11 
Under consideration, it does not bear a greater proportion to the 
weiV/^^ than the numerical relation of one io Jive; and I am sustained 
in this view by the following statement relative to this matter, and 
which, I understand, is regarded as being true by men of science, 
namely, " that the wings of birds of extensive flight are loaded with 
the weight of only one pound to each square foot," or in that proportion. 
And in repeated and careful examinations of the structure of many 
such birds, I have invariably found that the relative weight and area 
of wing conformed strictly to both my own view and the rule just 
cited. In the case of the largest wild duck, the weight is about j^ye 
pounds, and the area of wing one square foot ; and hence it may be 
fairly inferred, that, while traversing the atmosphere on a horizontal 
line, this bird exerts a muscular force equal to one pound only in im- 
parting motion to his organs of flight. In that of the wild goose, 
whose average weight is about ten pounds, with two feet of wing, the 
power expended is equal to two pounds. And in that of the largest 
North American eagle, whose weight is about twenty pounds, with/I/wr 
feet of wing, the strength employed is equal to four pounds. 
And here I close my remarks upon this important element in the 
process ot flight, after having shown, as I think, the truth of my third 
proposition — " that the velocity attained in flight proper is incidental j 
and that it equalizes the amount of power exerted in imparting motion 
to the wings, in proportion to the weight carried, with that employed 
by all other animals in their respective modes of locomotion on the 
earth and in the water," 
Having now passed rapidly over the whole field of flight proper, I 
will conclude the first part of this inquiry, with a restatement of the 
process by which that motion is produced, disconnected with the 
reasons and illustrations adduced in support of the theory. 
In the foregoing description of the manner in which the bird starts 
from the earth and ascends to the proper elevation for making long 
excursions on the wing, it will be remembered that, after having ex- 
plained how he attained that position, he was left in transitu on a 
horizontal line. And now, returning to and considering him as still 
being on the wing, he may be justly regarded as a falling body, con- 
stantly interfered with and diverged from the line of perpendicular 
descent by the vertical action of the wings, resulting in a constrained 
tendency of his body towards the earth in the direction of a point 
at a considerable distance in front of Iiim, and thus indefinitely pro- 
