[Illustration: FIG. 31. Borelli's bird with artificial wings.
r e, Anterior margin of the right wing, consisting of a rigid rod.
o a, Posterior margin of the right wing, consisting of flexible feathers.
f, Posterior margins of the left wing same as the right.
r g, d h, Vertical direction of the down stroke of the wing.]
The same argument is restated in different words as under:--"If 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_."
There are three points in Borelli's argument to which it is necessary to draw attention: (1) the direction of the down stroke: it is stated to be _vertically downwards_; (2) the construction of the anterior margin of the wing: it is stated to consist of _a rigid rod_; (3) the function delegated to the posterior margin of the wing: it is said _to yield in an upward direction_ during the down stroke.
With regard to the first point. It is incorrect to say the wing strikes vertically downwards, for, as already explained, the body of a flying bird is a body in motion; but as a body in motion tends to fall downwards and forwards, the wing must strike downwards and forwards in order effectually to prevent its fall. Moreover, in point of fact, all natural wings, and all artificial wings constructed on the natural type, invariably strike downwards and forwards.
With regard to the second point, viz. the supposed rigidity of the anterior margin of the wing, it is only necessary to examine the anterior margins of natural wings to be convinced that they are in every case flexible and elastic. Similar remarks apply to properly constructed artificial wings. If the anterior margins of natural and artificial wings were rigid, it would be impossible to make them vibrate smoothly and continuously. This is a matter of experiment. If a rigid rod, or a wing with a rigid anterior margin, be made to vibrate, the vibration is characterized by an unequal jerky motion, at the end of the down and up strokes, which contrasts strangely with the smooth, steady fanning movement peculiar to natural wings.