The corrections that must be applied to the reading of a mercury barometer in order that this observed value may be rendered accurate. There are four kinds. 1) The instrument correction is the mean difference between the readings of a given mercury barometer and those of a standard instrument. It is a composite correction, including the effects of capillarity (see capillarity correction), index misalignment, imperfect vacuum, and scale correction, which are the barometric errors. 2) The temperature correction is applied to account for the difference between the coefficient of expansion of mercury and that of the scale. 3) The gravity correction is necessary because the acceleration of gravity varies with both altitude and latitude. 4) The removal correction is applied when the barometer elevation differs from the adopted station elevation and/or climatological station elevation. See also capacity correction.