FLIGHT 



183 



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 



x Y 



Fig 50. 

 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 Fettigrew's description of the 

 wing's movement agrees with the account I have 



