175. The weight of the Air, at the Earth’s surface, is found by
experiments made with the air-pump; and also by the quantity of mercury
that the Atmosphere balances in the barometer; in which, at a mean
state; the mercury stands 29-1/2 inches high. And if the tube were a
square inch wide, it would at that height contain 29-1/2 cubic inches of
mercury, which is just 15 pound weight; and so much weight of air every
square inch of the Earth’s surface sustains; and every square foot 144
times as much, because it contains 144 square inches. Now as the Earth’s
surface contains about 199,409,400 square miles, it must be of no less
than 5,559,215,016,960,000 square feet; which, multiplied by 2016, the
number of pounds on every foot, amounts to 11,207,377,474,191,360,000;
or 11 trillion 207 thousand 377 billion 474 thousand 191 million and 360
thousand pounds, for the weight of the whole Atmosphere. At this rate, a
middle sized man, whose surface may be about 14 square feet, is pressed
by 28,224 pound weight of Air all round; for fluids press equally up and
down and on all sides. But, because this enormous weight is equal on all
sides, and counterbalanced by the spring of the internal Air in our
blood vessels, it is not felt.
[Sidenote: A common mistake about the weight of the Air.]
176. Oftentimes the state of the Air is such that we feel ourselves
languid and dull; which is commonly thought to be occasioned by the
Air’s being foggy and heavy about us. But that the Air is then too
light, is evident from the mercury’s sinking in the barometer, at which
time it is generally found that the Air has not sufficient strength to
bear up the vapours which compose the Clouds: for, when it is otherwise,
the Clouds mount high, the Air is more elastic and weighty about us, by
which means it balances the internal spring of the Air within us, braces
up our blood-vessels and nerves, and makes us brisk and lively.
[Sidenote: Without an Atmosphere the Heavens would always appear dark,
and we should have no twilight.]
177. According to [42]Dr. KEILL, and other astronomical writers, it is
entirely owing to the Atmosphere that the Heavens appear bright in the
day-time. For, without an Atmosphere, only that part of the Heavens
would shine in which the Sun was placed: and if an observer could live
without Air, and should turn his back towards the Sun, the whole Heavens
would appear as dark as in the night, and the Stars would be seen as
clear as in the nocturnal sky. In this case, we should have no twilight;
but a sudden transition from the brightest sunshine to the blackest
darkness immediately after sun-set; and from the blackest darkness to
the brightest sun-shine at sun-rising; which would be extremely
inconvenient, if not blinding, to all mortals. But, by means of the
Atmosphere, we enjoy the Sun’s light, reflected from the aerial
particles, before he rises and after he sets. For, when the Earth by its
rotation has withdrawn the Sun from our sight, the Atmosphere being
still higher than we, has his light imparted to it; which gradually
decreases until he has got 18 degrees below the Horizon; and then, all
that part of the Atmosphere which is above us is dark. From the length
of twilight, the Doctor has calculated the height of the Atmosphere (so
far as it is dense enough to reflect any light) to be about 44 miles.
But it is seldom dense enough at two miles height to bear up the Clouds.
[Sidenote: It brings the Sun in view before he rises, and keeps him in
view after he sets.]
178. The Atmosphere refracts the Sun’s rays so, as to bring him in sight
every clear day, before he rises in the Horizon; and to keep him in view
for some minutes after he is really set below it. For, at some times of
the year, we see the Sun ten minutes longer above the Horizon than he
would be if there were no refractions: and about six minutes every day
at a mean rate.
[Sidenote: Fig. IX.