TIGREW ON THE MECHANISM OF FLIGHT. 2 



is delivered obliquely downwards and forwards, and not vertically or slightly backwards, as is 

 generally stated. During the return or up stroke the movement is 

 the body, which may be regarded as running through the root of the 

 clined upwards as in flight ; d, root of wing; df, anterior or thick ma: 



x, long axis of 



ng in- 



df 



cr 



is depressed, 



ens at the termination of 

 margin of the wine: durin 



j its descent and ascent. Compare with wave-track described 

 by the insect's wing, fig. 59 a a\ Diagram 6a,b, p. 233, and with the terminal or free mar- 



gin of the screw propeller, fig. 52 a a ! . A careful examination of this figure (61) will show 

 that the anterior or thick margin, and the posterior or thin margin of the wing describe 



motion, crossing each other. It also slum 

 mcave or biting surface (af 1 ) to the horizon, 

 ted backwards. Lastly, it shows that the blur 



curves, when the wine is in 



(/) 



presented by the wing in motion is twisted upon itself precisely in the same manner as the 

 blade of the screw propeller (fig. 52) is twisted upon itself. Compare convex surface/ (of 

 fig. 61) with convex surface c of fig. 52 j then the concave surface / of fig. 61 with the eon- 

 cave surface d of fig. 52. Compare likewise the free spiral margin a a f of fig. 61 with eorre- 



marffin 



Pig. 62. Left wing of the Wild Goose, elevated preparatory to making the effective or down stroke. 



cle, anterior or thick margin of the pinion; b a c, posterior or thin margin, formed by the tij of 



fff. 



found on the under 

 down stroke. The 



, & , C*XWAig 



See also t of fis% 75. IVom 



om fi<r. 75, it will be evident that the wing, when full 



give the down stroke, presents a double curve— a large one, whose concavity i> directed forward 

 when the wing is raised (fig. 62 ba), and downwards, when the wing is on the MUM level as 

 the body (fig. 75 b a). This curve is formed by the forward inclination of the primary and 

 secondary feathers. The smaller curve is principally formed by the tertiary feather which arc 

 slightly inclined backwards. It has its convexity directed forwards when the wing is raised 

 (fig. 62 g), and downwards when the wing is on a level with the body (fig. 75 e). The tip 

 and body of the wing, as will readily be understood, seize the air with avidity, while the 

 root of the wing facilitates its escape when it can no longer be serviceable for elevation 

 and propulson. The wing may therefore be said to be seizing the wind at one part and 

 letting it eo at another at one and the same time; and there can be no doubt that the wing in 



to AU to 



great measui 



The grasping and disentangling power possessed by the wing is augmented by the pinion 



being more 



cur 



margin 



during flexion (fig. 73 b a c) are reversed during extension 



(fig. 75 bac). 



Shows 



iiglit wing ot the Kestrel, drawn irom tne specimen; wm*m*» *~~^ ~ o- 



how the primary (b), secondary (a), and tertiary (c) feathers overlap and buttress or support each 



other in every direction. Each set of feathers has further 



g conical from within 



'/ 



thin 



win 



form, it being neither rounded as in the Partridge (fig. 67), nor 



as 



Albatros (fig, 64), nor pointed as in tl 

 are unusually symmetrical and strong. 



The feathers of the Kestrel's wing 



