DR PETTIGREW ON THE PHYSIOLOGY OF WINGS. 417 



is inclined forwards in the direction of the head of the bird — the bases of the 

 cones formed by each natural wing being, on the contrary, directed outwards 

 {vide x b d of figure 12, page 342) and backwards (see qpn of same figure). 

 This arises from the fact that the wing rotates upon two axes (a b and c d of 

 figure 45, page 376); because it rotates on its root (a of figure 19, Plate XIV.) 

 to form one cone (a/e of same figure), and because, while it is rotating on 

 its root, it is also rotating along its anterior margin (a b) to form a second 

 cone, c h g. The wing, in forming the cone a f e elevates, and in forming 

 the cone dig propels. The base of the wedge which furnishes the horizontal 

 transference is, therefore, turned in the direction of the tail of the bird, 

 which is just the opposite of what Borelli maintains, the base of his wedge 

 being turned in the direction of the head. 



Borelli, and all who have written since his time, are unanimous in affirming 

 that the horizontal transference of the body of the bird is due to the perpen- 

 dicular vibration of the wings, and to the yielding of the posterior or flexible 

 margins of the wings in an upward direction as the wings descend. I am, how- 

 ever, disposed to attribute it to the fact (1st), that the wings, both when elevated 

 and depressed, leap forwards in curves, those curves uniting to form a con- 

 tinuous waved track ; (2d), to the tendency which the body of the bird has to 

 swing forwards, in a more or less horizontal direction, when once set in motion ; 

 (3d), to the construction of the wings (they are elastic helices or screws, which 

 twist and untwist while they vibrate, and tend to bear upwards and onwards any 

 weight suspended from them) ; (4th), to the reaction of the air on the under 

 surfaces of the wings ; (5th), to the ever- varying power with which the wings 

 are urged, this being greatest at the beginning of the down stroke, and least at 

 the end of the up one ; (6th), to the contraction of the voluntary muscles and 

 elastic ligaments, and to the effect produced by the various inclined surfaces 

 formed by the wings during their oscillations ; (7th), to the weight of the bird 

 — weight itself, when acting upon wings, becoming a propelling power, and so 

 contributing to horizontal motion. This is proved by the fact that if a sea 

 bird launches itself from a cliff with expanded motionless wings, it sails along 

 for an incredible distance before it reaches the water. 



The authors who have adopted Borelli's plan of artificial wing, and who 

 have indorsed his mechanical views of the wing's action most fully, are Chabrier, 

 Straus-Durckheim, Girard, and Marey. Borelli's artificial wing, as a 

 reference to fig 54, page 409, will show, consists of a rigid rod in front, and a 

 flexible sail, composed of feathers, behind. It acts upon the air, and the air 

 acts upon it, as occasion demands. 



Chabrier's Views. — Chabrier states that the wing has only one period of 

 activity — that, in fact, if the wing be suddenly lowered by the depressor muscles, 

 it is elevated solely by the reaction of the air. There is one unanswerable objec- 



