19 
I will now resume the investigation of the more intricate features 
•of this motion, in view of the various figures constituting this diagram, 
but with reference more particularly to that of No. 2. 
Among men of science the chief objection urged against my theory 
of flight is, that the power of gravitation cannot be made available in 
imparting horizontal moiion to a body in the air ; and believing, as 
I do, that the whole process of flight rests upon this principle, and that 
the horizontal progression of birds on the wing is due to its influence 
alone, I will attempt to remove this objection, together with all other 
doubts relative to its truth, by a more elaborate exposition of the 
principles and the various interesting incidents connected with the 
production of this admirable motion. 
The matters now claiming our particular attention are, first, the 
structure of the wing ; second, the alterations occuring in the super- 
ficial condition of those organs by their action in the air, and the 
eifects thereby produced ; and third, the modus operandi of gravitation 
and acceleration in producing the horizontal motion of birds on the 
wing : and which points will be discussed in the order here stated. 
It is well known to both the learned and unlearned, that the wings 
of birds are concave on one side and convex on the other ; and it is 
equally certain that the extreme ends of the quill-feathers are quite 
elastic. And the points to be more closely examined, are the peculiar 
condition of the upper and under surfaces of those organs ; the na- 
ture and the extent of the changes which they undergo by their 
action, and the direct and incidental efi'ects thereby produced. 
It scarcely needs to be mentioned, that there is a marked variation 
in the general outlines and departures from plane surfaces on both 
sides of the wings, from the points where they unite with the body to 
their extreme tips. And on repeated examinations of them I have 
invariably found, that those portions of the feathers of which they are 
composed which lie in close proximity to the body are but slightly 
curved downward, so that, in the case of a wing six inches in width 
at that point, the usual extent of this flexure is only about one inch. 
But the curvature in the succeeding quills gradually increases through- 
out the series, until the depression at the extremity of a wing eighteen 
inches in length, extends some four or five inches below the top sur- 
face at the opposite end near the body. And such is the unvarying 
