430 



DR PETTTGREW ON THE PHYSIOLOGY OF WINGS. 



piston (r r), and the gearing apparatus {y z), it is likewise extended or spread 

 out, the mere elevation of the piston rendering the cord or wire {I, n,) taught — 



Fig. 69.* 



the taughtness of the cord causing the different parts of the wing to fly out- 

 wards, while it at the same time puts the elastic substances ( h k) on the stretch. 



* Fig. 69. Wing which folds upon itself during the up stroke, and expands during the down one, made to vibrate 

 by a direct piston action. At A the wing is fully expanded and in the act of commencing the down stroke. At B the 

 wing is at mid stroke and very slightly folded. At C the wing is fully folded, and ready to begin the up stroke. It is 

 thus that the wing acts as a long lever at the beginning of the down stroke, and a short one at the beginning of the up one. 

 Compare with figs. 18 and 19, Plate XIV., and also with figs. 9, 10, and 11, Plate XII. The lettering of the wing in 

 the present fig. is the same as in fig. 68, p. 429 . 



x Universal joint at root of wing received into cup-shaped cavity (v) of cylinder (t). 



a Proximal, d central, and g distal portions of wing. 



6, e, Joints which unite the three portions of the wing to each other. 



/, q, Points to which the cord or wire of wing is fixed. 



c, Aperture through which cord or wire of wing glides as the wing ascends and descends. When the piston ascends 

 it elevates the wing by its gearing y z. It also renders the cord I n taught, the cord in its turn extending the wing (A) 

 and the elastic substance h 1c. When the piston descends to mid stroke the wing is very slightly folded (B) and the 

 cord I' n' somewhat relaxed. When the piston has quite descended the cord I" n" is very much relaxed, and as a conse- 

 quence the elastic substance extending between the different portions of the wing has contracted, the wing being thereby 

 folded upon itself (C). The elastic substance may be dispensed with, if a strong elastic cord be employed instead of the 

 non-elastic one I, n. If two cords be fixed to two points on the cylinder as at p and q, and the one cord be passed on 

 the upper surface of the wing, and the remaining one on the under surface, the wing will be under control during the 

 whole of the down and up strokes, the one cord extending the wing, the other flexing it. 



t, t, Cylinder, o, o, Stuffing boxes, r r, Piston, iv, w, Cross heads for driving gear, y z Driving gear, s Piston 

 head, v, Cup-shaped cavity for receiving root of wing. 



