Barrons SAT Subject Test Chemistry, 13th Edition

(C) The fact that the mercury level in the U-tube is higher in the right side of
the tube indicates that the pressure in the flask is higher than the atmospheric
pressure exerted on the open end of the tube on the right side. If the pressure
inside the flask were the same as the atmospheric pressure, the height of the
mercury would be the same in both sides of the U-tube.