theory of respiration in all classes of creatures possessing animal
life; and that we become acquainted with the migrations of animals,
as well as many of their peculiar instincts and habits. It is the
atmosphere that enables us to account for the periodical changes in
the plumage of birds and the furs of animals, and the variety of
colours to be found amongst them. By means also of the elasticity of
the atmosphere, sounds and odours are transmitted to sensitive beings.
Atmospherical phenomena, it may be safely inferred, attracted the
observation of mankind in the earliest ages: we know that the
Egyptians and the Greeks wrote upon the subject; the Jews too, a
pastoral people, "could discern the face of the sky;" and even in our
day, shepherds may be ranked among the weather-wise. "This is a fine
morning, a soft day, or a cold evening," are modes of salutation with
us, as commonly as is the "Salem Alikem" (Peace be with you!) amongst
the inhabitants of the more serene countries of the East. Shenstone
says, though with nearly equal spleen and truth: "there is nothing
more universally commended than a fine day: the reason is, that people
can commend it without envy."

_Why do we call the atmosphere a fluid?_

Because it has a tendency to move in all directions, and consequently
rushes in and fills every space not previously occupied by a more
solid substance. Hence we find, that every cave, crevice, place, and
vessel, having communication with the atmosphere, if it be not filled
with something else, is filled with air; against which it is no
argument that we do not see it, as it is perfectly transparent, and
consequently invisible.

_Why do birds fly?_