SOME COMMON BRITISH BIRDS 85 



'ew as sixteen, the largest as many as fifty of these " quills." But 

 there is a great constancy about the number of the quills borne 

 on the hand. These are known as the " primaries " or " primary 

 quills/' and never exceed twelve in number, or fall below ten. 

 So that the range in this difference of thirty-four is due to the 

 variations presented by the quills of the fore-arm, which are known 

 as the " secondary "quills, or " secondaries." When the fore-arm 

 is short there are few of these ; as the length of this support 

 increases, so the number of the secondaries increases. Next it 

 will be noticed that while the secondary quills are peculiar in that 

 the " vane " is of equal extent on both sides of the shaft in the 

 primaries, the outer vane is much narrower. Finally, in the 

 outermost primaries the inner and outer webs are often deeply 

 hollowed so as to give the wing, when fully extended, a 

 curiously fringed appearance, as in the crow when flying, for 

 example. 



These long feathers, when the wing is extended, form, on either 

 side of the body, a broad flat sheet comparable to that of the bat's 

 wing (p. 44), but with this very important difference. The 

 bat's wing is formed by a delicate sheet of skin stretched between 

 long and very delicate fingers; and any extensive injury to such 

 a wing would be irreparable, rendering its possessor helpless. The 

 bird's wing, on the other hand, being made up of a series of separate 

 parts, is easily repaired should any one of these parts become 

 injured in any way. 



But more than this, the bird's wing is superior to that of the 

 bat in this : that the resistance offered to the air in flight is less, 

 and for this reason. It will be remembered that the quill feathers 

 of the wing have a peculiar overlap, whereby the free edges of every 

 quill, when the wing is examined from its upper surface, overlap 

 the inner edge of the feather next outside it. Now, when the wing 

 is raised for the upstroke the air is driven through it ; that is to say, 

 it escapes by the pressure exerted on the inner webs of the feathers. 

 But on the down-stroke the resistance of the air to the under 

 surface of the wing forces the free edges of the quills upwards, and 

 so presses them against the outer edge of the feather to its inner 

 side, thereby offering a perfectly unyielding surface to the air. By 

 this arrangement the resistance to the air on the up-stroke is 





