234 



AERONAUTICS. 



During the vigorous depression of the wing the nervure has 

 the power of remaining rigid, whereas the flexible portion, 

 being pushed in an ujpward direction on account of the resist- 

 ance it experiences from the air, assumes an oblique position, 

 which causes the upper surface of the wing to looh forwards!^ 

 . . . "At first the plane of the wing is parallel with the body 

 of the animal. It lowers itself — the front part of the wing 

 strongly resists, the sail which follows it being flexible yields. 

 Carried by the ribbing (the anterior margin of the wing) which 

 lowers itself, the sail or posterior margin of the wing being 

 raised meanwhile by the air, which sets it straight again, 

 the sail will take an intermediate position, and incline itself 

 about 45° plus or minus according to circumstances. The 

 wing continues its movements of depression inclined to the hori- 

 zon, but the impulse of the air which continues its effect, and 

 naturally acts upon the surface which it strikes, has the power 

 of resolving itself into two forces, a vertical and a horizontal 

 ^'orce, the first suffices to raise the animal, the second to move 

 d along J' ^ The reverse of this, Marey states, takes place during 

 the elevation of the wing — the resistance of the air from above 

 causing the upper surface of the wing to look backwards. The 

 fallaciousness of this reasoning has been already pointed 



1 Compare Marey's description with that of Borelli, a translation of which 

 I subjoin. " Let a bird be suspended in the air with its wings expanded, 

 and first let the under surfaces (of the wings) be struck by the air ascending 

 perpendicularly to the horizon with such a force that the bird gliding down 

 is prevented from falling : I say that it (the bird) will be impelled with a 

 horizontal forward motion, because the two osseous rods of the wings are 

 able, owing tcf the strength of the muscles, and because of their hardness, to 

 resist the force of the air, and therefore to retain the same form (literally ex- 

 tent, expansion), but the total breadth of the fan of each wing yields to the 

 impulse of the air when the flexible feathers are permitted to rotate around 

 the manuhria or osseous axes, and hence it is necessary that the extremities 

 of the wings approximate each other : wherefore the wings acquire the form 

 of a wedge whose point is directed towards the tail of the bird, but whose 

 surfaces are compressed on either side by the ascending air in such a manner 

 that it is driven out in the direction of its base. Since, however, the wedge 

 formed by the wings cannot move forward unless it carry the body of the bird 

 along with it, it is evident that it (the wedge) gives place to the air impelling 

 it, and therefore the bird flies forward in a horizontal direction. But now let 

 the substratum of still air be struck by the fans (feathers) of the wings with 

 a motion perpendicular to the horizon. Since the fans and sails of the wings 



