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 :::::Mindbuilder, I can't really follow your arguments because I find them internally inconsistent and fluid. Consider these arguments that appear in your comments: 

 ::::::A) So you can say that gravity is entirely 100% responsible for ""driving"" the siphon or ""sustaining"" the siphon and I won't dispute that, but at the same time while gravity is entirely ""driving"" the siphon, it is still atmospheric pressure providing the force to ""lift"" the liquid up. … So I ask again: If a molecule is moving up a siphon at a fairly steady speed, is it not true that there must be SOME upward force acting on it other than gravity? This is basic physics. It's a simple question. 

 ::::::B) Though the low pressure in the crushing container is low, it is still higher than the pressure at the top. The low pressure in the crushing container gets much help in overcoming the higher atmospheric pressure at the exit from the pull of gravity on the liquid in the tall down leg. 

 :::::In A) you state that 'it is still atmospheric pressure providing the force to ""lift"" the liquid up'; yet in B) your arguement changes to, 'The low pressure in the crushing container gets much help in overcoming the higher atmospheric pressure at the exit from the pull of gravity on the liquid in the tall down leg'. 

 :::::This presents two things, firstly if A) atmospheric pressure is lifting the water what need is there for gravity? Water is fairly incompressible so if air pressure lifts the water it must eject it out of the other end of the siphon. Then in B) you write 'the pull of gravity on the liquid in the tall down leg'. This explicitly acknowledges that molecular cohesion is acting and so causing the water molecules to move and so answers your Question in A),'... If a molecule is moving up a siphon at a fairly steady speed, is it not true that there must be SOME upward force acting on it other than gravity?' Molecular cohesive forces provide for this.  

 ::::: Now, you have argued that atmospheric pressure lifts the water column (against gravity) and that gravity acts upon the downward draining tube. Logically they must balance or water would drain out under gravity faster than replaced by atmospheric pressure. If they balance it should be fairly easy, since we know g = 9.8 ms-2 and can measure the mass of the column of the downward tube, to convert that into the force that atmospheric pressure exerts on water in the upward tube.  

 :::::To summarise you have argued that atmospheric pressure lifts the up tube water column and gravity drains the down tube water column and molecular cohesion acts to draw the water on as a mass. You then introduce a third element in the case of a siphon that continues to work as atmospheric pressure drops. As you point out, 'It has often been mentioned that the flow rate of the siphon doesn't vary with altitude'. A siphon acting in that way disproves atmospheric pressure as the main force driving a siphon, however, you continue; 'That's true, but it does not follow that atmospheric pressure is providing no lifting force. With increasing altitude the atmospheric pressure exerts less upward force on the liquid going up, but it also exerts less upward force on the liquid coming down. These effects cancel overall in the end, but at intermediate points in the siphon, the atmospheric pressure forces are not entirely canceled.'  

 :::::I find this a difficult last statement as the reasoning is not clear, additionally you earlier state that gravity drives the downward draining tube flow but here introduce the idea of atmospherice pressure on the falling water column; if atmospheric pressure is the force that can lift the up column of water that same force must be acting on the down draining tube, which is confounding.  

 :::::The third element you add is that at the top of the siphon tube, where it switches from up to down, there is an area of lower pressure. Even as atmospheric pressure drops on the reservoir water this area of low pressure in the tube remains always lower than the atmospheric pressure.  

 :::::As you write; 'In the closed crushing upper container siphon demonstration, it is still the lower pressure in the crushing container pushing the liquid up the siphon. The reason it can do this is that the down leg of the siphon is tall enough that it lowers the pressure at the top of the siphon even much lower than the low pressure inside the crushing container.' Here there is a third force you introduce: It is liquids moving from high to low pressure. This appears to be suction; yet you argue that atmospheric pressure is doing the lifting in the up tube.  

 :::::To sum up again, you are saying tht atmospheric pressure lifts the water column in the up tube; gravity pulls the down tube water column; molecular cohesion keeps the water mass whole; and a low pres