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ANNUAL EEPORT SMITHSONIAN INSTITUTION, 19 3 2 



are opening, each whole feather, pivoting about its root in the hand 

 of the wing, is dragged forward by the force reacting on iis twisted 

 tip. When the limit of that movement has been reached, the flexible 

 separated tips bend forward, still in obedience to the reaction on the 

 twisted parts of the blades. The bending of separating primaries 



Figure S. — Diagram illustrating the reason for the upward and 

 forward bending of separated feathers 

 A, direction of air stream ; F, direction of movement of the 

 wing ; R, total resultant force on the main wing ; r, total result- 

 ant force on each separated feather-blade. 



adds greatly to the smoothness of the flight of short-winged birds. 

 The reaction in each wing beat is, as it were, cushioned. Instead of 

 commencing and ending sharply, it gradually rises to a maximum, 

 and gradually dies away as the feathers straighten themselves after 

 the termination of the down beat. 



V. THE SINGLE WING-TIP SLOT 



A good example of the simplest development of the slot is found in 

 the wings of some forms of duck, the teal {Anas crecca) for instance. 

 Figure 9 shows the first four flight feathers of a teal's wing. Num- 

 ber one feather's front web is very 

 narrow and very stiff from tip to 

 root; a suitable form for taking 

 the first blow of the air as the 



FiGUEB 9. — Under-surface of right wing 

 tip of teal 



wing cuts through it, and for 



dividing the stream ready for its 

 passage over and under the wing 

 surface that lies in rear of it. The front web of number two feather 

 is of identical construction, but only for a distance of 1% inches (AB 

 in the figure), measured inwards from the tip; that is, as far as the 

 step in the web. Inside that point it resembles the front webs of all 

 the other primary feathers that lie behind it in being comparatively 



