The pressure beneath a column of liquid is directly proportional to the density and depth of the liquid: p = hρg
Barometers
Barometers can be used to measure atmospheric pressure
A simple mercury barometer works by having a sealed tube of mercury sit upside-down in a reservoir of mercury; the mercury in the tube falls, creating a vacuum at the top, or rises until the pressure of the mercury matches atmospheric pressure
Manometers
A manometer measures the pressure of a fluid, usually a gas
It is a U-shaped tube containing a reference liquid, with one end open to the fluid whose pressure is to be obtained and the other open to a fluid with known concentration, such as the atmosphere; the difference in height of the two levels of liquid can be used to calculate the pressure of the unknown fluid, as the above equation reveals the pressure difference