8 
and thus forcing the body through and overcoming the resistance of 
the medium in which it floats. But not so in the atmosphere. Here, 
from the great rareness of the medium, it is absolutely necessary to 
impart to the wings in flight vertical motion, the chief elfect of which 
is simply to raise and sustain the vsreight of the animal in opposition 
to the constant influence of gravity, ever tending to reduce him back 
to the earth, as already partially explained. It follows from this, 
that the fish exerts the tail-fin in a horizontal direction, and thereby 
propels himself through the water, there being no necessity for a sus- 
taining power in the process of swimming ; and that the bird imparts 
to his wings vertical movements, and thereby raises and sustains his 
weight in the air, there being no necessity for a propelling force in 
the process of flight. And thus, the supposed identity of these two 
cases vanishes like the shadow of a flying cloud. 
Every just consideration of this subject goes to show, that the bird 
ought not to propel himself in flight ; but the leading and most pow- 
erful one, is, that there is no necessity for such an efi"ort in producing 
this motion, and if required, it is equally certain, that it would be im- 
possible to make it under the circumstances of the case, as I will now 
endeavor to show. 
It is generally supposed that there is a double effect produced by 
the action of the wings, the one sustaining and the other propelling 
the animal, under the popular idea that the laws of mechanics require 
the latter influence in this, as in all other cases of transient motion. 
But this is a great mistake ; for if the back extremities of those 
organs were either intentionally, or from the pressure of the air be- 
neath in making the downward strokes, incidentally raised, (and this 
must take place to produce the effect claimed,) they would retard in- 
stead of aid in effecting this motion ; because, the slightest elevation 
of those portions of their surfaces above the level of the front out- 
lines, from the force of the air through which they are carried with 
great velocity by the body, thus brought in contact with them, would 
result in the double injury of constantly depressing the animal below, 
and hence seriously impede his progress on the line of motion. And 
were it possible in this process to elevate those portions of the wings, 
as required in the supposed oblique movement of them, without pro- 
ducing these injurious consequences, no sort of advantage would re- 
sult from it, as will clearly appear from the following statements. 
