, and mobility, the barbs and barbules knitting the more
flexible parts together, so that they do not separate, but only expand,
when the wing is unfolded.

[Illustration: Barn Swallow]

While the primary purpose of wings is flight, there is quite a number
of notable exceptions. A concrete example is the ostrich, whose wings
are too feeble to lift it from the ground, but evidently aid the great
fowl in running, as it holds them outspread while it skims over the
plain, perhaps using them mainly as outriggers or balancing poles in
its swift passage on its stilt-like legs. The penguins convert their
wings into fins while swimming through the water, the feathers closely
resembling scales.

There are birds of many kinds, and therefore a great variety of wings
and modes of flight. Birds with short, broad, rounded wings, with the
under surface slightly concave and the upper surface correspondingly
convex, usually have comparatively heavy bodies, and race through the
air with rapid wing-beats and rather labored flight, and compass only
short distances. Among the birds of this kind of aerial movement may
be mentioned the American meadowlark, the bob-white, and the pheasant.
Other species propel themselves in rapid, gliding, and continued flight
by means of long, narrow, and pointed wings, like the swifts, swallows,
and goatsuckers, while many others, notably herons, hawks, vultures,
and eagles, are distinguished by a vast alar expansion in proportion to
their weight, and hence are able to sustain themselves in the air by
sailing, with only a slight stroke at rare intervals. Such birds as
the stormy petrel and the frigate-bird have wings that are broad,
convex, and of great length in contrast with the lightness and small
bulk of their bodies, for which reason they are able to sustain
themselves in the air for days without rest. It is even thought that
some of these wonderful birds of the limitless ocean sleep on the wing,
though how such an hypothesis could be proved it would be difficult to
say.

Even in this day of scientific research and astuteness, it must not be
supposed that everything about the mechanics of avicular flight is
understood. We may readily comprehend how a bird, without fluttering
its wings, can poise in the air; but how can it move forward or in a
circle, and even mount upward, without a visible movement of a pinion?
And this some birds are able to do without reference to the direction
of the ethe