222 



AERONAUTICS. 



lent to saying that the wings carry the body of the bird to 

 which they are attached in a horizontal direction^ 



Borelli restates the same argument in different words, as 

 follows : — 



" If," he says, " the air under the wings be struck by the 

 flexible portions of the wings (flabella, literally fly-flaps or 

 small fans) with a motion perpendicular to the horizon, the 

 sails (vela) and flexible portions of the wings (flabella) will 

 yield in an upward direction, and form a wedge, the point of 

 which is directed towards the tail. Whether^ therefore, the 

 air strikes the wings from below, or the wings strike the air 

 from above, the result is the same — the posterior or flexible 

 margins of the wings yield in an upward direction, and in 

 so doing urge the bird in a horizontal direction^ 



In his 197th proposition, Borelli follows up and amplifies 

 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 accom- 

 plished 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 per- 

 pendicular 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 man- 

 ner 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 im- 

 pelled 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, etc., never vibrate their wings 



