The Self-Flowing Flask

http://commons.wikimedia.org/wiki/File:Boyle'sSelfFlowingFlask.png

Denis Papin described this perpetual motion scheme in 1685. The pint of liquid in the goblet weighs more than the ounce in the tube, and so forces it down. When the liquid spills out the top of the tube and back into the goblet, it starts a cycle that must continue until the liquid finally evaporates. Where is the error?