SYPHON is a tube bent so that the two legs are either parallel, or incline at an acute angle, and is employed to draw off liquids from vessels which it is not convenient or desirable to move. If the end of the short leg of a siplioti be plunged into the liquid, and the other leg be suffered to hang outside the vessel,then, whenever the siphon is exhausted of air (a process which can be performed by suction by the mouth or a pump, or by fill in the tube with the liquid it is employed to decant, and keeping it so filled till it is placed in its proper position), the liquid will at once flow out of the vessel through the tube, and continue to do so either till it falls below the level of the outside end, or till the inside end cease's to be immersed.. The principle of this simple and efficient instru ment is easy of explanation: let ABC (fig.) be it siphon with one leg, BC, partially immersed in liquid, and suppose the whole siphon filled with the same liquid; then at A we have the pressure of the atmosphere acting upward into the tube in opposition to the pressure of the liquid iu the leg BA; at C we have the pressure of the atmosphere (transmitted through the liquid), and the pressure of the liquid in the vessel out side (which balances an equal height of liquid inside) the tube, acting upward into the tube in opposition to the pressure downward of the liquid in the leg BC. The effective pres sures inward at A and C arc, respectively, the atmospheric pressure less by the pressure of the liquid in BA, and the at mospheric pressure less by the pressure of the liquid in BD; and as the latter of these two is the greater, it overcomes the other, forces the liquid in the tube out atA, and that in the ves sel into the tube at C, the process continuing till the liquid falls to the level of C (when air is admited), or of A (when the two pressures become equal).
It is evident from the above explanation that when. A is on or above the level of D, the surface of the fluid, there can be no flow through the tube; also, that it is quite inune.te rial'whether the longer or the shorter leg be immersed. if only A be below the level of D. If the bend of the siphon be 33 ft. for water, or 30 in. for mercury, above D, the pressure at C, which produces the action of the siphon, becomes the weight of the atmosphere, diminished by an equal weight of a column of fluid. in which case the resulting pressure is zero, and there is no flow through the tube. The flow increases in rapidity and force as the difference of level between D and A increases, and as the dif ference of level between D and B diminishes. Many siphons have a suction-pump per manently attached to the end of the outer leg for the purpose of exhausting the air inside. variety. is the Malmberg siphon, which hastwo equal legs, the extremi ties of which arc bent upward, so that when the siphon is once filled with fluid ;1 remains full, and is always ready for use.