Pressure units:

How can Pressure be expressed in linear units, like mmHg?

From physics we learn that Pressure = Force/Area, or P = F/A, and F/A can also be written as distance*density*gravity.

now, when we recognize that gravity is constant at sea level, and density is relatively constant for any liquid, then we can see that,

P = distance*density*gravity = distance*constant,

This shows that pressure is a direct function only of the distance (vertical distance) of a liquid, so because the gravity and the density of liquid are constant, pressure can be expressed with linear units of a liquid, such as ftH2O, or mmHg.