416 BR PETTIGREW ON THE PHYSIOLOGY OF WINGS. 



direction x w, and if allowance be made for the operation of gravity, the 

 flight of the bird will fall somewhere between w and b, probably in the line x, r. 

 This arises from the wing acting as an eccentric — from the upper concave sur- 

 face of the pinion being always directed 

 \ m upwards, the under concave surface 



w ^v -•' N downwards — from the under surface, 



r ^ ^^\/ \. i \ which is a true kite, darting forwards in 



7 """"^""^^x L^^ '■ wave curves both during the down and 



^iV */ up strokes, and never making a back- 



y/ ward angle with the horizon ; and lastly, 



x --__ .!„._. ---'' from the wing employing the air under 



71 it as a fulcrum during the down stroke 



Fig. 56. ° y 



the air, on its part, reacting on the under 

 surface of the pinion, and when the proper time arrives, contributing to the 

 elevation of the wing. 



If, as Borelli and his successors believe, the posterior margin of the wing 

 yielded to any marked extent in an upward direction during the down stroke, 

 and more especially if it yielded to such an extent as to cause the under surface 

 of the wing to make a backward angle with the horizon of 45°, one of two things 

 would inevitably follow — either the air on which the wing depended for support 

 and propulsion would be permitted to escape before it was utilised, or the wing 

 would dart rapidly downward, and carry the body of the bird with it. If 

 the posterior margin of the wing yielded in an upward direction to any 

 marked extent during the down stroke it would be tantamount to removing 

 the fulcrum (the air) on which the lever formed by the wing operates. The 

 wing of the bird, as I have fully explained (see pages from 366 to 384 inclu- 

 sive), acts as a kite both during the down and up strokes, the ventral aspect 

 of the kite being always directed forwards [vide from c to m of fig. 15, 

 page 345). 



If a bird flies in a horizontal direction the angles made by the under sur- 

 face of the wing with the horizon are very slight, but they always look forwards. 

 If a bird flies upwards the angles in question are increased. In no instance, 

 however, unless when the bird is everted and flying downwards, is the posterior 

 margin of the wing on a higher level than the anterior one. This holds true of 

 natural flight, and, consequently, ought to hold true of artificial flight. 



With regard to the cone formed, according to Borelli, by the vertical 

 descent of the two wings, or what, in his opinion, is the same thing, the per- 

 pendicular ascent of the air, and which is represented aife c of figure 54, page 

 409 ; I think it would be more accurate to state that, instead of the two wings 

 taken together forming one cone, that each wing by itself forms two cones. 



The base of Borelli's cone (e b c, figure 54, p. 409), it will be remembered, 



