AERONAUTICS. 



221 



tion of its wings, the wings during their action forming a 

 wedge, the base of which (c h e) is directed towards the head 

 of the bird; the apex (af) being directed towards the tail. 

 This idea is worked out in propositions 195 and 196 of the 

 first part of Borelli's book. In proposition 195 he explains 

 how, if a wedge be driven into a body, the wedge will tend 

 to separate that body into two portions ; but that if the two 

 portions of the body be permitted to react upon the wedge, 

 they will communicate oblique impulses to the sides of the 

 wedge, and expel it, base first, in a straight line. 



Following up the analogy, Borelli endeavours to show in 

 his 196th proposition, " that if the air acts obliquely upon 

 the wings, or the wings obliquely upon the air (which is, of 

 course, a wedge action), the result will be a horizontal trans- 

 ferenee of the body of the bird^ In the proposition referred to 

 (196) Borelli states — " If the expanded wings of a bird sus- 

 pended in the air shall strike the undisturbed air beneath it 

 with a motion perpendicular to the horizon, the bird will fly 

 with a transverse motion in a plane parallel with the horizon." 

 In other words, if the wings strike vertically downwards, the 

 bird will fly horizontally forwards. He bases his argument 

 upon the belief that the anterior margins of the wings are 

 rigid and unyielding, whereas the posterior and after parts of 

 the wings are more or less flexible, and readily give way under 

 pressure. " If," he adds, " the wings of the bird be expanded, 

 and the under surfaces of the wings be struck by the air 

 ascending perpendicularly to the horizon, with such a force 

 as shall prevent the bird gliding downwards {i.e, with a 

 tendency to glide downwards) from falling, it will be urged 

 in a horizontal direction. This follows because the two 

 osseous rods (virgse) forming the anterior margins of the 

 wings resist the upward pressure of the air, and so retain 

 their original form (literally extent or expansion), whereas 

 the flexible after-parts of the wings (posterior margins) are 

 pushed up and approximated to form a cone, the apex of 

 which {vide af oi fig. 113) is directed towards the tail of the 

 bird. In virtue of the air playing upon and compressing the 

 sides of the wedge formed by the wings, the wedge is driven 

 forwards in the direction of its base {c b e), which is equiva- 



