Millimetre of mercury

[4][2] Although not an SI unit, the millimetre of mercury is still often encountered in some fields; for example, it is still widely used in medicine, as demonstrated for example in the medical literature indexed in PubMed.

The torr is about one part in seven million or 0.000015% smaller than the millimetre of mercury;[8] such difference is negligible for most practical uses.

[9] For much of human history, the pressure of gases like air was ignored, denied, or taken for granted, but as early as the 6th century BC, Greek philosopher Anaximenes of Miletus claimed that all things are made of air that is simply changed by varying levels of pressure.

He could observe water evaporating, changing to a gas, and felt that this applied even to solid matter.

In the 17th century, Evangelista Torricelli conducted experiments with mercury that allowed him to measure the presence of air.

Previously, the more popular conclusion, even for Galileo, was that air was weightless and it is vacuum that provided force, as in a siphon.