erally feeble, and, therefore, timid. Accordingly,
wings have been given them to enable them to fly through the air, and
thus elude the force which, by nature, they are unable to resist.
Notwithstanding the natural tendency of all bodies towards the centre of
the earth, birds, when raised in the atmosphere, glide through it with
the greatest ease, rapidity, and vigour. There, they are in their
natural element, and can vary their course with the greatest
promptitude--can mount or descend with the utmost facility, and can
light on any spot with the most perfect exactness, and without the
slightest injury to themselves.

918. THE MECHANISM WHICH ENABLES BIRDS to wing their course through the
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 aerial 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 partl