“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_). Third, That if the air strikes the under surface of the wing perpendicularly in a direction from below upwards, the flexible portion of the wing will yield in an upward direction, and form a wedge with its neighbour. Fourth, Similarly and conversely, if the wing strikes the air perpendicularly from above, the posterior and flexible portion of the wing will yield and be forced in an upward direction. Fifth, That this _upward yielding_ of the posterior or flexible margin of the wing results in and necessitates _a horizontal transference_ of the body of the bird. Sixth, That to sustain a bird in the air the wings must strike _vertically downwards_, as this is the direction in which a heavy body, if left to itself, would fall. Seventh, That to propel the bird in a horizontal direction, the wings must descend in a perpendicular direction, and the posterior or flexible portions of the wings _yield in an upward direction_, and in such a manner as virtually to communicate _an oblique action_ to them. Eighth, That the feathers of the wing are _bent in an upward direction_ when the wing _descends_, the upward bending of the elastic feathers contributing to the horizontal travel of the body of the bird. I have been careful to expound Borelli’s views for several reasons:-- _1st_, Because the purely mechanical theory of the wing’s action is clearly to be traced to him.