344 



DR PETTIGREW ON THE PHYSIOLOGY OF WINGS. 



and posterior margin, the root in like manner cleaving a passage from without 

 inwards and upwards for the body and tip. It is in this way that the wing 

 presents a sharp cutting edge during the up stroke, a remark which applies even 

 to the rowing feathers (quill feathers) of the wing of the bird. The ascent of 

 the wing, as will be subsequently explained, is favoured by the reaction of the air 

 on its under surface, and by the downward and forward fall of the body. If 

 the wing was not concavo-convex in form, and made to oscillate on the surface 

 of an imaginary sphere, it would be impossible for it alternately to avoid and 

 seize the air while it is rising and falling. When the wing descends or 

 makes the down stroke, as it is termed, it also rotates on the surface of the 

 imaginary sphere in question. In this case, however, it is the concave or under 

 surface of the wing which is active, and the rolling takes place in such a manner 

 (it is outwards, downwards, and forwards) as actually greatly to increase the sup- 

 port afforded — the air, which was dispersed and avoided during the up stroke, 

 being now collected together and seized with avidity. It would be difficult to 

 conceive a more simple or effective arrangement. 



The Natural Wing, ivlien Elevated and Depressed, must move Fomvards. — It 

 is a condition of natural wings, and of artificial wings constructed on the prin- 

 ciple of living wings, that when forcibly elevated or depressed, even in a strictly 

 vertical direction, they inevitably dart forward. This is well shown in figure 14. 



Fig. 14. 



If, for example, the wing is suddenly depressed in a vertical direction, as 

 represented at a b, it at once darts downwards and forwards in a curve to c, thus 

 converting the vertical down stroke into a doivn oblique forward stroke. If, 

 again, the wing be suddenly elevated in a strictly vertical direction, as at c d, the 

 wing as certainly darts upwards and forwards in a curve to e, thus converting 

 the vertical up stroke into an upivard oblique forward stroke. The same 

 thing happens when the wing is depressed from e to /, and elevated from g to 

 h. In both cases the wing describes a waved track, as shown at e g, g i, which 

 clearly shows that the wing strikes downwards and fomvards during the down 

 stroke, and upwards and forwards during the up stroke. The wing, in fact, is 

 always advancing, its under surface attacking the air like a boy's kite. If, on 

 the other hand, the wing be forcibly depressed, as indicated by the heavy waved 

 line a c, and left to itself, it will as surely rise again, and describe a waved 

 track, as shown at c e. This it does, in virtue of its flexibility and elasticity, 



