CONTRIVANCE A NECESSITY. 163 



The second fact observable in reference to birds 

 of easy and powerful flight — namely, that their 

 wings are all sharply pointed at the end — will lead 

 us still further into the niceties of adjustment 

 which are so signally displayed in the machinery 

 of flight. 



The feathers of a bird's wing have a natural 

 threefold division, according to the different wing- 

 bones to which they are attached. The quills 

 which form the end of the wing are called the 

 Primaries : those which form the middle of the 

 vane are called the Secondaries ; and those 

 which are next the body of the bird are called 

 the Tertiaries. The motion of a bird's wing in- 

 creases from its minimum at the shoulder-joint to 

 its maximum at the tip. The primary quills which 

 form the termination of the wing are those on 

 which the chief burden of flight is cast. Each 

 feather has less and less weight to bear, and less 

 and less force to exert in proportion as it lies 

 nearer the body of the bird ; and there is nothing 

 more beautiful in the structure of a wing than the 

 perfect gradation in strength and stiffness, as well 

 as in modification of form, which marks the series 



