Thanks for any replies in advance. I am currently trying to discern the theory behind an air-lift. A very large group of people are currently debating exactly how an air-lift works. Imagine a pond with a pipe in it. An airstone is place at the bottom end of the pipe and air is pumped into the pipe. the result is a lift causing the water to rise up the pipe and expel at a level higher than the surface of the pond. At question is why. A basic theory given is: air bubbles act as pneumatic pistons, pushing or drawing water up a pipe or stack as the rise and expand. A more advanced theory given, and the one I am more prone to subscribe to describes the pumping action as the result of an air-water mixture. The air-water mixture is less dense than (and therefore is displaced by) the surrounding water of higher density. I propose that the air displaces the water, the surrounding pressure of the pond/reservoir is greater than that in the tube thus the water must move up the pipe to provide space for the air. However alternate arguements state that the air itself pushes the water and drags water behind it. I know both theories sound similar but one is based on pressure of the surrounding water where as the other seems to subscribe to aerodynamics; the bubble itself pushes the water rather than displaces it. Thus forcing the water to go upwards because the air itself is traveling that direction. The issue in my opinion with that arguement is that the air itself is being farced that direction because it is lighter and less dense than the water and must escape so how could it push the water if the water is pushing it? Could someone provide a definitive answer to the theory? Our group is grinding to a halt on this issue. Thanks.