 Evangelista Torricelli

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]

Evangelista Torricelli (1608–1647) was an Italian physicist and mathematician, best known for his work in fluid dynamics and for his invention of the barometer. He is often credited with the discovery of the principle of atmospheric pressure. One of his significant contributions to science is the Torricelli’s law, which describes the speed of fluid flowing out of an orifice under the influence of gravity.