DR PETTTGKEW ON THE PHYSIOLOGY OF WINGS. 



379 



by the rowing feathers with the horizon (see straight dotted line) in a direction 

 from within outwards is greater than those made by the same feathers (r), 

 in extension, as represented at figure 47. In figure 46 the wing is folded upon 

 itself at x, and presents two arches, a larger (r) and a smaller (s), the numbers 

 123456789 giving the position of the primary feathers when counted from 

 without inwards, the arrows indicating the direction in which the primary and 

 secondary feathers open up and cut into the air from below upwards and from 

 within outwards during the up stroke. This figure shows that the primary and 

 secondary feathers (particularly the former), when viewed from the tip, or when 

 cut across, present a spiral contour (c g of figure 49). This arises from the 

 primary and secondary feathers being twisted upon themselves, as represented 

 at a b, c d of figure 50. 



.9 b 



Fig. 49. Frimary Feather, showing double curves at anterior margin (c d), posteiior margin (a b), and across (cj). 



Fig. 50. The same, seen from hefore edgeways. Here the anterior (c d) and posterior (a b) margins cross 



each other. 



Figure 47 shows that the primary and secondary feathers of the wing of 

 the piet are thrown into a beautiful groined arch in extension, preparatory 

 to the clown stroke, the advantage in favour of a concave surface over a 

 convex one for seizing air or water being something like 2 to 1. It also 

 shows that the primary (r) and secondary (s) feathers in extension, and during 

 the down stroke, rotate upon their long axes in a direction from below 

 upwards, as indicated by the arrows abed efg h ij klmnop q, so as to form 

 an arch which cannot be destroyed so long as the individual feathers remain 

 intact. In fact, the integral parts of the arch are so disposed that the greater 

 the pressure the greater the strength. Figure 48 shows a similar groined 

 arch formed by the roots of the primary and secondary feathers, the spirals 

 constituting the arch {abed efg hij klmnop q) running in an opposite direc- 

 tion to those seen in figure 47, from the fact of the primary and secondary 

 feathers being twisted upon themselves as already explained (compare b c with 

 a d of figure 50). Fig. 20, Plate XIV. shows how the air is forced during 

 the down stroke in a spiral direction (vide arrows) from without inwards, 



VOL. XXVI. PART II. 5 F 



