Force of Air in Motion-Ascensional, Power of Warm Air-Lifting Power of Hydrogen Gas Types of Dirigibles-Why the Airplane is Best-Attraction of Gravity-Elementary Airplane Principles-Kite Supported by Air in 'Motion-Air Resistance-Table 1, Resistance of Aerofoil Sections-Table 2, Wind Pressure at Various Velocities-How Airplanes Differ.
THE navigation of the air, which has been the dream of mankind for ages, has only been realized in recent years. Practical aircraft have been built in definite forms that can easily be classified, and also in several experimental types that are little known and which have been discarded in favor of the types known to be practical. The air is a gas that surrounds the earth and which is said to extend above the earth's surface for about 40 miles, though the density becomes less and the air becomes rarer as the distance above the earth's surface increases. Above a certain height, about four or five miles from sea level, it is very difficult for human beings to breathe because of the rarity of the air. We are so used to moving about in the air that many consider it an almost intangible substance and do not realize that 16 cubic feet of air will weigh about a pound and that it exerts a pressure of about 15 pounds per square inch surface on everything. We are so constituted that this load is not appreciable to us any more than the force of gravity.
Force of Air in Motion.-Air in motion may exert considerable force. A gentle breeze creates very slight pressure, but a cyclone or hurricane, which means air travelling at a rate of from 75 to 100 miles per hour, can do considerable damage. Much destruction is caused by tornadoes due to the great pressure of air travelling at a high speed, and which has sufficient velocity to uproot large trees and tear buildings