Structure and Flight 



edge stiff, the posterior yielding ; hence, with the 

 beat of the wing, almost all the imprisoned air 

 escapes backwards, and in compliance with the 

 law, there must be an equal force acting in the 

 opposite direction, and this urges the bird forward. 

 It is the same principle as that which enables a 

 boat to " tack." 



We may here note, in passing, another point of 

 similarity between the structure of a bird and a 

 boat. The skeleton or framework is made as 

 light as possible, consistent with strength, and 

 the heaviest bone in the bird's body is the breast 

 bone, which corresponds in position and function 

 to the keel of the boat. 



Such, in brief, are the principles which govern 

 flight. We have already seen what possibilities 

 there are of variation in structure to suit the 

 habits of different birds, and the comparison of a 

 Swift, which can, when travelling at upwards of 

 a hundred miles an hour, deviate from its course 

 to snap up the minutest insect, with a Puffin, 

 which has difficulty in starting flight at all, is 

 sufficient to prove that this province of bird-life 

 is equally subject to modification. 



We now notice that all swift fliers have sharp- 

 pointed wings. Compare for a moment a Pere- 

 grine Falcon with an Owl. The former overtakes 

 and strikes down his prey ; speed is essential. 

 Not so the Owl ; he sails noiselessly along and 

 drops on his prey unawares. The Peregrine's wing 



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