232 



AERONAUTICS. 



the wing lias to elevate as well as propel ; the oar of a boat 

 when employed as a scull only propelling. In order to elevate 

 more effectually, the oars formed by the wings are made to 

 oscillate on a level with and under the volant animal rather 

 than above it; the posterior margins of the wings being made 

 to oscillate on a level with and below the anterior margins 

 (pp. 150, 151). 



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 for- 

 wards, 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 ; 6 th, 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 liga- 

 ments; 7th, to the effect produced by the various inclined 

 surfaces formed by the wings during their oscillations ; Sth, 

 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 ex- 

 panded 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 ex- 

 plained (p. 220, fig. 113), consists of a rigid rod (e^r) in 



