PETTIGREW ON THE MECHANISM OF FLIGHT. 251 



had occasion to state (pages 241, 242, 243, and 244), that the primary, secondary, and 

 tertiary feathers (if allowance he made for a slight decree of elasticity, necessary to 

 prevent shock) do not yield in an upward direction during the downward stroke, a- 

 many believe *. A reference to Diagram IS a, p. 253, and to the figures in Plate ~X 1 V., 

 all of which are accurately drawn, will he found to corroborate this belief, the uplifting 

 referred to being in every instance quite inappreciable. If the tips of the primary, 

 secondary, and tertiary feathers, did yield, which I am not prepared to admit 



p. 244), they would lose much of the levering power which they undoubtedly posses. To 

 bend, and so permit the air to escape, would be tantamount to removing the fulcrum 

 on which the wing acts. As proof that the feathers do not yield, I may stale that I 

 have never observed them give way in the living bird, and that all wings are more or 

 less scooped out on their inferior surface expressly to prevent the escape of the air 

 (PI. XIV. figs. 28, 31, 32 & 38), the difference in favour of a concave surface over a 

 convex one (PL XIV. figs. 35 & 37), when applied to air or water, being as 1 wo to one. 



The only part where the wind can escape, as I have already pointed out (p. 2\~>), is 

 towards the root of the wing, where the feathers are feeble, and there is a bn b 

 naturally (Plate XIV. figs. 28, 30, 35, and 37, c). In order to prevent shock and undue 

 straining of the primary and secondary feathers during extension, the bones of the 

 forearm and hand are made to rotate to a slight extent, as already mentioned. 



Lax condition of the Pinion in flexion. The imbricating and disimbri eating action of the 

 feathers. — The wing, which was rigid or tense during extension, presents an opposite con- 

 dition during flexion (Plate XIV. figs. 33 & 40), the pinion becoming limp, the feathers 

 losing their bony and fibrous supports, and the wind escaping. Here again, however, 

 there is a limit, the elastic ligament of the wing, which is variously constructed, 

 contracting more or less completely in different birds, the amount of contraction (as 

 has been explained) being regulated by the weight to be supported and by the rapidity 

 with which the wing requires to be driven. In the Swallow e. g. (Plate XV. fig. 58) 



bird that slides or skims along for great distances, the wing is only very slightly 



gXXUCO V3. 9AUUO «^~ 



This slat 



flexed by the elastic ligament, whereas in the Partridge and some of the other Gallina 



ceous birds it is semiflexed. In the majority of quick-flying birds, where the wing 



requires to lay hold of the wind and disengage itself suddenly and the birds do not 

 glide or skim, considerable advantage is gained by the primary and secondary feathers 



being thrown out of position during flexion, this arrangement preventing retardation by 

 diminishing the amount of air displaced in the upward or back stroke 

 overlapping) and unslating action of the feathers during extension and flexion, however, 

 is one of the peculiarities or refinements of flight, and not necessarily an essential 

 feature in it-since flight can be efficiently performed by the insect and bat, whei no 



* His Grace the Duke of Argyll, in speaking of the wind compressed in the hollow of the wing, says that, as thi 

 cannot pass through the wing « owing to the closing upwards of the feathers against each other, or escape forwards, 



r . . & . . ... „ , . i a „f the mull s in this direction, it passes backwards, and in so doing hfts 



by 



Mr 



direction it is necessary 



in a horizontal 

 jree of the fr 



yf the quills r— Hist, of 



