DR PETTIGREW ON THE PHYSIOLOGY OF WINGS. 411 



111 his 197tli proposition, Borelli follows up and amj)lifies the arguments 

 contained in propositions 195 and 196. Thus, he observes, "It is evident that 

 the object of flight is to impel birds upwards, and keep them suspended in the 

 air, and also to enable them to wheel round in a plane parallel to the horizon. 

 The first (or upward flight) could not be accomplished unless the bird were 

 impelled upwards by frequent leaps or vibrations of the wings, and its descent 

 prevented. And because the downward tendency of heavy bodies is perpen- 

 dicular to the horizon, the vibration of the plain surfaces of the wings must be 

 made by striking the air beneath them in a direction perpendicular to the 

 horizon, and in this manner nature produces the suspension of birds in the air. 



With regard to the second or transverse motion of birds (i.e., horizontal 

 flight) some authors have strangely blundered ; for they hold that it is like that 

 of boats, which, being impelled by oars, moved horizontally in the direction of 

 the stern, and pressing on the resisting water behind, leaps with a contrary 

 motion, and so are carried forward. In the same manner, say they, the wings 

 vibrate towards the tail with a horizontal motion, and likewise strike against 

 the undisturbed air, by the resistance of which they are moved forward by a 

 reflex motion. But this is contrary to the evidence of our sight as well as to 

 reason ; for we see that the larger kinds of birds, such as swans, geese, &c, 

 never vibrate their wings, when flying, towards the tail with a horizontal 

 motion like that of oars, but always bend them downwards, and so describe 

 circles raised perpendicularly to the horizon." 



Besides, in boats the horizontal motion of the oars is easily made, and a 

 perpendicular stroke on the water would be perfectly useless, inasmuch as their 

 descent would be impeded by the density of the water. But in birds such a 

 horizontal motion (which indeed would rather hinder flight) would be absurd, 

 since it would cause the ponderous bird to fall headlong to the earth ; whereas 

 it can only be suspended in the air by constant vibration of the wings perpen- 

 dicular to the horizon. Nature was thus forced to show her marvellous skill 

 in producing a motion which, by one and the same action, should suspend the 

 bird in the air, and carry it forward in a horizontal direction. This is effected 

 by striking the air below perpendicularly to the horizon, but with oblique 

 strokes — an action which is rendered possible only by the flexibility of the 

 feathers, for the fans of the wings in the act of striking acquire the form of a 

 wedge, by the forcing out of which, the bird is necessarily moved forwards in a 

 horizontal direction." 



The points which Borelli endeavours to establish are these : — 



First, That the action of the wing is a wedge action. 



* It is clear from the above that Borelli did not know that the Avings of birds strike forwards 

 as well as downwards during the down stroke. He seems to have been equally ignorant of the fact 

 that the wings of insects vibrate in a more or less horizontal direction. 



VOL. XXVI. PART II. R o 



