Capacity correction

From Glossary of Meteorology

capacity correction

The correction applied to a mercury barometer with a nonadjustable cistern in order to compensate for the change in level of the cistern as the atmospheric pressure changes.

Thus, as the pressure falls, the height of the cistern increases, due to the exchange of mercury between the barometer tube and its cistern. This correction is not required if the scale is calibrated as in the Kew barometer.
See also barometric corrections.