rce to
keep up the momentum of a body moving in a horizontal plane in the air
(in which there is so little friction) cannot be great, and this force
is all that is wanted. The movement of the neck and body of the condor,
we must suppose, is sufficient for this. However this may be, it is
truly wonderful and beautiful to see so great a bird, hour after hour,
without any apparent exertion, wheeling and gliding over mountain and
river.

The airplane has a propelling power in its motor, and it shifts its
wings to take advantage of the currents. The buzzard and condor do the
same thing. They are living airplanes, and their power is so evenly and
subtly distributed and applied, that the trick of it escapes the eye.
But of course they avail themselves of the lifting power of the
air-currents.

All birds know how to use their wings to propel themselves through the
air, but the mechanism of the act we may not be able to analyze. I do
not know how a butterfly propels itself against a breeze with its
quill-less wings, but we know that it does do it. As its wings are
neither convex nor concave, like a bird's, one would think that the
upward and downward strokes would neutralize each other; but they do
not. Strong winds often carry them out over large bodies of water; but
such a master flyer as the monarch beats its way back to shore, and,
indeed, the monarch habitually flies long distances over salt water when
migrating along our seacoast in spring and fall.

At the moment of writing these paragraphs, I saw a hen-hawk flap heavily
by, pursued by a kingbird. The air was phenomenally still, not a leaf
stirred, and the hawk was compelled to beat his wings vigorously. No
soaring now, no mounting heavenward, as I have seen him mount till his
petty persecutor grew dizzy with the height and returned to earth. But
the next day, with a fairly good breeze blowing, I watched two hawks for
many minutes climbing their spiral stairway to the skies, till they
became very small objects against the clouds, and not once did they flap
their wings! Then one of them turned toward the mountain-top and sailed
straight into the face of the wind, till he was probably over his mate
or young, when, with half-folded wings, he shot down into the tree-tops
like an arrow.

In regard to powers of flight, the birds of the air may be divided into
three grand classes: those which use their wings simply to transport
themselves from one place to another,--the sam