396 Introduction to the Study of Science 



Glenn H. Curtiss, whose name and work are widely known, 

 won the race for speed at Rheims, France, attaining a speed of 

 47 miles an hour. To-day machines are made to carry several 

 passengers, to move at an almost incredible rate of speed, to 

 remain in the air for many hours, tests have now reached more 

 than twenty-four hours' continuous flight, and to attain 

 altitudes formerly supposed possible only for gas balloons. 

 The control of the machine in the air is remarkable, as shown in 

 the marvelous feats by expert airmen, such as looping the loop, 

 flying upside down, and apparently leaping from place to 

 place. The uses of the airplane and the hydro-airplane are 

 familiar to all, but the extent of their services to man can only 

 with difficulty be realized. 



184. Airplanes and kites. Some of the chief difficulties 

 met in the development of flying machines have been mentioned. 

 The principle on which the flight of the airplane depends may be- 

 comparatively easy to understand, but its realization in machin- 

 ery and planes was not easily or quickly made. A flat stone 

 when properly thrown sails a long distance, and illustrates some 

 of the points of the flying machine. But the simplest device by 

 which the principle of flight may be made clear is the kite, 

 familiar in scientific investigations as well as in the play of 

 children. The kite depends for its flight upon the moving air or 

 wind against which its lower surface is presented at a suitable 

 angle ; but the airplane makes its own wind by moving forward, 

 its planes being presented against this wind. This difference 

 may be neglected for the present. 



Exercise: Why kites fly. In the diagram (Fig. 139) the entire 

 force of the wind is represented as located at the center of the kite. 

 A line representing the wind force or pressure is drawn at right .angles 

 to the surface of the kite. The action of the string is shown similarly. 

 We have to find what the lifting force is when the wind pressure and 

 the position of the kite are known. The procedure is this : from the 

 center of the kite, which may be represented by a point, draw a line 

 of indefinite length and at right angles to the kite. Select a scale to 

 be employed throughout, as 10 grams to one centimeter, in order to 



