FLYING AND FLYING-MACHINES. 331 



whether the air may not resist descent so strongly that 

 with comparatively small effort a horizontal or even 

 ascending motion may be effected. 



A familiar illustration of this supporting power of 

 the atmosphere is given in the flight of an oyster-shell 

 or piece of thin slate, deftly thrown from a schoolboy's 

 practised hand. Such a missile, instead of following 

 the parabolic path traversed by an ordinary projectile, 

 is seen to skim along almost like a bird on resting 

 pinions. It will sometimes even ascend (after the 

 projectile force has ceased to act in raising it), as 

 though in utter disobedience to the laws of gravi- 

 tation. 



The fact appears to be, that when a horizontal plane 

 traverses the air in a horizontal direction, the sup- 

 porting power of the air is increased in proportion as 

 the plane moves more quickly, or in proportion to the 

 actual quantity of air it glides over, so to speak. 

 Indeed, we have clear evidence to this effect in the 

 behaviour of the common toy-kite, the supporting 

 power of which is increased in proportion to the force 

 of the wind. For a kite held by a string in a strong 

 horizontal current of air, corresponds exactly to an 

 inclined plane surface drawn swiftly in a horizontal 

 direction during a calm. The same supporting power 

 which results from the rapid passage of the air under 

 the kite will be obtained during the rapid passage of 

 the kite over still air. 



When we study the flight of birds we are confirmed 

 in the opinion that velocity of horizontal motion 



