The open-tube manometer is a device for measuring pressure. It is a U-shaped tube, partially filled with Mercury. We can find the atmospheric pressure by finding the difference in the heights of the liquid in each 'branch' of the tube. The pressure is related to the difference by the formula:

P = P0 + ρgh

where P0 is atmospheric pressure and ρ is the density of Mercury at whatever temperature you are performing the measurement at. ρgh is known as the guage pressure, obviously the difference between atmospheric pressure and P. However, often only the difference in heights is given, in a unit known as millimetres of Mercury (mm-Hg). This unit of measure is also known as the torr, after 17th century chemist, Evangelista Torricelli. The SI unit of measure of pressure is the Pascal or Newtons per metre.