Operation
There are two main issues in the operation of a siphon:
- why liquid flows from the higher reservoir to the lower reservoir, which is basic; and
- why liquid flows up the siphon, which is subtler.
The first issue is basic: liquid flows from the higher level to the lower level because the lower location has lower potential energy – water flows downhill. This is independent of the particular connection – liquids will also flow from higher to lower if there is a direct path (a canal), or if there is a tube that goes below the reservoirs (an "inverse" siphon), and these do not depend on siphon effect. Note that this is due to different heights (moving in the direction of gravity), not due to differences in atmospheric pressure at different heights (in fact, lower locations will, all else equal, have higher atmospheric pressure, due to a longer column of air above).
The second issue, why liquid flows up, is due primarily to atmospheric pressure (in ordinary siphons), and is the same mechanism as in suction pumps, vacuum pumps, and barometers, and can be replicated in the simple experiment of placing a straw in water, capping the top, and pulling it up (leaving the bottom tip submerged).
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