242 NATURAL THEOLOGY. 



quills, it becomes a wonderful instrument, more 

 artificial than its first appearance indicates, though 

 that be very striking ; at least, the use which the 

 bird makes of its wings in flying is more compli- 

 cated and more curious than is generally known. 

 One thing is certain, that if the flapping of the 

 wings in flight were no more than the reciprocal 

 motion of the same surface in opposite directions, 

 either upwards and downwards, or estimated in 

 any oblique line, the bird would lose as much by 

 one motion as she gained by another. The sky- 

 lark could never ascend by such an action as this ; 

 for, though the stroke upon the air by the under- 

 side of her wing would carry her up, the stroke 

 from the upper-side, when she raised her wing 

 again, would bring her down. In order, there- 

 fore, to account for the advantage which the bird 

 derives from her wing, it is necessary to suppose 

 that the surface of the wing, measured upon the 

 same plane, is contracted, whilst the wing is drawn 

 up ; and let out to its full expansion, when it de- 

 scends upon the air for the purpose of moving the 

 body by the re-action of that element. Now, the 

 form and structure of the wing, its external con- 

 vexity, the disposition, and particularly the over- 

 lapping, of its larger feathers, the action of the 

 muscles and joints of the pinions, are all adapted 

 to this alternate adjustment of its shape and dimen- 

 sions. Such a twist, for instance, or semirotatory 

 motion, is given to the great feathers of the wing, 

 that they strike the air with their flat side, but rise 



