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 
Cc B 
A D 
Fig. 3. 
into two forces represented in magnitude and direc- 
tion by the linesc 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 
