THE CONQUEST OF THE AIR 79 



to carry messages out of besieged cities, but these things are all 

 better done by small balloons, which will be considered later. 



Lawrence Hargrave, of Sidney, New South Wales, invented 

 the box kite. This kite, it was discovered, has greater lifting 

 power than a kite that presents only one plane surface; besides 

 which it flies much more steadily, needing no tail. This dis- 

 covery was very suggestive to those inventors who were working 

 on the aeroplane at that time. In fact, the principles underlying 

 the flight of a kite are the ones that make possible the flight of 

 the aeroplane, and to understand why a kite flies is to understand 

 in large measure the principles upon which operate all those 

 machines of men that depend on currents of moving fluids such 

 as winds and streams of water for their motive or sustaining 

 power. 



Every lad who has flown a kite knows it will fly well only in 

 a wind. By running swiftly while you hold one end of the long 

 string to the other end of which the kite is attached, you may 

 make the kite rise a bit on a still day, but it drops back to the 

 ground again the minute you stop running. While running you 

 pull the kite through the air, but when the wind is blowing the 

 air streams past the kite, sending it up. But just how does it 

 operate to accomplish this ? 



When the wind is blowing, particles of the air in their forward 

 trike -against -the face of the kite. If the kite were 



not held by the string, it would just blow along on the ground 

 in the direction in which the wind is blowing, like a loose sheet of 

 paper. But the string is so tied to the kite by means of the bridle 

 that the kite's surface stands inclined to the wind, and so the 

 moving air particles strike the kite at an angle, hitting a glancing 

 blow. When that happens the force with which the wind strikes 

 the kite is broken up into two components, one of which lifts the 

 kite up into the air. 



A simple experiment may be readily performed to illustrate 

 the law of the composition and decomposition of forces. Set 

 three small nails into a large drawing-board or the floor at the 



