The Bird Book 



possess a Great Auk's egg may be judged from 

 the fact that several hundred pounds have often 

 been paid in order to gratify it. 



But we digress. We have seen that weight 

 gives momentum, and momentum obviously helps 

 to overcome the resistance of the air which, 

 in its turn, is used to combat the tendency 

 of gravitation. It is this resistance of the air 

 to a sufficiently vigorous beat of the wing 

 that enables the bird to get enough leverage to 

 raise itself in the air, though the upward beat of 

 the wing would, but for a special contrivance, 

 neutralise the effect of the downward, and flight 

 would be impossible. 



This contrivance is two-fold. The wing is 

 curved, not flat, and as the upward beat is made 

 with the convex side, it encounters little resist- 

 ance from the air compared with the downward 

 beat with the concave side a difference which 

 may easily be tested by first pushing and then 

 pulling an open umbrella through the air. 

 Secondly, the feathers under-l&p, and so, in the 

 downward beat, become locked together in full 

 resistance to the air, while they separate on the 

 upward beat, allowing the air to escape between 

 them. 



So far, however, our imaginary bird would 



only be able to rise perpendicularly, and now the 



third law comes into operation. The whole wing, 



as indeed each feather, is formed with the anterior 



8 



