trength in some experiments (such as with the z-tube), and siphons in vacuum rely on such cohesion, common siphons can easily be demonstrated to need no liquid tensile strength at all to function. Furthermore, since common siphons operate at positive pressures throughout the siphon, there is no contribution from liquid tensile strength, because the molecules are actually repelling each other in order to resist the pressure, rather than pulling on each other. To demonstrate, the longer lower leg of a common siphon can be plugged at the bottom and filled almost to the crest with liquid as in Figure 5, leaving the top and the shorter upper leg completely dry and containing only air. When the plug is removed and the liquid in the longer lower leg is allowed to fall, the liquid in the upper reservoir will then typically sweep the air bubble down and out of the tube. The apparatus will then continue to operate as a siphon. As there is no contact between the liquid on either side of the siphon at the beginning of this experiment, there can be no cohesion between the liquid molecules to pull the liquid over the rise. It has been suggested by advocates of the liquid tensile strength theory, that the air start siphon only demonstrates the effect as the siphon starts, but that the situation changes after the bubble is swept out and the siphon achieves steady flow. But a similar effect can be seen in the flying droplet siphon of figure 2. The flying droplet siphon works continuously without liquid tensile strength pulling the liquid up. The siphon in the video demonstration operated steadily for more than 28 minutes until the upper reservoir was empty. Another simple demonstration that liquid tensile strength isn't needed in the siphon is to simply introduce a bubble into the siphon during operation. The bubble can be large enough to entirely disconnect the liquids in the tube before and after the bubble, defeating any liquid tensile strength, and yet if the bubble isn't too big, the siphon will continue to operate with little change.
Another common misconception about siphons is that because the atmospheric pressure is virtually identical at the entrance and exit, atmospheric pressure cancels, and therefore atmospheric pressure can't be pushing the liquid up the siphon. But equal and opposite forces may not completely cancel if there is an intervening force that counters some or all of one of the forces. In the siphon, the atmospheric pressure at the entrance and exit are both lessened by the force of gravity pulling down the liquid in both tubes, but the pressure on the down side is lessened more by the taller column of liquid on the down side. In effect, the atmospheric pressure coming up the down side doesn't entirely "make it" to the top to cancel all of the atmospheric pressure pushing up the up side. This effect can be seen more easily in the two carts being pushed up a hill example. Even though the guy on the left seems to have his push canceled entirely by the equal and opposite push from the guy on the right, the guy on the left's seemingly canceled push is still the source of the force to push the left cart up.
In some situations siphons do function in the absence of atmospheric pressure and via tensile strength – see vacuum siphons – and in these situations the chain model can be instructive. Further, in other settings water transport does occur via tension, most significantly in transpirational pull in the xylem of vascular plants. While water and other liquids may seem to have no tensile strength, because when a handful is scooped up and pulled on, the liquids narrow and pull apart effortlessly, liquid tensile strength in a siphon is possible when the liquid adheres to the tube walls. Any contamination on the tube walls, such as grease or air, or other minor influences such as turbulence or vibration, can cause the liquid to detach from the walls and lose all tensile strength.
In more detail, one can look at how the hydrostatic pressure varies through a static siphon, considering in turn the vertical tube from the top 