es: in a boat, the air strikes the
sail; in a bird, the sail strikes the air: in a boat, the force is
lateral, and in a bird downwards; and it has its sail on both sides. I
shall leave you to follow out the mechanical problem for yourselves, as
far as the mere resolution of force is concerned. My business, as a
painter, is only with the exquisite organic weapon that deals with it.

67. Of which you are now to note farther, that a bird is required to
manage his wing so as to obtain two results with one blow:--he has to
keep himself up, as well as to get along.

But observe, he only requires to keep himself up _because_ he has to
get along. The buoyancy might have been given at once, if nature had
wanted _that_ only; she might have blown the feathers up with the hot
air of the breath, till the bird rose in air like a cork in water. But
it has to be, not a buoyant cork, but a buoyant _bullet_. And therefore
that it may have momentum for pace, it must have weight to carry; and
to carry that weight, the wings must deliver their blow with effective
vertical, as well as oblique, force.

Here, again, you may take the matter in brief sum. Whatever is the
ship's loss, is the bird's gain; whatever tendency the ship has to
leeway, is all given to the bird's support, so that every atom[13] of
force in the blow is of service.

[13] I don't know what word to use for an infinitesimal degree or
divided portion of force: one cannot properly speak of a force
being cut into pieces; but I can think of no other word than
atom.

68. Therefore you have to construct your organic weapon, so that this
absolutely and perfectly economized force may be distributed as the
bird chooses at any moment. That, if it wants to rise, it may be able
to strike vertically more than obliquely;--if the order is, go-ahead,
that it may put the oblique screw on. If it wants to stop in an
instant, that it may be able to throw its wings up full to the wind; if
it wants to hover, that it may be able to lay itself quietly on the
wind with its wings and tail, or, in calm air, to regulate their
vibration and expansion into tranquillity of gliding, or of pausing
power. Given the various proportions of weight and wing; the conditions
of possible increase of muscular force and quill-strength in proportion
to size; and the different objects and circumstances of flight,--you
have a series of exquisitely complex problems, and exquisitely perfect
so