a vessel, the fluid would begin to issue from the other branch, though the orifice of the latter were a little above the level of that surface.
The effect of a siphon may be produced by capillary attraction alone ; for if a piece of cotton cloth have one of its extremities in a vessel of water, and part of it be made to hang over the edge of the vessel, the water will be attracted along the threads of the cloth, and will descend from thence in drops, provided the extremity of the part. thus hanging over be below the surface of the water in the vessel.