[James C. Bennett]

But a siphon has both ends open to the atmosphere. The barometer doesn’t.

Besides, just by an energy equivalence analysis, atmospheric pressure cannot drive a work process in that manner. Gravity-based potential energy by way of the relative difference in liquid levels, is the main source of energy causing the flow.

[BitWielder1]

The top of the siphon has a lower pressure than the ends.
Make the siphon tall enough and that low pressure will reach zero.
Make it even taller and the liquid will split and stop moving.

[dsrtsage]

“Besides, just by an energy equivalence analysis, atmospheric pressure cannot drive a work process in that manner. Gravity-based potential energy by way of the relative difference in liquid levels, is the main source of energy causing the flow.”

The problem with that is the reason you are “Siphoning” is because in order to get the liquid from the higher spot to the lower spot, you need to get the liquid from the higher spot a little bit higher first.

In order to do that you are decreasing the pressure in the tube (relative to the pressure pushing down on the top of the liquid) in order to bring liquid to the high point in the tube, where after the force of gravity will keep the pressure in the tube at its lesser state by itself (which continues to draw more liquid uphill.

If it was in a vacuum, there would be no way to force the liquid to defy gravity and go up, either to start the process, or after the process got started. You are relying on differential pressure between the atmosphere and the tube to make the liquid go “uphill” first.