The atmosphere in which we fly is an envelope of air which surrounds the earth and rests upon its surface. It is as much a part of the earth as the seas or the land. However, air differs from land and water inasmuch as it is a mixture of gases. It has mass weight and indefinite shape.

   Air, like any other fluid, is able to flow, and change its shape when subjected to even minute pressures because of the lack of strong molecular cohesion. For example, gas will completely fill any container into which it is placed, expanding or contracting to adjust its shape to the limits of the container.
 

The atmosphere is composed of 78 percent nitrogen, 21 percent oxygen, and 1 percent other gases, such as argon, helium, etc. As some of these elements are heavier than others, there is a natural tendency of these heavier elements, such as oxygen, to settle to the surface of the earth, while the lighter elements are lifted up to the region of higher altitude. This explains why most of the oxygen is contained below 35,000 feet altitude. Because air has mass and weight, it is a body, and as a body, it reacts to the scientific laws of bodies in the same manner as other gaseous bodies. This body of air resting upon the surface of the earth has weight (Fig. 17-1) and at sea level develops an average pressure of 14.7 pounds on each square inch of surface, or 29.92 inches of mercury, - but as its thickness is limited, the higher we go the less air there is above us. For this reason, the weight of the atmosphere at 18,000 feet is only one-half what it is at sea level.