explain this to readers a little versed in the elements of
mechanics,) if B is the locus of the center of gravity of the bird,
moving in slow flight in the direction of the arrow, w is the locus of
the leading feather of its wing, and _a_ and _b_, roughly, the
successive positions of the wing in the down-stroke and recovery.

71. I say the down-stroke is as effective as the bird chooses; that is
to say, it can be given with exactly the quantity of impulse, and
exactly the quantity of supporting power, required at the moment. Thus,
when the bird wants to fly slowly, the wings are fluttered fast, giving
vertical blows; if it wants to pause absolutely in still air, (this
large birds cannot do, not being able to move their wings fast enough,)
the velocity becomes vibration, as in the humming-bird: but if there is
wind, any of the larger birds can lay themselves on it like a kite,
their own weight answering the purpose of the string,[14] while they
keep the wings and tail in an inclined plane, giving them as much
gliding ascent as counteracts the fall. They nearly all, however, use
some slightly gliding force at the same time; a single stroke of the
wing, with forward intent, seeming enough to enable them to glide on
for half a minute or more without stirring a plume. A circling eagle
floats an inconceivable time without visible stroke: (fancy the pretty
action of the inner wing, _backing_ air instead of water, which gives
exactly the breadth of circle he chooses). But for exhibition of the
complete art of flight, a swallow on rough water is the master of
masters. A sea-gull, with all its splendid power, generally has its
work cut out for it, and is visibly fighting; but the swallow plays
with wind and wave as a girl plays with her fan, and there are no words
to say how many things it does with its wings in any ten seconds, and
does consummately. The mystery of its dart remains always inexplicable
to me; no eye can trace the bending of bow that sends that living
arrow.

But the main structure of the noble weapon we may with little pains
understand.

[14] See App. p. 112, Sec. 145.

72. In the sections _a_ and _b_ of Fig. 5, I have only represented the
quills of the outer part of the wing. The relation of these, and of the
inner quills, to the bird's body may be very simply shown.

Fig. 6 is a rude sketch, typically representing the wing of any bird,
but actually founded chiefly on the sea-gull's.

[Illustration