[117] 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 to 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 extent, 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 _manubria_ 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 acquire the form of a wedge, the point of which is turned towards the tail (of the bird), and since they suffer the same force and compression from the air, whether the vibrating wings strike the undisturbed air beneath, or whether, on the other hand, the expanded wings (the osseous axes remaining rigid) receive the percussion of the ascending air; in either case the _flexible feathers yield to the impulse_, and hence approximate each other, and thus the bird moves in _a forward direction_.”--De Motu Animalium, pars prima, prop. 196, 1685.
_The Author’s Views:--his Method of constructing and applying Artificial Wings as contra-distinguished from that of Borelli, Chabrier, Durckheim, Marey, etc._--The artificial wings which I have been in the habit of making for several years differ from those recommended by Borelli, Durckheim, and Marey in four essential points:--