I have here an ordinary pop-gun. If I push the cork in very
tight, and then force the piston slowly inwards, I can compress
the air a good deal. Now I am forcing the atoms nearer and
nearer together, but at last they rebel so strongly against being
more crowded that the cork cannot resist their pressure. Out it
flies, and the atoms spread themselves out comfortably again in
the air all around them. Now, just as I pressed the air together
in the pop-gun, so the atmosphere high up above the earth presses
on the air below and keeps the atoms closely packed together.
And in this case the atoms cannot force back the air above them
as they did the cork in the pop-gun; they are obliged to submit
to be pressed together.
Even a short distance from the earth, however, at the top of a
high mountain, the air becomes lighter, because it has less
weight of atmosphere above it, and people who go up in balloons
often have great difficulty in breathing, because the air is so
thin and light. In 1804 a Frenchman, named Gay-Lussac, went up
four miles and a half in a balloon, and brought down some air;
and he found that it was much less heavy than the same quantity
of air taken close down to the earth, showing that it was much
thinner, or rarer, as it is called;* and when, in 1862, Mr.
Glaisher and Mr. Coxwell went up five miles and a half, Mr.
Glaisher's veins began to swell, and his head grew dizzy, and he
fainted. The air was too thin for him to breathe enough in at a
time, and it did not press heavily enough on the drums of his
ears and the veins of his body. He would have died if Mr.
Coxwell had not quickly let off some of the gas in the balloon,
so that it sank down into denser air. (*100 cubic inches near the
earth weighed 31 grains, while the same quantity taken at four
and a half miles up in the air weighed only 12 grains, or two-
fifths of the weight.)
And now comes another very interesting question. If the air gets
less and less dense as it is farther from the earth, where does
it stop altogether? We cannot go up to find out, because we
should die long before we reached the limit; and for a long time
we had to guess about how high the atmosphere probably was, and
it was generally supposed not to be more than fifty miles. But
lately, some curious bodies, which we should have never suspected
would be useful to us in this way, have let us into the secret of
the height of the atmosphere. These bodies are the meteors, or
falling stars.
Most people, at one time or another, have seen what looks like a
star shoot right across the sky, and disappear. On a clear
starlight night you may often see one or more of these bright
lights flash through the air; for one falls on an average in
every twenty minutes, and on the nights of August 9th and
November 13th there are numbers in one part of the sky. These
bodies are not really stars; they are simply stones or lumps of
metal flying through the air, and taking fire by clashing against
the atoms of oxygen in it. There are great numbers of these
masses moving round and round the sun, and when our earth comes
across their path, as it does especially in August and November,
they dash with such tremendous force through the atmosphere that
they grow white-hot, and give out light, and then disappear,
melted into vapour. Every now and then one falls to the earth
before it is all melted away, and thus we learn that these stones
contain tin, iron, sulphur, phosphorus, and other substances.
It is while these bodies are burning that they look to us like
falling stars, and when we see them we know that hey must be
dashing against our atmosphere. Now if two people stand a
certain known distance, say fifty miles, apart on the earth and
observe these meteors and the direction in which they each see
them fall, they can calculate (by means of the angle between the
two directions) how high they are above them when they first see
them, and at that moment they must have struck against the
atmosphere, and even travelled some way through it, to become
white-hot. In this way we have learnt that meteors burst into
light at least 100 miles above the surface of the earth, and so
the atmosphere must be more than 100 miles high.
Our next question is as to the weight of our aerial ocean. You
will easily understand that all this air weighing down upon the
earth must be very heavy, even though it grows lighter as it
ascends. The atmosphere does, in fact, weigh down upon land at
the level of the sea as much as if a 15-pound weight were put
upon every square inch of land. This little piece of linen
paper, which I am holding up, measures exactly a square inch, and
as it lies on the table, it is bearing a weight of 15 lbs. on its
surface. But how, then, comes it that I can lift it so easily?
Why am I not conscious of the weight?
To understand this you must give all your attention, for it is
important and at first not very easy to grasp. you must
remember, in the first place, that the air is heavy because it is
attracted to the earth, and in the second place, that since air
is elastic all the atoms of it are pushing upwards against this
gravitation. And so, at any point in air, as for instance the
place where the paper now is as I hold it up, I feel no pressure
because exactly as much as gravitation is pulling the air down,
so much elasticity is resisting and pushing it up. So the
pressure is equal upwards, downwards, and on all sides, and I can
move the paper with equal ease any way.
Even if I lay the paper on the table this is still true, because
there is always some air under it. If, however, I could get the
air quite away from one side of the paper, then the pressure on
the other side would show itself. I can do this by simply
wetting the paper and letting it fall on the table, and the water
will prevent any air from getting under it. Now see! if I try to
lift it by the thread in the middle, I have great difficulty,
because the whole 15 pounds' weight of the atmosphere is pressing
it down. A still better way of making the experiment is with a
piece of leather, such as the boys often amuse themselves with in
the streets. This piece of leather has been well soaked. I drop
it on the floor and see! it requires all my strength to pull it
up. (In fastening the string to the leather the hole must be
very small and the know as flat as possible, and it is even well
to put a small piece of kid under the knot. When I first made
this experiment, not having taken these precautions, it did not
succeed well, owing to air getting in through the hole.) I now
drop it on this stone weight, and so heavily is it pressed down
upon it by the atmosphere that I can lift the weight without its
breaking away from it.
Have you ever tried to pick limpets off a rock? If so, you know
how tight they cling. the limpet clings to the rock just in the
same way as this leather does to the stone; the little animal
exhausts the air inside it's shell, and then it is pressed
against the rock by the whole weight of the air above.
Perhaps you will wonder how it is that if we have a weight of 15
lbs. pressing on every square inch of our bodies, it does not
crush us. And, indeed, it amounts on the whole to a weight of
about 15 tons upon the body of a grown man. It would crush us if
it were not that there are gases and fluids inside our bodies
which press outwards and balance the weight so that we do not
feel it at all.
This is why Mr. Glaisher's veins swelled and he grew giddy in
thin air. The gases and fluids inside his body were pressing
outwards as much as when he was below, but the air outside did
not press so heavily, and so all the natural condition of his
body was disturbed.
I hope we now realize how heavily the air presses down upon our
earth, but it is equally necessary to understand how, being
elastic, it also presses upwards; and we can prove this by a
simple experiment. I fill this tumbler with water, and keeping a
piece of card firmly pressed against it, I turn the whole upside-
down. When I now take my hand away you would naturally expect
the card to fall, and the water to be spilt. But no! the card
remains as if glued to the tumbler, kept there entirely by the
air pressing upwards against it. (The engraver has drawn the
tumbler only half full of water. The experiment will succeed
quite as well in this way if the tumbler be turned over quickly,
so that part of the air escapes between the tumbler and the card,
and therefore the space above the water is occupied by air less
dense than that outside.)
And now we are almost prepared to understand how we can weigh the
invisible air. One more experiment first. I have here what is
called a U tube, because it is shaped like a large U. I pour
some water in it till it is about half full, and you will notice
that the water stands at the same height in both arms of the
tube, because the air presses on both surfaces alike. Putting my
thumb on one end I tilt the tube carefully, so as to make the
water run up to the end of one arm, and then turn it back again.
But the water does not now return to its even position, it
remains up in the arm on which my thumb rests. Why is this?
Because my thumb keeps back the air from pressing at that end,
and the whole weight of the atmosphere rests on the water at the
other end. And so we learn that not only has the atmosphere real
weight, but we can see the effects of this weight by making it
balance a column of water or any other liquid. In the case of
the wetted leather we felt the weight of the air, here we see its
effects.
Now when we wish to see the weight of the air we consult a
barometer, which works really just in the same way as the water
in this tube. An ordinary upright barometer is simply a straight
tube of glass filled with mercury or quicksilver, and turned
upside-down in a small cup of mercury. The tube is a little more
than 30 inches long, and though it is quite full of mercury
before it is turned up, yet directly it stands in the cup the
mercury falls, till there is a height of about 30 inches between
the surface of the mercury in the cup, and that of the mercury in
the tube. As it falls it leaves an empty space above the mercury
which is called a vacuum, because it has no air in it. Now, the
mercury is under the same conditions as the water was in the U
tube, there is no pressure upon it at the top of the tube, while
there is a pressure of 15 lbs. upon it in the bowl, and therefore
it remains held up in the tube.