GLIDING 7 



slanting- wise at a window — thrown by a dexterous, 

 mischievous urchin standing far off beneath the 

 wall of the house — and the fragments of glass as 

 they go flying into the room make a right angle 

 with the plane of the window. 



The air, then, striking against the plane presented 

 by the bird acts at right angles to it, and there 

 comes into play a resolution of one force into two. 

 This introduces what is known as the parallelogram 

 of forces. 



There is a force acting along a b (fig. 3), and if 

 resistance is in this direction it may resolve itself 



C ^. 



Fig. 3. 



into two forces represented in magnitude and direc- 

 tion by the lines c b, d b. Take the case of a boat 

 tacking. The wind acts at right angles to the sail, 

 but the boat refuses to move much in that direction 

 (i.e. broadside on) ; she makes only a little leeway. 

 The force of the wind, therefore, acting towards x 

 (fig. 4), is broken up into two forces acting towards 

 d and l, and that towards l, as I have said, does not 

 count for much, since the boat will not readily move 

 broadside on. So there is much headway and a 

 little leeway. The principle at work is the same 

 when a bird is gliding horizontally. His body is 

 inclined slightly upward. The force of the air acts 



