Evangelista Torricelli (1608-1647)

Evangelista Torricelli interpreted the support of a Mercury column in terms of external atmospheric pressure. His 11 June 1644 letter to Michelangelo Ricci describes equal column heights in vessels with different upper spaces, testing whether the support came from outside the vessel. In modern hydrostatic pressure notation, $p_{\mathrm{atm}}-p_{\mathrm{top}}=\rho gh$, where $\rho$ is the liquid's density and $g$ is gravitational acceleration. When the upper pressure is small, the column height measures atmospheric pressure, as in a barometer. Comparing liquids of different densities explains why water requires a much taller column than Mercury.^[ref]

A physicist and mathematician associated here with Torricelli's law.^[ref]