Measuring Atmospheric Pressure:

At sea level, the atmospheric pressure is about 101,300 Pa or 101,300 $Nm^{-2}$. 

Barometer:

The instruments that measure atmospheric pressure are called barometers. One of the simple barometers is a mercury barometer. It consists of a glass tube 1meter long closed at one end.  

Construction:

After filling it with mercury, it is inverted in a mercury trough. Mercury in the tube descends and stops at a certain height. The column of mercury held in the tube exerts pressure at its base. At sea level, the height of the mercury column above the mercury in the trough is found to be about 76 cm. The pressure exerted by 76 cm of the mercury column is nearly 101,300 $Nm^{-2}$ equals atmospheric pressure. It is common to express atmospheric pressure in terms of the height of the mercury column. As the atmospheric pressure at a place does not remain constant, hence, the height of the mercury column also varies with atmospheric pressure.

DO YOU KNOW

The fan in a vacuum cleaner lowers the air pressure in its bucket. The atmospheric air rushes into it carrying dust and dirt with it through its intake port. The dust and dirt particles are blocked by the filter while the air escapes out.