TEATHERED WINGS. 37 



though the flight of bats, unless when they are agi- 

 tated, is comparatively noiseless. So also those 

 reptiles which fly by means of membranous appen- 

 dages, are obliged to flutter these very much in pro- 

 portion to the rate of their progressive motion. 



Now the difference of action in these two textures 

 of wings in the other classes of animals, shows us 

 the advantages which birds derive from their fea- 

 thery covering and feathery organs of flight. These 

 feathers, even to the minutest fibre on the plumes or 

 webs, are tubular, consisting of only a thin film of 

 solid matter, filled with air within, though strength- 

 ened by partitions of cellular substance, more or less 

 close together, according to the strain v^-hich the 

 feathers have to bear. From the mode in which the 

 feathers, and all their parts, are laid upon the bird, 

 it presents a smooth surface upv/ards and forwards, 

 so that the animal can move in either of these direc- 

 tions, with very little resistance from the friction of 

 the air. When it moves in either of them, the resist- 

 ance of friction does not increase so rapidly as the 

 rate of motion ; because the pressure smooths the 

 feathers, and causes the air to take less hold on them. 

 This property, which arises in part from the texture 

 of the upper surface of the feathers, but chiefly from 

 the way in which they are formed and placed, is of 

 equal service to birds when they must perch, or 

 otherwise remain at rest so as to abide the blast, as 

 when they fly exposed to it. Perching or flying, when 

 a bird is in the wind it always faces the current ; and 

 thus off'ers the least resistance both by its form and 

 its feathers. 



When, however, the feathers are taken in the 

 opposite directions, they offer as much increase of 

 resistance as ^they offer diminution when they are 



