its extreme tenuity, and the
ease with which bodies move through it, people will scarcely believe
that they are living at the bottom of an aerial ocean, like fishes at
the bottom of the sea. We become, however, very sensible of it, when
it flows rapidly in streams or currents, so as to form what is called
a wind, which will sometimes act so violently as to tear up the
strongest trees by the roots, and blow down to the ground the best
and firmest buildings.

Some may still be inclined to ask, what is this air in which we are
said to live? We see nothing; we feel nothing; we find ourselves at
liberty to move about in any direction, without any hindrance. Whence
then comes the assertion, that we are surrounded by a fluid, called
air? When we pour water out of a vessel, it appears to be empty; for
our senses do not inform us that any thing occupies the place of the
water, for instance, when we pour water out of a vial. But this
operation is exactly similar to pouring out mercury from a vial in a
jar of water, the water gets in and supplies the place of the
mercury; so does the air which supplies the place of the water; and
this air will prevent water from rising, or filling a vessel which
contains it.

Hence we see that air possesses similar appearances of
impenetrability with other matter: for it excludes bodies from the
space which itself occupies.

Air being therefore material must have weight; and we shall
accordingly find, that a quart of it weighs about fifteen grains. But
a quart of water weighs about two pounds; this fluid therefore is
nearly a thousand times heavier than air.

But though air is so much lighter than water, yet, because it extends
to a considerable height above the surface of the earth, it is
evident, that it must press strongly on the surfaces of bodies. It is
thought to extend nearly fifty miles above the surface of the earth,
and must therefore press heavily on this surface. This may be evinced
by different experiments, performed by means of the air pump.

Another property of the air, by which it is distinguished from most
other fluids, is its elasticity. It may be compressed into a less
space than it naturally occupies, and when the compressing force is
removed, it expands to its former bulk, by its spring or elasticity.
Indeed it is always compressed into less space than it would
naturally occupy, by the weight of the superincumbent air.

The trachea, or windpipe, commences at the furthe