It must be kept lower than the open end of the short branch.
While the water is running out of the long branch, what is produced at the bend of the tube?
The tendency is to produce a vacuum at the bend of the tube.
What is the result of this?
The water is continually forced up the short branch to fill this vacuum, so that it runs out of the long branch in a steady stream.
How do boys often suck cider from a barrel?
By means of a straw.
How is this done?
One end of the straw is placed in the cider, and the other end in the mouth; then the air is sucked out of the straw, and the cider is forced up to fill the vacuum. When the cider is sucked out, more cider is forced up, so that the straw is kept full.
What makes the cider rise to fill the vacuum in the straw?