e is
much larger in proportion to the bulk of the head, than in any of these.
This is a superiority conferred upon them not without a corresponding
utility: it seems even indispensable to their safety and subsistence.
Were this organ in birds dull, or in the least degree opaque, they would
be in danger, from the rapidity of their motion, of striking against
various objects in their flight. In this case their celerity, instead of
being an advantage, would become an evil, and their flight be restrained
by the danger resulting from it. Indeed we may consider the velocity
with which an animal moves, as a sure indication of the perfection of
its vision. Among the quadrupeds, the sloth has its sight greatly
limited; whilst the hawk, as it hovers in the air, can espy a lark
sitting on a clod, perhaps at twenty times the distance at which a man
or a dog could perceive it."

920. AMONGST THE MANY PECULIARITIES IN THE CONSTRUCTION OF BIRDS, not
the least is the mode by which their respiration is accomplished. This
is effected by means of air-vessels, which extend throughout the body,
and adhere to the under-surface of the bones. These, by their motion,
force the air through the true lungs, which are very small, and placed
in the uppermost part of the chest, and closely braced down to the back
and ribs. The lungs, which are never expanded by air, are destined to
the sole purpose of oxidizing the blood. In the experiments made by Mr.
John Hunter, to discover the use of this general diffusion of air
through the bodies of birds, he found that it prevents their respiration
from being stopped or interrupted by the rapidity of their motion
through a resisting medium. It is well known that, in proportion to
celerity of motion, the air becomes resistive; and were it possible for
a man to move with the swiftness of a swallow, as he is not provided
with an internal construction similar to that of birds, the resistance
of the air would soon suffocate him.

921. BIRDS ARE DISTRIBUTED OVER EVERY PART OF THE GLOBE, being found in
the coldest as well as the hottest regions, although some species are
restricted to particular countries, whilst others are widely dispersed.
At certain seasons of the year, many of them change their abodes, and
migrate to climates better adapted to their temperaments or modes of
life, for a time, than those which they leave. Many of the birds of
Britain, directed by an unerring instinct, take their departure from the