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.
2d, Because his doctrines have remained unquestioned for nearly two centuries, and have been adopted by all the writers since his time, without, I regret to say in the majority of cases, any acknowledgment whatever.
3d, Because his views have been revived by the modern French school; and