22 
its lateral pressure, also affords considerable aid in maintaining the 
equilibrium of the animal in flight. 
This brings us to the consideration of the process of flight on a 
horizontal line, the extremely intricate nature of which I have sought 
to illustrate to some extent by means of that portion of the diagram 
indicated at No. 2. 
It will be observed that the central line of this figure is subdivided 
into sections by a succession of doited and drawn lines of equal length, 
and which, together with the serpentine line, are designed to show 
the precise relation subsisting between the impulses of the wings and. 
the motion of the body at every instant of time during the progress 
of the bird through the atmosphere. 
Now, let it be supposed that a bird of the form and weight of the 
canvas-back duck, having one foot area of wiag exerted at the rate 
of three strokes per second of time, after raising himself to the 
desired elevation upon the ascending line, as seen at No. 1, and ad- 
justing his body in a horizontal position, be moving with the u&ual 
velocity of that animal on the wing of one hundred miles per hour 
upon the central line of this figure, from left to right. It will be per- 
ceived that he traverses the first dotted section, which is designed to 
represent the space of twenty-five feet, whilst raising the wings, and 
thence passing on through the length of the succeeding drawn section 
while making the downward stroke, he thus overcomes the space of 
fifty feet during the time of one revolution of those organs. And 
commencing again to raise them from the point opposite the numeral 
I, the same process is repeated whilst passing onward to II, and thence 
to III, the distance of one hundred and fifty feet in one second of 
time. Now, it may be safely inferred, I think, that when the wings 
are at the highest points of elevation, as indicated by the angles of 
the serpentine line, the animal is in a situation to begin to depart 
from the line of motion in the direction successively of the diverging 
lines A, B, C ; because after having traversed the space of twenty-five 
feet under the influence of his current velocity alone, and with noth- 
ing to sustain his weight besides the force of the air exerted against 
the under surfaces of those organs, as already stated, it is evident 
that the body must incline to fall in this manner, and that it would 
soon exhibit a marked departure below that line, but for the check it 
receives from the downward strokes of wing recurring regularly at 
