Because of the many factors which affect atmospheric pressure ... the density of air on its summit. As air becomes less dense, it contains less gases per unit of volume, and therefore less ...

A lower air pressure caused the mercury to drop. Historically, the changes in the height of the mercury were measured with a ruler, so barometric pressure used "inches of mercury" as a unit.

[Andrew]’s Air filtering unit & positive pressure supply might look like something off the set of Ghostbusters, but it’s an experiment in making a makeshift (but feasible) positive pressure suit.