286 ANNUAL REPORT SMITHSONIAN INSTITUTION, 193 2 



sectional view of two feathers in this position. The narrow parts of 

 the feathers outside the steps in the webs are, of course, in the fully 

 separated position at this final stage of the spreading of the wing. 

 The arrow marked Z shows where the friction is greatest. 



This friction business does not apply only to slotted wings, for 

 ordinary ones, such as those of the swallow {Hlrundo rust'ica), 

 woodcock {Scolopax imstlcola) and gull, disptlay varying degrees of 

 this locking tendency at the moment when the tips of their feathers 

 are about to separate. The secondary feathers, and also the unslot- 

 ted primary feathers of birds that have slotted wings, are subject to 

 it as well. 



In most wings the engagement of the friction areas is made the 

 more certain by the upward curl of the rear margins of the feathers, 

 which assists the air pressure to bring the two surfaces into contact. 



One other interesting point about the device is that the front edge 

 of the front web of an emarginated feather is always sharply curved 

 down in the unemarginated part, but in nearly all such feathers (the 

 vulture and perhaps some other soaring birds are exceptions, as was 

 mentioned before) it is to all intents and purposes flat from the step 

 outwards to the tip, that is to say, along the " cutting edge." This 

 peculiarity is quite helpful when one is trying to measure the total 

 length of cutting edge that any wing possesses. The reason for this 

 difference is that inside the step, the front web has to do the work of 

 digging down into the friction area of the feather in front of it, and 

 working as a limit stop to prevent overspreading; but, outside the 

 step, its purpose is simply to cut the air. 



In a table to be given at the end of this paper will be found the 

 proportion of cutting edge to length of wing in a number of repre- 

 sentative types of birds. It is there called the " slot factor." 



The apparent action of the opening of the wing-tip slots can now 

 be summed up as follows : When the wing has spread so far that the 

 emarginated parts of the feathers are about to separate, the air 

 stream, if it has sufficient incidence, forces the broader rear webs of 

 the separated parts of the feathers upw\ard, so that the blades are 

 twisted toward the line of the air stream. In this manner the inci- 

 dence of these separate feathers becomes less than that of the main 

 wing; and, consequently, the direction of the force reacting on them 

 is more forward. The result is that they all move forward and the 

 slots open wide. At the same time the tips of the feathers bend 

 upward, owing to the absence of mutual support. At a certain 

 moment during this process the individual forward movement of 

 each feather is checked and finally stopped by the arrival of the 

 curved-down leading edges of the still overlapping parts of the 

 feathers at the front margin of the friction areas of the feathers 



