Many later experiments by scientists have proved that at sea level the mercury always stands about 30 inches high in a barometer tube such as Torricelli used. It has been found, also, that a column of mercury 30 inches high and 1 inch square weighs 14.7 pounds.
Therefore the atmospheric pressure at sea level is about 15 pounds per square inch. Thus we say that a pressure of 15 pounds per square inch is 1 atmosphere of pressure. Air pressures are usually expressed in atmospheres.