[107] Borelli, De Motu Animalium. Sm. 4to, 2 vols. Romæ, 1680.
Indeed it will not be too much to affirm, that to this distinguished physiologist and mathematician belongs almost all the knowledge we possessed of artificial wings up till 1865. He was well acquainted with the properties of the wedge, as applied to flight, and he was likewise cognisant of the flexible and elastic properties of the wing. To him is to be traced the purely mechanical theory of the wing’s action. He figured a bird with artificial wings, each wing consisting of _a rigid rod in front_ and _flexible feathers_ behind. I have thought fit to reproduce Borelli’s figure both because of its great antiquity, and because it is eminently illustrative of his text.[108]
[108] De Motu Animalium, Lugduni Batavorum apud Petrum Vander. Anno MDCLXXXV. Tab. XIII. figure 2. (New edition.)
[Illustration: FIG. 113.--Borelli’s Artificial Bird.]
The wings (_b c f_, _o e a_), are represented as striking vertically downwards (_g h_). They remarkably accord with those described by Straus-Durckheim, Girard, and quite recently by Professor Marey.[109]
[109] Revue des Cours Scientifiques de la France et de l’Etranger. Mars 1869.
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_.”