THE FLIGHT OF KIKDB. 200 



is preserved, for tlie wing retains the conditions essential to 

 this function namely, a rigid main-rib and a flexible sur- 

 face. Lastly, in some species the combination of two wings 

 is indispensable to flight; a kind of pseudo- elytron consti- 

 tutes the nervure, and behind this is extended a membranous 

 wing, which is locked in with the posterior border of the 

 anterior one. This second wing does not present sufficient 

 rigidity to enable it to strike the air with advantage, and in 

 these insects flight is rendered impossible, if we cut off the 

 false wing-case ; it is as if we had destroyed the main-rib of 

 a perfect wing. 



CHAPTER III. 



OF THE FLIGHT OF BIRDS. 



Conformation of the bird with reference to flight Structure of the wing, 

 its curves, its muscular apparatus Muscular force of the bird; 

 rapidity of contraction of its muscles Form of the bird ; stable 

 equilibrium ; conditions favourable to change of plane Proportion of 

 the surface of the wings to the weight of the body in birds of different 

 size. 



THE plan by which we have been guided in the study of 

 insect flight must also be followed in investigating that of 

 birds. It will be necessary to determine, by a delicate mode 

 of analysis, the movements produced by the wing during 

 flight ; from these movements we may draw a conclusion as 

 to the resistance of the air which affords the bird a fulcrum 

 on which to exert its force. Then, having propounded cer- 

 tain theories respecting the mechanism of flight, the force 

 required for the work effected by the bird, &c., we will under- 

 take to represent these phenomena by means of artificial 

 instruments, as we have already done with respect to insects. 

 But, before we enter methodically on this study, it will be 

 useful to prepare ourselves for it by some general observa- 

 tions on the organization of the bird, the structure of its wings, 

 the force of its muscular system, its conditions of equilibrium 

 in the air, &c. 



