21 
of the quills, and which latter takes place at the instant that the 
direction of the former is altered ; whereby they not only reassume 
the natural form but, from the resistance of the air against their top 
surfaces, they are subjected to a still greater degree of curvature, 
resulting in a corresponding diminution of the obstruction encountered 
in raising them, and an equal reduction of the amount of power em- 
ployed in this moTement, compared with what is expended in making 
the downward strokes. 
And I will barely mention here a most singular and interesting 
incident in this process, connected with the upward movement of the 
wings, and resulting from the pressure of the air brought in contact 
with the extended curvature of their under surfaces by the velocity 
of the body just noticed, and which materially aids in sustaining his 
weight. An explanation of the manner in which this occurs, together 
with the probable extent of its influence in flight, will be presently 
given. And in this connection I will also explain certain effects in- 
cidentally arising from the action of the wings and the motion of the 
animal in flight, but which are not included among the matters that 
still remain to be discussed, under the enumeration made in the outset* 
I allude to the manner in which the air is distributed upon the under 
surface of the wings, and its pressure equalized, in view of the great 
difference between the extent of the movement and the far greater 
velocity of their extreme tips, compared with that of the more inward 
and broader portions of them, and arising from their hinge-like con-* 
nection with the body; and which latter circumstance is well calculated 
to lead the mind of the casual observer to believe that there is but 
little of this kind of force distributed over those parts lying inside of 
the points of percussion. But not so ; for it will be perceived that 
the air embraced by the more outward portions of those organs in 
making the downward strokes is thus forced inward towards the sides 
of the body, and being thereby slightly condensed it must therefore 
contribute towards the equalization of the pressure upon the whole 
of the middle portions of their surfaces. And in regard to the dis- 
tribution of this force upon those portions lying still nearer to the 
sides of the animal, it only needs to be stated that the air which is 
displaced by the moving body passes in two currents under the 
wings, right and left, whereby it is likewise partially condensed, and 
thus not only tends to equalize this influence at those points, but, by 
