FLYING BY JERKS. 45 



in the mere motion of the wings in flying. They do 

 not proceed upon a level, but by a series of flat ver- 

 tical parabolas, each as long as one of the jerks ; and 

 the motion is upward while the strength of the jerk 

 lasts, and downward as it weakens. The tail feathers 

 of such birds are also frequently flirted out horizon- 

 tally, so that the tail may either resist too great an 

 upward motion from the jerk, or act as a parachute 

 against the downward motion. 



The head and neck also come into action, both in 

 altering the lateral course of the bird, and in shifting 

 its centre of gravity, with reference to the central 

 line of action in the wings ; and the last alteration 

 has no inconsiderable influence in its ascents and its 

 descents. Indeed, a bird when flying is so much in 

 action in all its parts, that it is impossible to point 

 .out the specific action of each. Thus we cannot ex- 

 plain the rationale of flight in any thing like a satis- 

 factory manner, and therefore though the operation 

 is not only possible, but performed habitually and 

 with ease, the precise mode in which it is done still 

 remains one of the wonders of nature. 



We have made these general observations on the 

 operation of flying and the organs of flight, not only 

 because flying is peculiarly the motion of birds, and 

 the one from which they are named, but also to show 

 how much there is to be learned from the operations 

 of nature, which we all have daily opportunities of ob- 

 serving, and yet upon which comparatively few of us 

 ever bestow a single thought. But it is not from 

 this species of motion alone that we can obtain any 

 thing like a knowledge of birds, neither can we 

 found wholly or chiefly upon it that classification 

 which is calculated to assist us in our inquiries. 



The general definition of a bird, at least as depend- 



