8 



THE FLIGHT OF BIRDS 



at right angles to the surface he presents to it. 

 And this force is broken up into two, the one 

 supporting him, the other tending to drive him 

 backward, or, as it is now briefly and clearly put, 

 the force is resolved into lift and drift. 



Fig. 4. 



Boat tacking. — w, the wind which acts at right angles to sl 

 (the sail), towards x. The force is broken up into two forces, 

 acting towards d and l. 



A paper kite supplies us with another and perhaps 

 still more apt illustration. If the air is still, the 

 kite-flyer may supply a force by running with the 

 string ; if there is a wind, he has merely to hold 

 the string. Let us imagine that the wind is blowing 

 horizontally. It will, nevertheless, lift the kite, as 

 long as the string is held firmly. It strikes against 

 the oblique surface which the kite presents, and 



