23 
those points. And I deduce from this, that in the process of flight 
on a horizontal line, the bird begins to depart from the line of motion 
at intervals of one third of a second of time, and at three successive 
and equidistant points within the space of every hundred and fifty 
feet ; while he is as constantly sustained upon that line by a similar 
frequency of the impulses of the wings, made within the same period 
of time and occurring at successive intervals, which equally subdivide 
the same distance. 
Another feature of this process, as may be inferred from what has 
just been said, is the wide difference between the velocity of the body 
and that of the movement of the wings ] but which may be rendered 
entirely certain by a glance at figure No. 14, the chief object of which 
I will now explain : 
Adhering to the case of the wild duck, for example, I have ascer- 
tained that in raising his wings, their extreme tips describe arcs of 
circles of about fifteen inches in length, extending from a point seven 
or eight inches below, to a similar height above, the line of motion, 
their thin front edges cleaving the air with little or no resistance, 
describing a line passing from right to left and crossing the path of 
flight at the centre of the section traversed by the body during the 
time of this movement, the distance of twenty-five feet, as accurately 
represented in this figure. And in this manner those organs escape 
that degree of resistance which would unavoidably result from raising 
them if the animal progressed very slow, as we know always occurs in 
rapidly moving, up and down, any such article held in the hand, but 
without at the same time imparting to it similar horizontal motion. 
And it is equally certain that whilst being thus raised, the extended 
curvature of the extreme ends of the quills, under the rapid motion 
of the body, must meet with a considerable amount of resistance from 
their contact with the air, and thereby exert a proportionate and suf- 
ficient influence to prevent the body from exhibiting any perceptible 
departure below the line of flight for the time being, as already stated. 
It follows from this that the extremities of the wings pass through 
the space of about thirty inches during each revolution, and altogether 
about seven and a half feet, per second of time, one half of which only 
being in the downward direction, and thereby effectively sustaining 
the weight, while the body within the same period of time traverses 
the distance of one hundred and fifty feet ; thus showing the velocity 
