vii FLIGHT 189 



the front. The difference is far more apparent when 

 we take the whole wing. Above, a bird s wing is con- 

 vex, so that it passes easily through the air ; the 

 under-surface is concave and lays hold of it. I am 

 referring mainly to the part of it which is nearest to 

 the body and which forms a kind of irregular cup, its 

 hinder side gently sloping away. Experiments have 

 been made with a view to measuring the resistance of 

 the air to concave surfaces, but the results do not help 

 us much. The irregularity of the wings has not been 

 reproduced, the velocity has been uniform, whereas 

 that of the wing is very different at different stages of 

 the stroke, and no account has been taken of the 

 variation of the curves during flight owing to the 

 elasticity of the great feathers. 



The duty of the near part of the wing is to a great 

 extent that of a parachute. Between the strokes, the 

 bird drops, and were it not for these umbrella-like 

 supports, the drop would be greater than it is. The 

 work of propelling is, as we have seen, done mainly 

 by the extremities of the wings, which move with far 

 greater rapidity. It is possible to find approximately 

 the centre of the action of the air — to find a point in 

 the wing so situated that the air shall act with equal 

 force on the near and far sides of it. If the wing were 

 a triangle, this point would be in a line drawn from 

 the middle of the base to the apex, f of its length 

 from the shoulder. If it were a rectangle, it would be 

 at a point on the corresponding line f of its length 

 out. Professor Marey estimates that in the actual 

 wing it is at a point about | of its length from the 

 base. It must vary much in wings of different shape. 



