Borelli, and all who have written since his time, are unanimous in affirming that the horizontal transference of the body of the bird is due to the perpendicular vibration of the wings, and to the yielding of the posterior or flexible margins of the wings in an upward direction as the wings descend. I am, however, as already stated, disposed to attribute the transference, _1st_, to the fact that the wings, both when elevated and depressed, _leap forwards in curves_, those curves uniting to form a continuous waved track; _2d_, to the tendency which the body of the bird has to swing forwards, in a more or less horizontal direction, when once set in motion; _3d_, to the construction of the wings (they are elastic helices or screws, which twist and untwist when they are made to vibrate, and tend to bear upwards and onwards any weight suspended from them); _4th_, to the reaction of the air on the under surfaces of the wings, which always act as kites; _5th_, to the ever-varying power with which the wings are urged, this being greatest at the beginning of the down stroke, and least at the end of the up one; _6th_, to the contraction of the voluntary muscles and elastic ligaments; _7th_, to the effect produced by the various inclined surfaces formed by the wings during their oscillations; _8th_, to the weight of the bird--weight itself, when acting upon inclined planes (wings), becoming a propelling power, and so contributing to horizontal motion. This is proved by the fact that if a sea bird launches itself from a cliff with expanded motionless wings, it sails along for an incredible distance before it reaches the water (fig. 103, p. 186). The authors who have adopted Borelli’s plan of artificial wing, and who have indorsed his mechanical views of the action of the wing most fully, are Chabrier, Straus-Durckheim, Girard, and Marey. Borelli’s artificial wing, as already explained (p. 220, fig. 113), consists of _a rigid rod_ (_e_, _r_) in front, and _a flexible sail_ (_a_, _o_) composed of feathers, behind. It acts upon the air, and the air acts upon it, as occasion demands. _Chabrier’s Views._--Chabrier states that the wing has only one period of activity--that, in fact, if the wing be suddenly lowered by the depressor muscles, it is elevated solely by the reaction of the air. There is one unanswerable objection to this theory--the bats and birds, and some, if not all the insects, have distinct elevator muscles. The presence of well-developed elevator muscles implies an elevating function, and, besides, we know that the insect, bat, and bird can elevate their wings when they are not flying, and when, consequently, no reaction of the air is induced.