Let’s see how Mercury Barometer expresses atmospheric pressure in terms of the length or height of the mercury column as a variant of the manometer.
Mercury Barometer can be regarded as a variant of the manometer, in which the U-tube has been straightened out and one end submerged in mercury (figure 1).
This means that the mercury level at A is always at atmospheric pressure.
The other end of the manometer has been sealed, trapping a vacuum at V above the mercury column. Following the idea of the manometer, the height difference h between the mercury levels is now equivalent to the pressure difference between a vacuum (≡ 0 Pa) and the atmosphere.
So, this is the atmospheric pressure expressed as a length h of mercury.