The layer of air that surrounds the earth is several miles deep. The
weight of the air above exerts pressure in all directions. This pressure
is called atmospheric pressure. Atmospheric pressure at sea level is
14.7 psi, which converts to 1.013 × 105 N/m2.
The instrument used to measure atmospheric pressure is called a barometer. Two common barometers are the aneroid barometer and the mercury barometer. The aneroid barometer has a sealed chamber containing a partial vacuum. As the atmospheric pressure increases, the chamber is compressed causing the needle to move. As the atmospheric pressure decreases, the chamber expands, causing the needle to move in the other direction. Adial on the meter is calibrated to indicate the correct pressure.
The mercury barometer has a glass tube about 34-in. long. The tube holds a column of mercury. The height of this column reflects the atmospheric pressure. Standard atmospheric pressure at sea level is indicated by 29.92 in. of mercury, which converts to 759.96 mm.