AERONAUTICS. 



223 



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 per- 

 fectly 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 head- 

 long to the earth ; whereas it can only be suspended in the 

 air by constant vibration of the wings ^erjpendicular 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 efi'ected 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 nioved 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. 



Second, That the wing consists of two portions — a rigid 

 anterior portion, and a non-rigid flexible portion. The rigid 

 portion he represents in his artificial bird (fig. 113, p. 220) as 

 consisting of a rod {e r), the yielding portion of feathers {a o). 



Third, That if the air strikes the under surface of the 

 wing perpendicularly in a direction from below upwards, the 

 flexible portion of the wing will yield in an upward direction, 

 and form a wedge with its neighbour. 



Fourth, Similarly and conversely, if the wing strikes the 



1 It is clear from the above that Borelli did not kftow that the wings of 

 birds s,tYi\<iQ forioarcls as well as downwards during the down stroke, and for- 

 wards as well as upwards during the up stroke. These points, as well as the 

 twisting and untwisting figure-of-8 action of the wing, were first described by 

 the author. Borelli seems to have been equally ignorant of the fact that the 

 wings of insects vibrate in a more oi; less horizontal direction. 



