The sensitivity of a mercury barometer

 

This barometer was on the wall at Lamb House in Rye where Henry James used to live. The top section got me thinking about how much the mercury would be expected to rise or fall due to normal changes in atmospheric pressure. Using the formula for pressure at the bottom of a fluid = height x density x g, with atmospheric pressure given as 101325Pa and density of mercury as 13546kg/m^3, you get a height of 762mm. This is said to be 1013.25mBar so 1mm is worth 1.33mBar or 1mBar is worth 0.75mm. Say a low pressure was 980mBar, then the mercury would need to go down by about 25mm. It looks like the display at the top has the capacity to go up and down by that amount.