[Sidenote: _Atmosphere._]
The _Atmosphere_ is that thin body of air which surrounds the Earth,
in which the clouds hover, and by which in their descent they are
broke into drops of rain; which sometimes, according to the warmth or
coldness of air, are froze into _Snow_, or _Hailstones_. _Thunder_ and
_Lightning_ are also made in the _Atmosphere_, and wind is nothing
else but a percussion of the air, occasioned by its different density
in different places. The benefits we receive from the atmosphere are
innumerable; without air no earthly creature could live, as is plainly
proved by experiments made by the _Air-Pump_; and the wholsomeness
of a climate chiefly depends upon that of its air: If there was no
atmosphere to reflect the rays of the Sun, no part of the heavens would
be lucid and bright, but that wherein the Sun was placed; and if a
spectator should turn his back towards the Sun, he would immediately
perceive it to be quite dark, and the least Stars would be seen shining
as they do in the clearest night; and the Sun immediately before his
setting would shine as brisk as at noon, but in a moment, as soon as
he got below the horizon, the whole hemisphere of the Earth would be
involved in as great a darkness as if it were midnight.
But by means of the atmosphere it happens, that while the Sun is above
the horizon, the whole face of the heavens is strongly illuminated by
its rays, so as to obscure the faint light of the Stars, and render
them invisible; and after Sun-setting, though we receive no direct
light from him, yet we enjoy its reflected light for some time: For
the atmosphere being higher than we are, is a longer time before it
is withdrawn from the Sun (as if a man was to run to the top of a
steeple, he might see the Sun after it had been set to those at the
bottom.) The rays which the atmosphere receives from the Sun, after he
is withdrawn from our sight, are by refraction faintly transmitted to
us; until the Sun having got about 18 degrees below the horizon, he
no longer enlightens our atmosphere, and then all that part thereof
which is over us becomes dark. After the same manner in the morning,
when the Sun comes within 18 degrees of our horizon, he again begins
to enlighten the atmosphere, and so more and more by degrees, until he
rises and makes full day.
[Sidenote: _Twilight_, or _Crepusculum_.]
This small illumination of the atmosphere, and the state of the Heavens
between day and night, is called the _Twilight_, or the _Crepusculum_.
The duration of twilight is different in different climates, and in
the same place at different times of the year. The beginning or ending
of twilight being accurately given, we may from thence easily find the
height of the atmosphere, which is not always the same. The mean height
of the atmosphere is computed to be about 40 miles; but it is probable,
the air may extend itself a great deal further, there being properly no
other limits to it, as we can conceive, but as it continually decreases
in density the farther remote it is from the Earth, in a certain ratio;
which at last, as to our conception, must in a manner terminate.
[Illustration]