vu FLIGHT 183 
4 
into two forces, one represented by XM, the other by 
XR. But the force XR will have little effect since the 
water offers great resistance to the movement of a 
boat sideways through it ; at any rate if it has a keel or 
a centreboard. The force XM will cause the boat to 
move in the direction she is meant to go. This is an 
excellent illustration of what happens in the flight of 
a bird. When the wing is descending its front 
margin is lower than its hind margin ; it is turned so 
that the long feathers slope upwards. For simplicity 
Fic 50. 
X Y=section of wing ; X, front margin; Y, hind margin; W = wind caused by 
the down stroke ; BA, resultant force, which is resolved into BC, BD. 
we must imagine that the air is quite still. The wing 
descends with great velocity through it, “ making its 
own wind” like a bicyclist. The resistance of the 
air will be equal to a wind blowing vertically upward 
(W in Fig. 50). When it strikes the inclined surface 
of the wing its force will act in a direction at right 
angles to it (BA). The force BA may be divided 
into two forces, BD, which supports the bird, BC, 
which drives him onward. 
Whether Professor Pettigrew’s description of the 
wing’s movement agrees with the account I have 
