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.