Pour water in the tube. Notice how the water levels in either
side of the tube are always the same regardless of the tube
position or diameter. Now plug one side - this is no longer
true. Finally, add oil to one side - notice the two levels are
The air pressure on both sides
initially are atmospheric, so the levels are the same. When
one end is plugged, the air pressure in the tube varies with
the position of the tube and thumb - hence the different heights.
In the last case the oil is less dense than water and therefore
the oil level is slightly higher (since the oil is lighter,
less water is needed on the other side to balance it).