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_.”