What is altitude and pressure?

Air pressure varies at different altitudes, or heights above sea level. Experiments by Torricelli and other scientists have proved that the height of the column of mercury in a barometer becomes less if the barometer is carried up a mountain or toward the top of a high building.

At three miles above sea level the barometer column is somewhat less than 15 inches high.

Therefore about half of all the air is below an altitude of three and a half to four miles. Aviators have found that above three miles the air “thins out” rapidly. If you were down in a deep mine, you would find the air pressure greater than 1 atmosphere because you would be below sea level. There you would find the air pressure greater than 15 pounds per square inch.