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
_4th_, Because, in commenting upon and differing from Borelli, I will necessarily comment upon and differ from all his successors.
_As to the Direction of the Stroke, yielding of the Wing, etc._--The Duke of Argyll[111] agrees with Borelli in believing that the wing invariably strikes _perpendicularly downwards_. His words are--“Except for the purpose of arresting their flight birds can never strike except _directly downwards_; that is, against the opposing force of gravity.” Professor Owen in his Comparative Anatomy, Mr. Macgillivray in his British Birds, Mr. Bishop in his article “Motion” in the Cyclopedia of Anatomy and Physiology, and M. Liais “On the Flight of Birds and Insects” in the Annals of Natural History, all assert that the stroke is delivered _downwards_ and more or less _backwards_.
To obtain an _upward_ recoil, one would naturally suppose all that is required is a _downward_ stroke, and to obtain an _upward and forward_ recoil, one would naturally conclude a _downward and backward_ stroke alone is requisite. Such, however, is not the case.