20 
condition, to a greater or less extent, of the top surface of the wings 
among all birds of extensive flight 5 and of course there is a corres- 
ponding concavity on the opposite sides, with this difi'erence, however, 
in regard to the strength of the quills, which, owing to their peculiar 
form, yield more readily from pressure against the upper than 
upon the under sides to them. 
These peculiarities of the wings of birds are both novel and instruc- 
tive when wisely considered, there being nothing within the wide range 
of locomotive organs bearing the least resemblance of them. 
And in regard to the nature and extent of the alterations produced 
in the position of the quills, and consequently the form of the wings 
by their action in flight, I remark, in the first place, that, in making 
the downward strokes, from the pressure of the air beneath, the cur- 
vature of the back and outer portions of the latter is entirely removed, 
or, more plainly, the extreme ends of the quills are forced upward to 
a level position with the solid portions of the wings, but never above 
that point. And in this straightened condition they exert a consider- 
able influence on the body of the animal, besides that of sustaining 
it in the air, the direct eff'ect of which is to depress the head below 
the line of motion ; but a feeling of insecurity in the first efforts at 
flight of young birds, and the force of habit among the mature ones, 
compels the former, and enables the latter, alike, to counteract this 
tendency to derangement of the equilibrium, by a general muscular 
exertion whilst on the wing; and the consequence is, that, in the strug- 
gle between these two influences, there is imparted to the body a 
pretty strong tendency to rise obliquely above the line of motion, 
upon an angle as indicated by the dotted line at No. 6. 
The precise manner in which this occurs is more easily conceived 
than expressed, farther than the explanation contained in the statement 
just made goes. For this is one of the most intricate features in the 
process of flight ; and I can only repeat that the eff'ect stated is pro- 
duced by the threefold influence of the downward strokes of wing — 
the force of the straightened quills, in the tendency of each to reas- 
sume the curved form, acting upon the air, and the muscular exertion 
of the animal preventing the depression of his head, which latter dis- 
turbance, otherwise, would inevitably ensue from this cause. 
Secondly, another important temporary change in the area of the 
wings, is produced by the reverse movement of them, and the reaction 
