26 
a given time than that of the organs or apparata by which it is pro- 
pelled. On the contrary, it seems to me that the requirement of 
the second law of motion, just cited, or the nature of propulsion and 
the assurance of the strongest and most rational inference, drawn from 
the highest and most convincing probability, unitedly concur in estab 
lishing the truth that the vertical action of the wings only serves to 
elevate and sustain the animal in the air ; a truth, moreover, which 
rests upon the just consideration of the evident impossibility, from the 
very nature of things, of any other result ensuing from that process. 
And here 1 close my remarks upon the several matters con- 
nected with the action of the wings and the various eflFects thereby 
produced on the body of the animal while in transitu. 
And finally, we now come to reconsider more particularly the man- 
ner in which the horizontal motion of birds on the wing is produced. 
And here again, I repeat my first proposition — " That gravitation is 
the primary and efiicient cause of the progression of birds on the 
wing," and hold that in the process of flight proper there incidentally 
arises from the force of this principle, and combines with it, a certain 
amount of what I have called acceleration — though the term is hardly 
admissible, and Is employed merely for want of a more suitable one, 
but which our language does not afford — and to the united effect of 
these influences the great velocity of those animals is entirely due. 
Now, let us again turn to figure No. 2 of the diagram, and in view 
of it make a further and final analysis of the process by which this 
motion is produced, and thus if possible ascertain its true cause. 
It will be perceived that at the time the bird attains the desired 
point of elevation, he is moving with all the velocity acquired in 
making his ascent, and which motion he never loses so long as he con- 
tinues on that plane in a still atmosphere. And here, after partially 
relaxing the action of his wings which, as seen in the measurably de- 
pressed angles of the serpentine line now takes place, and also ad- 
justing his body in a horizontal position — from both of which circum- 
stances he at once becomes, so to speak, more directly under the influence 
of gravity than had previously existed in ascending to that point — 
the true process of flight proper begins, and with it both the origin 
and influence of acceleration. That the animal whilst thus progress- 
ing on the central line of this figure, constantly tends to descend 
towards a point at a considerable distance in front of him is evident 
