(b) The measurement of pressure
The pressure exerted by the atmosphere is measured with a barometer. The original
version of a barometer (which was invented by Torricelli, a student of Galileo) was an
inverted tube of mercury sealed at the upper end. When the column of mercury is in
mechanical equilibrium with the atmosphere, the pressure at its base is equal to that
exerted by the atmosphere. It follows that the height of the mercury column is proportional
to the external pressure.