PKOGKESSION IN OR THKOUGH THE AIR. 



165 



organ, the jesiliency of the air, and the shortening and 

 elongating of the elastic ligaments and muscles all co-operating 

 and reciprocating in such a manner that the descent of the 

 wing elevates the body; the descent of the body, aided by the 

 reaction of the air and the shortening of the elastic ligaments 

 and muscles, elevating the wing. The wing during the up 

 stroke arches above the body after the manner of a parachute, 

 and prevents the body from falling. The sympathy which 

 exists between the parts of a flying animal and the air on 

 which it depends for support and progress is consequently of 

 the most intimate character. 



The up stroke (5, D of figs. 84 and 85, p. 160), as will 

 be seen from the foregoing account, is a compound movement 

 due in some measure to recoil or resistance on the part of the 

 air ; to the shortening of the muscles, elastic ligaments, and 

 other vital structures ; to the elasticity of the wing ; and to 

 the falling of the body in a downward and forward direction. 

 The wing may be regarded as rotating during the down 

 stroke upon 1 of figs. 84 and 85, p. 160, which may be taken 

 to represent the long and short axes of the wing; and during 

 the up stroke upon 2, which may be taken to represent the 

 yielding fulcrum furnished by the air. A second pulsation is 

 indicated by the numbers 3 and 4 of the same figures (84, 85). 



The Wing acts u^on yielding Fidcra. — The chief peculiarity 

 of the wing, as has been stated, consists in its being a twisted 

 flexible lever specially constructed to act upon yielding 

 fulcra (the air). The points of contact of the wing with the 

 air are represented at abcdefghijkl respectively of 

 figs. 84 and 85, p. 160; and the imaginary points of rotation 

 of the wing upon its long and short axes at 1,2, 3, and 4 of 

 the same figures. The assumed points of rotation advance from 

 1 to 3 and from 2 to 4 (vide arrows marked r and 5, fig. 85); 

 these constituting the steps or pulsations of the wing. The 

 actual points of rotation correspond to the little loops abed 

 f g h ij I of fig. 85. The wing descends at A and (7, and 

 ascends at B and D, 



The Wing acts as a true Kite both during the Down and Up 

 Strokes. — If, as I have endeavoured to explain, the wing, even 

 when elevated and depressed in a strictly vertical direction, 

 inevitably and invariably darts forward, it follows as a con- 



