2i0 



ANIMAL MECHANISM. 



Conformation of the bird. — By the simple inspection of a 

 bird's wing, it is easy to see that the mechanism of its flight 

 is altogether different from that of an insect. From the 

 manner in which the feathers of its wing lie upon each other, 

 it is evident that the resistance of the air can only act from 

 below upwards, for in the opposite direction the air would 

 force for itself an easy passage by bending the long barbs of 

 the feathers, which would no longer sustain each other. This 

 well-known arrangement, so carefully described by Prechlt,^' 

 has caused persons to suppose that the wing only needed to 

 oscillate in a vertical plane in order to sustain the weight of 

 the bird, because the resistance of the air acting from be- 

 low upwards is greater than that which it exerts in the 

 opposite direction. 



This writer has been wrong in basing on the inspection of 

 the organ of flight all the theory of its function. We shall find 

 that experiment contradicts in the most decided manner these 

 premature inductions. 



If we take a dead bird, and spread out its wings so as to 

 place them in the position represented in flg. 89, we see that at 



Fjg. 89. — Various curves of the wing of a bird at different points in its 

 length. 



different points in its length, the wing presents very remark- 

 able changes of plane. At the inner part, towards the body, 

 the wing inclines considerably both downwards and back- 

 wards, while near its extremity, it is horizontal and some- 

 times slightly turned up, so that its under surface is directed 

 somewhat backward. 



Dr. Pettigrew thought that he could find in this curve a 

 surface resembling a left-handed screw propeller ; struck with 

 the resemblance between the form of the wing and that of 

 the screw used in navigation, he considered the wing of a 



* Untersuchungen iiber den Flug der Vogel. 8vo. Vienna : 1846. 



