A FEATHER. 83 



gap, and much of the effectiveness of the feather would be 

 lost. If a bird is to fly well it must have firmly webbed 

 feathers, and all flying birds have them. If the ostrich had 

 wings as large as thunderclouds, he could not fly unless his 

 airy plumes were replaced by good quills fit to beat down 

 the air under them. 



We observe that all the long quills overlap each other like 

 the shingles of a roof, and that the unevenly webbed primaries 

 lie with their narrow edge uppermost, and their wide web 

 caught under the quill next nearer to the body. This greatly 

 aids in making the wing air-tight; for, on the downward 

 stroke, the wide web is pressed so firmly against the strong 

 quill and stiff outer web of the next feather that the air cannot 

 pass through. 



But on the up stroke there is nothing to hold the weak 

 web, which is borne down by the air, and thus the pressure 

 on the wing is relieved. While this would happen anyway, 

 it is such a help to the bird in flying that a special appa- 

 ratus is provided along the back of the forearm for turning 

 the secondaries on edge to let the air pass through on the 

 upward stroke. By these arrangements the bird is able to 

 press down a large quantity of air with every wing-beat, but 

 is not required to lift an almost equally large amount when 

 the wing rises. The inability to do this is what makes the 

 bat so much less swift and capable upon the wing, although 

 in comparison with the weight of his body his wing area is 

 very much greater than the bird's. 



