This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
This can best be illustrated by a few simple diagrams showing the principles involved. In Fig. 65 is shown a U tube with legs of equal length and filled with water. If we invert the tube, as shown in Fig. 66, the water will not run out, because the legs are of equal length, and contain equal weights of water, which pull downward from the top with the same force, tending to form a vacuum at the point A. If one of the legs is lengthened, as in Fig. 67, so that the column of water is heavier on one side than on the other, it will run out, while atmospheric pressure will force the water in the shorter tube up over the bend, as there would be no pressure to resist this action should the column of water break at this point. This action is also assisted by the adhesion of the particles of water to each other. The column of water in the tube may be likened to a piece of flexible rope hanging over a pulley; when equal lengths hang over each side it will remain stationary, but if drawn over one side slightly, so that one end is heavier than the other, the whole rope will be drawn over the pulley toward the longer and heavier end. The first cause, due to atmospheric pressure, is the principal reason for the action of siphons, but the latter assists it to some extent. If the shorter leg of the siphon be dipped in a vessel of water, as shown in Fig. 68, the atmospheric pressure, which before acted on the bottom of the water in the tube, is transferred to the surface of the water in the vessel, and the flow through the tube will continue until the water level in the vessel falls slightly below the end of the tube and admits air pressure, which breaks the siphon action. Fig. 69 shows the same principle applied to the trap of a sink or bowl. If the bowl is well filled with water, so that when the plug is removed from the bottom, the waste pipe for some distance below the trap is filled with a solid column of water, a siphon action will be set up like the one just described, and the trap will be drained. Frequently a sufficient amount of water runs down from the fixture and sides of the pipe above the trap to partially restore the seal. This direct action of the water of a fixture in breaking its own trap seal by siphoning is called "self-siphonage."
A more common form, where two or more fixtures connect with the same waste pipe, is shown in Fig. 70. In this case the seal of the lower closet is broken by the discharge of the upper. The falling column of water leaves behind it a partial vacuum in the soil pipe, and the outer air tends to rush into the pipe through the way of least resistance, which is often through the trap seals of the fixtures below. The friction of the rough sides of a tall soil pipe, even though it be open at the roof, will sometimes cause more resistance to air flow than the trap seals of the fixtures, with the result that they are broken, and gases from the drain are free to enter the building.
Three methods have been employed to prevent the destruction of the seal by siphonage. The first method devised was what is known as "back venting," and this is largely in use at the present time, although careful experiments have shown that in many cases it is not as effective as it was at first supposed to be, and is considered by some authorities to be a useless complication. It is, however, called for in the plumbing regulations of many cities, and will be taken up briefly in connection with other methods.