If you fall off the double bag that you hold under your seat, see that with it you strike the ground.

n – a – d c – f – b f f – f – g

Because the wings have to row downward and backward in order to hold the instrument up on high and to make it travel forward, the motion of the lever c d is made along an oblique path, guided by the strap a b. I could arrange that the foot which presses the stirrup g should be the one that, beyond its ordinary office, also pulls the lever f downward. But this would not be to our purpose, because we need the lever f to rise or descend before the stirrup g moves from its place, so that the wing, in throwing itself forward and lifting up on high (during the time that the already acquired impetus drives the bird forward of itself without beating of wings), may set the wings edgewise into the air; for, if it did not do so, the face of the wings would strike against the air, would impede the motion, and would not let the impetus carry the bird forward.