Principles of Flight Chapter 4  Introduction  Structure of the Atmosphere  This chapter examines the fundamental physical laws  governing the forces acting on an aircraft in flight, and  what effect these natural laws and forces have on the  performance characteristics of aircraft. To control an aircraft,  be it an airplane, helicopter, glider, or balloon, the pilot  must understand the principles involved and learn to use or  counteract these natural forces.  The atmosphere is an envelope of air that surrounds the Earth  and rests upon its surface. It is as much a part of the Earth as  the seas or the land, but air differs from land and water as it is  a mixture of gases. It has mass, weight, and indefinite shape.  The atmosphere is composed of 78 percent nitrogen, 21  percent oxygen, and 1 percent other gases, such as argon  or helium. Some of these elements are heavier than others.  The heavier elements, such as oxygen, settle to the surface  of the Earth, while the lighter elements are lifted up to the  region of higher altitude. Most of the atmosphere’s oxygen  is contained below 35,000 feet altitude. 