211 



DR. PETTIGREW ON THE MECHANISM OF FLIGHT. 



posterior portions to assume opposite curves. The longitudinal and lateral pulsations 

 occasioned by the wing in action may be fitly represented by spiral wave-tracks running 



The lateral wave-track (PL XY. figs. 73 & 75 b a c) is 



at riulit angles to each other. 



the more distinct. It is usually correctly portrayed by artists in their landscapes and 

 la-pieces, and is ascribed by His Grace the Duke of Argyll * to the bending upwards 

 of the feathers during the descent of the wing. I am, however, disposed to attribute 

 it not so much to the bending of the feathers, which would imply weakness or imper- 

 fection in the materials employed, as to the general arrangement of the feathers, to the 

 peculiar movements of the articular surfaces of the bones of the wing, and to the confor- 

 mation of the wing as a whole. 



The Ci^ves formed by the Wing in motion not due to the upbending of the Feathers. 



That the double curve presented by the wing in action is not referable to adventitious 

 pressure and yielding, but to the peculiar structure of the pinion, is rendered extremely 

 probable by the fact that it is found in the wing of the insect (PI. XV. fig. 68 cb) and 

 bat (PL XV. fig. 69 c 5), where no feathers are present, and in the wing of the bird when 

 it is artificially dried in the extended position (PL XV. fig. 62 gab), and where conse- 

 quently neither pressure nor yielding could occur. The wing, moreover, in the dried 

 state (when the joints are immobile), is so rigid that, strike the air with it as you will, 

 it is impossible to detect any upbending of either the primary or secondary feathers. 



Perhaps the most conclusive argument that can be adduced against the theory of the up- 

 ward bending of the feathers while the wing is descending, is to be found in the fact (vide 

 experiments pp.219, 220, and 221) that the posterior margin of the pinion, both in the in- 

 sect and bird may be completely removed without impairing the function of flight. This 

 circumstance would certainly lead us to infer that the wing derives its power from inherent 

 or structural, rather than accidental properties— the curves and the fundamental form of 



the wing being in no wise impaired by the mutilation. 



In corroboration of the opinion 



that no upbending of the feathers occurs during the effective or down stroke of the wing, 



it may be stated that the pressure which would produce it, does not naturally fall on 

 the posterior 



leathers, but upon the anterior or thick 



margin of the pinion, formed by the 



primary, secondary, and tertiary 



margin composed of the integument, 



alar 



ligament, bones, muscles, and the roots or stronger portions of the feathers in question. 

 When, moreover, the wing is driven with great velocity, and the speed attained by its 



rapid oscillation, by gliding or otherwise, is hi 

 more and more on 



o> 



the pressure, curiously enough, falls 

 the anterior or front edge of the icing , where it is strongest, and 

 where consequently it is best able to withstand the strain to which it is subjected. The 

 law regulating the accumulation of the pressure on the anterior or front edge of the wing 

 it high velocities has been worked out by Mr. P. H. Wenham f in a very able and inge- 

 nious paper, and is more fully explained in a footnote to p. 258. The bending upwards of 

 the wing towards its tip is usually observed when the bird is flying from or towards the 

 spectator, and may be referred either to the posterior distal curve pointing downwards 



* c 



The Reign of Law,' p. 143. London: 1867. 



\£££ZZ^^z??Jr>™- ■—* *-* * « — ■*■ *— 



Report 



Society of Great Britain, 1867. 



