INTRODUCTION. 11 



that the bird appears as if it had four wings, or rather had a 

 double wing on each side. This form of wing answers some 

 important purposes in their economy, which can be more 

 satisfactorily noticed in the particular accounts of those birds 

 in which it is found. 



From the manner in which the several coverts support each 

 other and the quills, the wing is a finer combination of light- 

 ness, strength, elasticity, and stiffness, than could be produced 

 by ? ny other means. The different coverts support each other, 

 and admit of motion upon each other, not unlike that which 

 takes place in a coach spring, which is about the best of our 

 mechanical contrivances for rendering motion smooth and 

 free from jolts, at the same time that the compound spring 

 is much less liable to be broken than if it were formed of a 

 single piece, containing the same quantity of metal. 



9. The rump feathers, and upper tail-coverts. The first of 

 these are a continuation of the covering of the back; and the 

 second support the tail feathers on the upper side, and are 

 strong in proportion as these are adapted for action in flying. 

 In birds which use the tail as the means of support in fixing 

 themselves upon upright surfaces, as is the case in the wood- 

 peckers, the upper tail-coverts are very strong and close, and 

 so thick in their array as to give that organ the same stiffness 

 as a powerful wing. In those birds also which have to take 

 wing from the water, and yet have not the feet webbed and 

 placed far backwards, so as to support them when rising, the 

 tail-coverts are peculiarly strong, in order to support the tail 

 feathers, which are the principal fulcra from which the bird 

 gets into the air. 



10. The vent feathers, and under tail-coverts, which cover 

 the hinder part of the bird ; and, in those birds in which the 

 tail feathers are long and stiff and much used in the act of 

 flying, support the under side of these feathers, in the same 

 manner as the under-coverts support the quills of the wings. 



