air, is both singular and instructive. Their bodies are covered with
feathers, which are much lighter than coverings of hair, with which
quadrupeds are usually clothed. The feathers are so placed as to overlap
each other, like the slates or the tiles on the roof of a house. They
are also arranged from the fore-part backwards; by which the animals are
enabled the more conveniently to cut their way through the air. Their
bones are tubular or hollow, and extremely light compared with those of
terrestrial animals. This greatly facilitates their rising from the
earth, whilst their heads, being comparatively small, their bills shaped
like a wedge, their bodies slender, sharp below, and round above,--all
these present a union of conditions, favourable, in the last degree, to
cutting their way through the aërial element to which they are
considered as more peculiarly to belong. With all these conditions,
however, birds could not fly without wings. These, therefore, are the
instruments by which they have the power of rapid locomotion, and are
constructed in such a manner as to be capable of great expansion when
struck in a downward direction. If we except, in this action, the slight
hollow which takes place on the under-side, they become almost two
planes. In order that the downward action may be accomplished to the
necessary extent, the muscles which move the wings have been made
exceedingly large; so large, indeed, that, in some instances, they have
been estimated at not less than a sixth of the weight of the whole body.
Therefore, when a bird is on the ground and intends to fly, it takes a
leap, and immediately stretching its wings, strikes them out with great
force. By this act these are brought into an oblique direction, being
turned partly upwards and partly horizontally forwards. That part of the
force which has the upward tendency is neutralized by the weight of the
bird, whilst the horizontal force serves to carry it forward. The stroke
being completed, it moves upon its wings, which, being contracted and
having their edges turned upwards, obviate, in a great measure, the
resistance of the air. When it is sufficiently elevated, it makes a
second stroke downwards, and the impulse of the air again moves it
forward. These successive strokes may be regarded as so many leaps taken
in the air. When the bird desires to direct its course to the right or
the left, it strikes strongly with the opposite wing, which impels it to
the proper side. In the motions of the animal, too, the tail takes a
prominent part, and acts like the rudder of a ship, except that, instead
of sideways, it moves upwards and downwards. If the bird wishes to rise,
it raises its tail; and if to fall, it depresses it; and, whilst in a
horizontal position, it keeps it steady. There are few who have not
observed a pigeon or a crow preserve, for some time, a horizontal flight
without any apparent motion of the wings. This is accomplished by the
bird having already acquired sufficient velocity, and its wings being
parallel to the horizon, meeting with but small resistance from the
atmosphere. If it begins to fall, it can easily steer itself upward by
means of its tail, till the motion it had acquired is nearly spent, when
it must be renewed by a few more strokes of the wings. On alighting, a
bird expands its wings and tail fully against the air, as a ship, in
tacking round, backs her sails, in order that they may meet with all the
resistance possible.