It is the difference in heights of the two ends of the mercury that enables us to calculate the
pressure of the gas in the vessel. If the end exposed to the atmosphere is x mm lower than the other
end (as in the diagram above), then the pressure of the gas is (760 – x) mm Hg: The pressure is lower
than atmospheric pressure, hence the mercury is pushed down further on the open-end side.
Conversely, if the exposed end is x mm higher than the other end, the pressure of the gas is higher
than atmospheric pressure, more exactly (760 + x) mm Hg.
A closed-tube manometer setup, considered a hybrid between the barometer and the open-end
manometer, is also possible. The end that used to be exposed is now sealed off as well, with any
trapped air again evacuated. This time, the difference in heights is read off directly as the pressure
of the gas, without having to reference the atmospheric pressure. In the diagram below, then, the
pressure of the gas is simply x mm Hg.
This setup is generally used only to measure pressures that are lower than atmospheric pressure.