DP PETTIGREW ON THE PHYSIOLOGY OF WINGS. 413 



one would naturally conclude a doivnivard and backivard stroke alone is requisite. 

 This reasoning is true of water and wings, but it is not true of air and wings. 



In the first place, a natural wing, or a properly constructed artificial one, 

 cannot be depressed either vertically downwards, or downwards and backwards. 

 It will of necessity descend doivnivards and forwards in a curve. This arises 

 from its being flexible and elastic throughout, and in especial from its being 

 carefully graduated as regards thickness, the tip being thinner and more elastic 

 than the root, and the posterior margin than the anterior margin. 



In the second place, there is only one direction in which the wing could 

 strike so as at once to support and carry the bird forward. The bird, when 

 flying, is a body in motion. It has therefore acquired momentum. If a grouse 

 is shot on the wing it does not fall vertically downivards, as Borelli and his 

 successors assume, but doivnivards and forwards. The flat surfaces of the 

 wings are consequently made to strike downwards and forwards, as they in this 

 manner act as kites to the falling body, which they bear, or tend to bear, upwards 

 and forwards. So much for the direction of the stroke during the descent of 

 the wing. Let us now consider to what extent the posterior margin of the wing- 

 yields in an upward direction when the wing descends. Borelli does not 

 state the exact amount. The Duke of Argyll, who agrees with Borelli 

 that the posterior margin of the wing is elevated during the clown stroke, avers 

 that, whereas the air compressed in the hollow of the wing cannot pass through 

 the wing owing to the closing upwards of the feathers against each other, or 

 escape forwards because of the rigidity of the bones and of the quills in this 

 direction, it passes backwards, and in so doing lifts by its force the elastic ends 

 of the feathers. In passing backwards it communicates to the whole line of 

 both wings a corresponding push forwards to the body of the bird. The same 

 volume of air is thus made, in accordance with the law of action and reaction, 

 to sustain the bird and carry it forward* Mr M'Gillivray observes that "to 

 progress in a horizontal direction it is necessary that the downward stroke 

 should be modified by the elevation in a certain degree of the free extremities of 

 the quills.^ 



Marey's Views. — Professor Marey states that during the down stroke the 

 posterior or flexible margin of the wing yields in an upward direction, to such 

 an extent as to cause the under surface of the wing to look backwards, and make 

 a backward angle with the horizon of 45° plus or minus according to circum- 

 stances.! That the posterior margin of the wing yields in a slightly upward 

 direction during the down stroke to prevent shock, I admit. The amount of 



* Eeign of Law. " Good Words," February 1865, p. 1 28. 

 t History of British Birds. Lond. 1837, p. 43. 



\ Mechanisme du vol chez les insectes. Comment se fait la propulsion, by Professor E. J. Marey. 

 Revue des Cours Scientifiques de la France et de l'Etranger for 20th March 18G9, p. 254. 



