ATMOSPHERIC PRESSURE AND CIRCULATION
Gas Pressure. --The attraction of gravity on the gases of the atmosphere causes them to exert a pressure on the surface of the earth. The actual pressure at any given place is directly due to the elastic pressure (moleeular bombardment) of the gases present, and is distributed among them according to their volume percentages at that particular place, but the sum total of these several partial pressures per unit area in the open atmosphere, as measured by the barometer, is always just equal to the combined weight of all the gases in a vertical column of unit cross section directly above this area.
The circulation of the atmosphere, and, therefore, to a great extent the precipitation, are intimately related to the pressure distribution.
The average pressure at sea level is about 14.7 pounds per square inch. The reason that ordinarily this pressure is hardly noticeable is that it is exerted on all sides of a body, and generally on the inside as well as on the outside. If we remove the air from the inside of a glass vessel by means of the air pump or otherwise, then the external pressure becomes at once apparent.
Mercurial Barometer.--Torricelli, a pupil of Galileo, was one of the earliest investigators to demonstrate that the atmosphere exerts pressure. He also discovered the principle of the mercurial barometer, which is now universally employed to measure this pressure. His experiment, performed as early as 1643, consisted in filling with mercury a long glass tube sealed at one end. The tube was then inverted and the open end immersed in a cup of mercury. The mercury in the tube no longer completely filled it, neither did it all run out; but the top of the mercurial column came to rest at a point about 30 inches above the level of the mercury in the cup. Since there was no air in the tube to press downward on the mercurial column, the weight of the mercury in the tube was just balanced by the weight of