Borelli is of opinion that flight results from the application of an inclined plane, which beats the air, and which has a wedge action. He, in fact, endeavours to prove that a bird wedges itself forward upon the air by the perpendicular vibration of its wings, the wings during their action forming a wedge, the base of which (c b e) is directed towards the head of the bird; the apex (a f) being directed towards the tail. This idea is worked out in propositions 195 and 196 of the first part of Borelli’s book. In proposition 195 he explains how, if a wedge be driven into a body, the wedge will tend to separate that body into two portions; but that if the two portions of the body be permitted to react upon the wedge, they will communicate oblique impulses to the sides of the wedge, and expel it, base first, in a straight line.
Following up the analogy, Borelli endeavours to show in his 196th proposition, “that if the air acts obliquely upon the wings, or the wings obliquely upon the air (which is, of course, a wedge action), the result will be a horizontal transference of the body of the bird.” In the proposition referred to (196) Borelli states—“If the expanded wings of a bird suspended in the air shall strike the undisturbed air beneath it with a motion perpendicular to the horizon, the bird will fly with a transverse motion in a plane parallel with the horizon.” In other words, if the wings strike vertically downwards, the bird will fly horizontally forwards. He bases his argument upon the belief that the anterior margins of the wings are rigid and unyielding, whereas the posterior and after parts of the wings are more or less flexible, and readily give way under pressure. “If,” he adds, “the wings of the bird be expanded, and the under surfaces of the wings be struck by the air ascending perpendicularly to the horizon, with such a force as shall prevent the bird gliding downwards (i.e. with a tendency to glide downwards) from falling, it will be urged in a horizontal direction. This follows because the two osseous rods (virgæ) forming the anterior margins of the wings resist the upward pressure of the air, and so retain their original form (literally extent or expansion), whereas the flexible after-parts of the wings (posterior margins) are pushed up and approximated to form a cone, the apex of which (vide a f of fig. 113) is directed towards the tail of the bird. In virtue of the air playing upon and compressing the sides of the wedge formed by the wings, the wedge is driven forwards in the direction of its base (c b e), which is equivalent to saying that the wings carry the body of the bird to which they are attached in a horizontal direction.”
Borelli restates the same argument in different words, as follows:—
“If,” he says, “the air under the wings be struck by the flexible portions of the wings (flabella, literally fly-flaps or small fans) with a motion perpendicular to the horizon, the sails (vela) and flexible portions of the wings (flabella) will yield in an upward direction, and form a wedge, the point of which is directed towards the tail. Whether, therefore, the air strikes the wings from below, or the wings strike the air from above, the result is the same—the posterior or flexible margins of the wings yield in an upward direction, and in so doing urge the bird in a horizontal direction.”
In his 197th proposition, Borelli follows up and amplifies the arguments contained in propositions 195 and 196. “Thus,” he observes, “it is evident that the object of flight is to impel birds upwards, and keep them suspended in the air, and also to enable them to wheel round in a plane parallel to the horizon. The first (or upward flight) could not be accomplished unless the bird were impelled upwards by frequent leaps or vibrations of the wings, and its descent prevented. And because the downward tendency of heavy bodies is perpendicular to the horizon, the vibration of the plain surfaces of the wings must be made by striking the air beneath them in a direction perpendicular to the horizon, and in this manner nature produces the suspension of birds in the air.”
“With regard to the second or transverse motion of birds (i.e. horizontal flight) some authors have strangely blundered; for they hold that it is like that of boats, which, being impelled by oars, moved horizontally in the direction of the stern, and pressing on the resisting water behind, leaps with a contrary motion, and so are carried forward. In the same manner, say they, the wings vibrate towards the tail with a horizontal motion, and likewise strike against the undisturbed air, by the resistance of which they are moved forward by a reflex motion. But this is contrary to the evidence of our sight as well as to reason; for we see that the larger kinds of birds, such as swans, geese, etc., never vibrate their wings when flying towards the tail with a horizontal motion like that of oars, but always bend them downwards, and so describe circles raised perpendicularly to the horizon.[110]
“Besides, in boats the horizontal motion of the oars is easily made, and a perpendicular stroke on the water would be perfectly useless, inasmuch as their descent would be impeded by the density of the water. But in birds, such a horizontal motion (which indeed would rather hinder flight) would be absurd, since it would cause the ponderous bird to fall headlong to the earth; whereas it can only be suspended in the air by constant vibration of the wings perpendicular to the horizon. Nature was thus forced to show her marvellous skill in producing a motion which, by one and the same action, should suspend the bird in the air, and carry it forward in a horizontal direction. This is effected by striking the air below perpendicularly to the horizon, but with oblique strokes—an action which is rendered possible only by the flexibility of the feathers, for the fans of the wings in the act of striking acquire the form of a wedge, by the forcing out of which the bird is necessarily moved forwards in a horizontal direction.”
The points which Borelli endeavours to establish are these:—
First, That the action of the wing is a wedge action.
Second, That the wing consists of two portions—a rigid anterior portion, and a non-rigid flexible portion. The rigid portion he represents in his artificial bird (fig. [113], p. 220) as consisting of a rod (e r), the yielding portion of feathers (a o).