THE MEASUREMENT OF PRESSURE: THE MERCURY BAROMETER
Two terms may be used to express the gravity of the air—weight and pressure. The weight of air is used more properly to express the gravimetric force of a given volume: thus, 1 cubic foot of air at normal pressure and temperature of 32° F weighs 1.29 troy ounces (1 cu. cm. = 0.0013 gram). In meteorology it is more convenient to consider the weight of a column of air throughout its whole extent, from sea level upward. At sea level such a column of air presses upon the surface with an average force of 14.7 pounds, a pressure empirically termed 1 atmosphere.
The pressure of such a column varies, however, not only from day to day, but from hour to hour. If a mass or a wave of air accumulates over a given locality the pressure increases; conversely, if a depression, or a trough, occurs, the pressure decreases. As the observer goes from sea level to a higher altitude the pressure decreases also. At an altitude of 19,000 feet, a height occasionally reached by airmen, the pressure is about half of that at sea level.
The pressure of the air may be determined by weighing it; it is much more convenient to compare it with a column of mercury which balances it. A balance constructed for this purpose is a barometer. The theoretical construction of the barometer is simple; it consists of a glass tube about 33 inches long, closed at one end, filled with mercury, and inverted with the open end in a cup of mercury. The column of mercury within the tube exactly balances a column of air of equal section. If air accumulates, the increased pressure on the mercury in the cup forces the column higher in the tube; conversely, a decrease in pressure causes the mercury column to shorten;