412 DP PETTIGREW ON THE PHYSIOLOGY OF WINGS. 



Second, That the wing consists of two portions — a rigid anterior portion, 

 and a non-rigid flexible portion. The rigid portion he represents in his artificial 

 bird (figure 54, page 409) as consisting of a rod (e, r), the yielding portion of 

 feathers (a, o). 



Third, That if the air strikes the under surface of the wing perpendicularly 

 in a direction from below upwards, the flexible portion of the wing will yield in 

 an upward direction, and form a wedge with its neighbour. 



Fourth, Similarly and conversely, if the wing strikes the air perpendicularly 

 from above, the posterior and flexible portion of the wing will yield and be forced 

 in an upward dirction. 



Fifth, That this upward yielding of the posterior or flexible margin of the 

 wing results in and necessitates a horizontal transference of the body of the bird. 



Sixth, That to sustain a bird in the air the wings must strike vertically 

 downwards, as this is the direction in which a heavy body, if left to itself, would 

 fall. 



Seventh, That to propel the bird in a horizontal direction, the wings must 

 descend in a perpendicular direction, and the posterior or flexible portions of 

 the wings yield in an upward direction, and in such a manner as virtually 

 to communicate an oblique action to them. 



Eighth, That the feathers of the wing are bent in an upivard direction when 

 the wing descends, the upward bending of the elastic feathers contributing to 

 the horizontal travel of the body of the bird. 



I have been careful to expound Borelli's views for several reasons : — 



1st, Because the purely mechanical theory of the wing's action is to be 

 traced to him. 



2d, Because his doctrines have remained unquestioned for nearly two 

 centuries, and have been adopted by all the writers since his time, without, I 

 regret to say in the great majority of cases, any acknowledgment whatever. 



3d, Because his views have been revived by the modern French school, and 



toh, Because in commenting upon and differing from Borelli I will neces- 

 sarily comment upon and differ from all his successors. 



The Duke of Argyll agrees with Borelli in believing that the wing 

 invariably strikes perpendicularly downivards. His words are — " Except 

 for the purpose of arresting their flight birds can never strike except directly 

 downwards ; that is, against the opposing force of gravity. " Professor Owen 

 in his " Comparative Anatomy," Mr M'Gillivray in his "British Birds," Mi- 

 Bishop in his article Motion in the " Cyclopedia of Anatomy and Physiology," 

 and M. Liais " on the flight of birds and insects" in the '* Annals of Natural 

 History," all assert that the stroke is delivered downwards and more or less 

 backwards. To obtain an upward recoil, one would naturally think all that is 

 required is a downward stroke, and to obtain an upward and forward recoil, 



