CONTRIVANCE A NECESSITY. 165 



escape is not useless for progression ; but the air 

 acts less favourably for this purpose than when 

 struck by a more compact set of feathers. The 

 common Rook and all the Crows are examples 

 of this. The Peregrine Falcon, the common Swal- 

 low, and all birds of very powerful flight have been 

 provided with the sharp-pointed structure* 



The object of this structure, and the mechanical 

 laws to which it appeals, will be apparent when 

 we recollect what it is on which the propelling 

 power, as distinct from the sustaining power, of a 

 bird's wing depends. It depends on the reaction 

 of the air escaping backwards — that is, in the direc- 

 tion exactly opposite to that of the intended 

 motion of the bird. Any air which escapes from 

 under the wing, in any other direction, will of 

 course react with less advantage upon that motion 

 But from under a round wing a good deal of air 

 must necessarily escape along the rounded cud — 

 that is, in a direction at right angles to the line of 

 intended flight. All the reaction produced by this 



* The illustrations of Mr Wolf will here again be the best explana- 

 tion to the reader of the difference between the sharp and the round 

 structure, p. 166. 



