410 DE PETTIGREW ON THE PHYSIOLOGY OF WINGS. 



vibration of its wings, the wings during their action forming a wedge, the base 

 of which (c b e) is directed towards the head of the bird, the apex (af) being 

 directed towards the tail (d). This idea is worked out in propositions 195 and 

 196 of the first part of Borelli's book. In proposition 195 he explains how, 

 if a wedge be driven into a body, the wedge will tend to separate that body into 

 two portions ; but that if the two portions of the body be permitted to react 

 upon the wedge, they will communicate oblique impulses to the sides of the 

 wedge, and expel it, base first, in a straight line. 



Following up the analogy, Borelli endeavours to show in his 196th pro- 

 position, " that if the air acts obliquely upon the wings, or the wings obliquely 

 upon the air (which is, of course, a wedge action), the result will be a horizontal 

 transference of the body of the bird." In the proposition referred to (196) 

 Borelli states — " If the expanded wings of a bird suspended in the air shall 

 strike the undisturbed air beneath it with a motion perpendicular to the horizon, 

 the bird will fly with a transverse motion in a plane parallel with the horizon." 

 In other words, if the wings strike vertically doivnwards, the bird will fly horizon- 

 tally forwards. He bases his argument upon the belief that the anterior 

 margins of the wings are rigid and unyielding, whereas the posterior and after 

 parts of the wings are more or less flexible, and readily give way under pres- 

 sure. If, he adds, the wings of the bird be expanded, and the under surfaces 

 of the wings be struck by the air ascending perpendicularly to the horizon, with 

 such a force as shall prevent the bird gliding downwards {i.e., with a tendency 

 to glide downwards) from falling, it will be urged in a horizontal direction. 

 This follows because, in Borelli's opinion, the two osseous rods (virgm) forming 

 the anterior margins of the wings resist the upward pressure of the air, and so 

 retain their original form (literally extent or expansion), whereas the flexible after 

 parts of the wings (posterior margins) are pushed up and approximated to form a 

 cone, the apex of which (vide af of figure 54, p. 409) is directed towards the 

 tail of the bird. In virtue of the air playing upon and compressing the sides of 

 the wedge formed by the wings, the wedge is driven forwards in the direction of 

 its base (c, b, e), which is equivalent to saying that the wings carry the body of 

 the bird to which they are attached in a horizontal direction." Borelli restates 

 the same argument in different words, as follows : — 



" If," he says, " the air under the wings be struck by the flexible portions of 

 the wings (fabella, literally fly flaps or small fans) with a motion perpendicular 

 to the horizon, the sails (vela) and flexible portions of the wings (fabella) will 

 yield in an upward direction, and form a wedge, the point of which is directed 

 towards the tail. Whether, therefore, the air strikes the wings from below, or 

 the wings strike the air from above, the result is the same — the posterior or 

 flexible margins of the wings yield in an upivard direction, and in so doing urge 

 the bird in a horizontal direction." 



