408 DR PETTIGR.EW ON THE PHYSIOLOGY OF WINGS. 



In the helecopteric models made by M. M. Nadar, Pontin d'Amecourt, and 

 de la Landelle, the screws (m n o pq r s t, figure 53, p. 407) are arranged in 

 tiers, i.e., the one screw is placed above the other. In this respect they resemble 

 the aero-planes recommended by Mr Wenham, and tested by Mr Stringfellow, 

 (compare m n o p q r s t of fig. 53. p. 407, with a b c of fig. 52, p. 404). The 

 superimposed screws, as already explained, were first figured and described 

 by Sir George Cayley. The French screws, and that employed by Mr 

 Phillips, are rigid or unyielding, and strike the air at a given angle, and herein, 

 I believe, consist their principal defect. This arrangement results in a ruinous 

 expenditure of power, and is accompanied by a great amount of slip. The 

 aerial screw, and the machine to be elevated by it, can be set in motion without 

 a preliminary run, and in this respect it has the advantage over the machine 

 supported by sustaining planes. It has, in fact, a certain amount of inherent 

 motion, its sustaining surfaces being active or moving surfaces. It is accordingly 

 more independent than the machine designed by Henson, Stringfellow, and 

 Wenham. 



I may observe with regard to the system of rigid inclined planes wedged 

 forward at a given angle in a line or in a circle, that it does not embody the 

 principle carried out in nature. 



The wing of a flying creature, as I have taken pains to show, is not rigid; 

 neither does it always attack the air at one angle. On the contrary, it is capable 

 of moving in all its parts, and attacks the air at an infinite variety of angles. 

 Above all, the surface exposed by a natural wing, when compared with the 

 great weight it is capable of elevating, is remarkably small. This is accounted 

 for by the length and the great range of motion of natural wings, the latter 

 enabling the wings to convert large tracts of air into supporting areas. It is 

 also accounted for by the multiplicity of the movements of natural wings, these 

 enabling the pinions to create and rise upon currents of their own forming, and 

 to select and utilise existing currents. 



If any one watches an insect, a bat, or a bird when dressing its wings, he 

 will observe that it can incline the under surface of the wing at a great variety 

 of angles to the horizon. This it does by causing the wing to rotate along its 

 anterior or thick margin, or by twisting the posterior or thin yielding margin 

 around the anterior or thick margin. As a result of this movement, the two 

 margins are forced into double and opposite curves, and the wing converted 

 into a plastic helix or screiv. He will further observe that the bat and bird, and 

 some insects, have, in addition, the power of folding and drawing the wing 

 towards the body during the up stroke, and of pushing it away from the body 

 and extending it during the down stroke, so as alternately to diminish and 

 increase its area, arrangements necessary to decrease the amount of resistance 

 experienced by the wing during its ascent, and increase it during its descent. It 



