Tube in the form of an inverted U with
unequal arms. When it is filled with liquid
and the shorter arm is placed in a tank or
reservoir, liquid flows out of the longer arm
provided that its exit is below the level of
the surface of the liquid in the tank. It
works on the principle that the pressure at
the liquid surface is atmospheric pressure,
whereas at the lower end of the longer arm it
is less than atmospheric pressure, causing
flow to occur.