AIRSHIPS AND AIRPLANES 31! 



principle. It is very much like the principle illustrated in 

 the flying of a kite. In the first attempts that were made to 

 fly, efforts were made to imitate the flapping action of a 

 bird's wing; but to-day airplanes are made in accordance 

 with the principles governing the soaring bird and the kite. 

 If a person holding the string of a flying kite runs against 

 the wind, the kite rises. There are three forces acting on 

 the surface of the kite : the wind, the string, and the weight 

 of the kite. In the airplane, the pull of the string is re- 

 placed by the motion of the propellers, which urge the air- 



FIG. 125. French fighting airplane, known as the S.P.A.D. 



plane onward until a sufficient wind is generated to lift the 

 airplane. 



Shape of planes. Experiments have shown that the 

 best shape for a plane is a long, narrow surface, with the long 

 edge at right angles to the direction of flight. Some birds 

 have wings that are fourteen times as long as broad. These 

 planes are now made with a slightly curved surface, as it 

 is found that these have greater lifting power than those 

 with flat surfaces. 



Stability. While flying, the planes encounter many cross 

 currents of air, which tend to make the airplanes unstable. 

 In order to give the plane more stability in a longitudinal 

 direction, in which the plane is traveling, one or two hori- 



