27 
from the nature of his position in the air, and which arises chiefly 
from the obstruction which the spread wings present to his perpen- 
dicular descent together with his velocity, both of which conspire to 
■disturb the vertical action of gravity upon the body for the time being; 
and that this inclination towards the earth does, or ought to, accel- 
erate the motion of the animal^ it seems to me, is equally certain. And 
to redeseribe this process in the most concise form of words of which 
I am capable, it occurs thus : The motion of the bird on a horizontal 
line in a case, for instance, where he starts from an elevated position, 
such as a tree or mountain top, originates primarily in the constant 
power of gravitation, the perpendicular action of which is deranged 
by the intervention of the spread wings and the velocity of the body, 
so far as to cause the weight to gravitate in a slanting forward direc- 
tion, as is constantly exhibited in the open air by the descending 
motion of those animals, whether under the control of the action or 
nonaction of those organs ; and in this peculiar condition of things, 
there incidentally arises at first a slight acceleration of the body and 
which continues to increase until the latter attains a certain degree 
of velocity, beyond which, from the resistance of the air in front, it 
cannot go. The precise extent to which this influence enters into the 
process of flight, would be very difficult to determine ; but in my 
judgment it is the cause of at least one half of the velocity attained 
by birds moving on a horizontal line. And this view of the matter 
may be considerably supported, if not rendered quite certain, by a 
brief consideration of the direct part which gravitation plays in 
effecting that result. 
Considering it as certain that gravity constantly tends to reduce 
the bird on the wing below the horizontal line of motion ; and re- 
garding it as true that the combined interposition of the spread wings 
and the velocity of the body prevent the perpendicular descent of the 
latter ; then, it follows that the animal must necessarily incline to- 
wards the earth on a line ranging a little below the plane of his ele- 
vation, or, in other words, he constantly tends to fall in the direction 
of a point upon the earth considerably in advance of his position in 
the air, and which point changes, toties qmties,mth. the motion of the 
body. In this manner only, and to this extent, whatever it may be, 
the force of gravity is directly concerned in producing the flight of 
birds ; and hence it becomes more than probable, I think, that under 
