7 
atmosphere at the rate of one hundred miles per hour. And so long 
as the wings are exerted with sufficient celerity to sustain the animal 
on the line of motion, he remains in this situation without any change 
in the degree of his uncontrollable velocity. 
So much for the present, in support of the assumption contained in 
my first proposition — " That the influence of gravitation is the primary 
and efficient cause of the progression of birds on the wing." And in 
regard to the precise manner in which the wings are exerted in flight 
proper, and the true effect thereby produced, as stated in my second 
postulate, I remark, in the first place, that the full realization of the 
truth of what has just been said is sufficient in itself, it should seem, 
to satisfy every intelligent mind, that nothing more than the simple 
vertical action of those organs is required in this process. But not 
so ] on the contrary, naturalists and philosophers generally, contend 
that volant animals propel themselves through the atmosphere by an 
oblique movement of the wings, in a similar manner to that of the fish 
in forcing his body through the water by the exertion of the tail-fin. 
And which, in turn, I deny, and hold that there is no such similarity 
in either the principles, or the manner of effecting these two motions . 
This position may be readily sustained by a brief reference to the 
circumstances in which they respectively occur, and a simple contrast 
of the manner of exerting the wings in the one case, and the fin in 
the other. 
It will be perceived from a single glance at this matter, that the 
condition of things, in all the essential particulars, as connected with 
the locomotion of these two animals in their appropriate elements, so 
called, is widely different. 
And in consideration of the single fact, that the specific gravity 
of water is about eight hundred times greater than that of the atmos- 
phere, it becomes evident at once that the principles of motion in 
these two fluids are, or ought to be, entirely distinctive. The density 
of the water of itself, sustains the weight of the fish without any effort 
on his part; while, on the other hand, the atmosphere will not 
support the smallest feather detached from the body of the bird. 
And hence, in traversing the water, whether beneath or upon its 
surface, propulsion is the principle of motion, and which, as exhibited 
in the swimming of the fish, is effected by means of the tail-fin exerted 
against the surrounding fluid, first on one side, and then on the other. 
