SAFETY DEVICES IN BIRDS ' WINGS GRAHAM 279 



broad and flexible, and only suited for working at an angle to the air 

 stream with the shield and support of another feather in front of it. 



This indicates that the outer part of number two's front web is a 

 " cutting edge," and that it serves a similar purpose to the whole of 

 the front web of number one; which, in fact, it does, for when the 

 slot is open it is left isolated through the bending up of number one, 

 to face the air stream on its own. (Fig. 10.) Number two 

 feather, itself, does not get bent or twisted, because its rear web is 

 supported above and behind by the front web of number three, there 

 being no slot between these two. And the same thing stands for all 

 the other feathers in the wing ; they give each other mutual support, 

 which prevents any part of them from being twisted round by the 

 force of the air stream. 



The teal's cutting edge is typical of the cutting edge of all birds, 

 though there is considerable variation in proportionate width in dif- 

 ferent species. There are other interesting variations, too : In most 

 game-birds and duck, for instance, the cutting edges appear to have 



Px^^f^ 



Figure 10. — Duck making a down-beat. The separated tip of the first 

 flight-feather has bent upwards and forwards. (Prom a chronophoto- 

 graph in Marey's Movement. Owing to the peculiar form of photogra- 

 phy, the series must be read from right to left.) 



a biconvex section, such as that used in the modern high-speed aero- 

 plane wing, and are about twice as thick in section as the rear webs 

 directly behind them. Wliere they are so thickened the under-sur- 

 1 aces of the feathers have an unmistakable silvery appearance. Then 

 there is the wing of the short-eared owl {Aslo flanwieus), in which 

 the one short piece of cutting edge is easily distinguished from all the 

 other leading edges by its comb-like appearance; no doubt this is 

 something to do with the general " muffling " of the tj'^pical owl's 

 wing. In the wing of a griffon vulture {Gyps fulvus) the cutting 

 edges are much more curved down than those of many other birds. 

 This, one suspects, may have something to do with the high lift 

 value required by that bird when soaring at low air-speeds. In fact, 

 cutting edges make a very interesting study in themselves alone, and 

 I have mentioned only a few of their peculiarities. 



When the single slot in the teal's wing is open, and the isolated 

 tip of the first feather has been bent and twisted by the air stream, a 

 section taken through the wing at the midpoint of the slot would 

 look something like Figure 11. The resemblance of this to a slotted 

 airplane wing (fig. 12) is quite evident. 



