When we have been laboring very hard, my dear child, and want to rest
for a minute, we say, _Let us take breath_; because breathing is
an action which takes place of itself, requiring neither effort nor
attention on our part.
But, if it takes place of itself, it does not explain itself;
consequently, when I say to you, _Now, let us take breath_, this
is not a signal for my having a rest, for I have undertaken to explain
Respiration to you.
If you were a German, I would remind you of what so often happens when
you put a fork into a dish of sour-krout. You want to lay hold of a
little bit merely, but the strips of cabbage-leaf are twisted one
within the other, and hang together in spite of you, so that
withoutintending it you get hold of a whole plateful at once.
Now this Respiration affair is something like the sour-krout
story--begging your pardon for the comparison. I should have liked to
give you only a small plateful--a child's plateful--of it; but I feel
the explanations coming, hanging one upon the other; and, whether I
will or no, I must treat you like a grown-up person, and we must give
up for once the nice little doll's dinners with which we began.
In my opinion, you will lose nothing by the change if you will but pay
attention; for about that soft little breath of yours, which is always
coming and going over your pretty lips, there are many more things to
be learnt than you have heard of yet. As I said just now, you will
find you have got hold of a plateful all at once. A good appetite to
you!
To prevent confusion we will divide the subject into two parts. I shall
explain to you first, _How we breathe?_--a very curious question,
as you will see. And afterwards we will examine, _Why we breathe?_--which
is still more interesting.
First, I must tell you that air is heavy, and very heavy too; a thousand
times more so than you may suppose. The air we breathe, through which
we move backwards and forwards, that air is _some_thing, remember,
although we do not see it; and when there is a wind, that is to say,
when the air is in motion, like a stream of water running down a hill,
we are forced to acknowledge its being something, for we see it throw
down the largest trees and carry along the biggest ships. But without
going so far out of the way for examples, try--you who run so well--to
run for two minutes against a strong wind: and then you shall tell me
whether the air is something or nothing. But if it be something it
must have weight, for all substances have; paper as well as lead; with
this sole difference, that the weight of lead is greater in proportion
to its size than that of paper. Now a sheet of paper is very light,
is it not? and you would be puzzled perhaps to say what it weighs. But
many sheets of paper placed one upon the other, end by forming a thick
book which has its undeniable weight; and if some one were to heap
upon your head a pile of large books, like those you see on your papa's
shelves, the end might be that you would be crushed to death.
In the same way, a small amount of air is by no means heavy; but you
can conceive that a great quantity of it gathered together may end by
weighing a great deal. Now get well into your head the fact, that we,
here, on the surface of the earth, are at the bottom of an immense
mass of air, extending to somewhere about forty or fifty miles above
our heads. Let us say forty to make more sure, for learned men have
not yet been able to calculate the precise height to a nicety; and for
my own part, I think we have done wonders to get so near the mark even
as this. But can you picture to yourself the distance which forty miles
high really is? I will help you to form some idea.
One mile contains 5,280 feet, and your papa is six feet high. One mile
high would therefore be 880 times as high as your papa, But this is
a mere nothing--only one mile's height. In forty miles there would be
no less than 211,200 feet; and setting papas aside, of whom it would
take 35,200, one on the top of the other, to go so far into the sky,
let us think of the height of the tallest buildings you know; church
and cathedral towers for instance. Now the towers of many parish
churches are 150 feet high; the towers of York Minister not 300. At
that rate it would take 1,408 ordinary parish church-towers, or upwards
of 704 York Minster towers, piled one above the other, to reach to the
end of the forty miles of air above our heads. I leave you to judge
what would be the weight of a mass of paper piled up as high as that.
You may safely grant then, that this mass or pile, or if you like it
better, this _column_ of air (for that is the proper expression),
must be of considerable weight; as is still further made certain by
the fact of its having been weighed, so that I can even name the weight
to you if you wish to hear it. Bear in mind too, that the weight of
a column of air will be in proportion to its _superficial extent_--to
its breadth and width, that is; for, as you may suppose, a column as
large in extent as one of the towers of York Minster will weigh a good
deal more than one the size of a single brick.
But wait; here is a book on the table which will serve me for a measure,
and as you will probably find the same on your mamma's table, you can
follow my measurement. It is a French Grammar. The back is seven inches
long and four and a quarter wide. That is, there are four and a quarter
rows, each seven inches long. In other words, the back contains
nearly--and let us call it quite, for convenience' sake--thirty inches
side by side. Thirty _square inches_ as it is called. Measure your
mamma's copy and you will see. Now, can you guess the weight of the
column of air forty miles high which this volume supports? Upwards
of four cwt.; 450 lbs., that is to say. If you want to be very exact,
here is the rule. Air presses on all bodies at the rate of fifteen
pounds to every square inch; so now you can make the calculation for
yourself.
But I suspect you had no idea you were so strong; for I see you tossing
up the book, heavily laden as it is, like a feather.
Comfort yourself. There is no magic in the matter. If a very strong man
were to push you on one side, could you resist him? Certainly not. But
if another man of equal strength were to push you at the same time on
the other side, what would happen? Well, you would remain quietly in
your place, without troubling yourself more about one than the other,
the two forces mutually destroying each other. And this is the case
here. While the air above your book is weighing down upon it with a
force of 450 lbs., the air below it presses against it underneath with
an equal weight, and this destroys the effect of the other. From 450
lbs. take 450 lbs., and nothing remains. Your grammar has nothing to
carry after all, and you may toss it about as you please, without
deserving much credit for the effort.
"What are you telling me?" you inquire. "If I put a stone on the top
of my head, I can feel its weight easily enough; but if I put my hand
on the top of the stone I no longer feel anything. How can the air
below the stone press against it? And talking of columns--how pleasant
it would be, for instance, if the people who go up the Monument were
to have the weight of it on their heads when they get to the top!"
Well said, little one. And your objection reminds me of an argument
which distracted my head as a lad, when I first heard the pressure of
air explained by a good fellow who did not trouble himself to be quite
as exact as you and I are in our discussions. I was told that the
surface of the body, or the skin of a large man, measured sixteen feet
square, which is equal to the surface of a table four feet long and
four broad. Now, you know that in four feet there are forty-eight
inches, and on the surface of the table are forty-eight rows, with
forty-eight inches in each, or 2,304 square inches; so that a man's
surface is 2,304 square inches, and the weight his body supports is
34,560 lbs., or upwards of fifteen tons--always at the rate of fifteen
pounds to every square inch, you understand. Now, I was constantly
asking myself how it happened that in entering a house one never seemed
to get rid of this almost fabulous weight, since the roof of the house
must naturally interpose itself between the air-column of forty miles
high and the man who would then only have some few feet of air above
his head. The roof would support the rest, that was clear. From whence,
then, came the 34,560 lbs. which seemed to weigh as heavily as before;
since, whether on the threshold of the door, while still under shelter
of the roof, or two steps outside in the open air, under the tremendous
column forty miles high, one never felt a bit lighter, not even to the
extent of the weight of a single sheet of paper? This was a difficulty
from which I could never extricate myself.
I found out the answer to the riddle afterwards, and a very simple one
it is.
Air does not, in point of fact, _weigh down_ like a solid fifty
pounds' weight, which has no impulse but to descend, and has nothing
to do with anything above it. It _presses against_ rather, like
a spring, which, having been compressed, tries to resume its natural
position with a force equal to that which holds it back. Ask some one
to show you the spring of a watch, and you will understand this better.
Each atom of air is a spring of matchless elasticity, which nothing
can break, which never wears out, which one can always compress, if
one employs force sufficient, and which is always ready to expand
indefinitely, in proportion as the compressing power is withdrawn.
Now, consider the column of air outside the door, where there is a
pile of such springs forty miles high. The lower ones have to bear up
all their comrades, which press upon them with their united weight,
and these make desperate efforts to repulse the tremendous pressure,
and to spread out in their turn. They endeavor to escape in every
direction--to the right, to the left, above, below; but caught between
the earth, which will not give way, and the compact mass of all the
columns of air which surrounds the earth in every direction, and of
which the lower part is equally compressed everywhere, they struggle
unceasingly, but in vain; indefatigable, but powerless. You live in
the midst of those little wrestlers, and naturally bear the punishment
of the injury done to them. They press against you as against every
thing else--before, behind, on all sides--with a force equal to thatwith
which they are themselves compressed, or I would say, equal to
the weight by which they are so horribly squeezed and contracted: so
that, in fact, you bear this weight not only on your head and shoulders,
as you might at first suppose, but also all along your body and limbs,
under your arms, under your chin, in the hollow of your nostrils,
everywhere.
Now we will suppose you to enter the house; and what do you find there?
Outer air, which on its part has got in by the door, the window, and
every little crevice in the wall. The column outside the roof no longer
presses upon it, but what is the gain of that?
It was compressed when it got in, and the little springs will struggle
as a matter of course, quite as much on this side of the door as on
the other. The protecting roof has so little power that were it not
itself protected by the air outside, the pressure of which keeps it
in its place, the air within would shiver it into a thousand fragments
in its efforts to get loose.
You laugh; but wait till I explain myself further. I will take the
case of a miniature house to make the matter pleasanter to you; one
fifteen feet long, fifteen feet wide, and with a flat roof, the most
economical plan as regards space. Fifteen feet are five yards, and as
the multiplication table tells us that five times five make twenty-five,
our roof will in this case be twenty-five square yards (_i. e._
225 square feet) in superficial extent, or _area_; it is not much,
and you will find few as small.
Would you like to calculate the force with which the millions and
thousand millions of little spring imps imprisoned under that poor
unfortunate roof would press against it? We settled before that the
quantity of them brought to bear upon a square inch had the power to
push at the rate of fifteen pounds. Were they to push against a square
yard (a surface 1296 times greater than the square inch) it would
therefore be 19,440 lbs. This being so for one square yard, calculate
for twenty-five square yards, and you will have the amount of pressure
against our roof--viz. 486,000 lbs--merely that! And now tell me what
cottage roof in the world was ever built so as to be able to stand
against such a weight?
Perhaps though, you can scarcely appreciate the amount of heaviness,
486,000 lbs. Well, 486,000 lbs. is nearly 217 tons; and one of those
railway trucks that you see laden with coals at the stations can carry,
perhaps, from eight to ten tons, without breaking down. Say ten tons
as an outside estimate, and then think of piling the contents of
twenty-one such trucks on your roof, and yet you would still be short
of the weight of air which is bearing down upon it. I need scarcely
say now that were you to take away the air from within the roof, theair
without would smash both it and the whole cottage flat, as a giant
at a fair strikes an egg flat with one blow of his fist. To show you
how in another way: take a moderate sized column or pillar, such as
you see sometimes in a nobleman's grounds, of about the weight of the
twenty-one tons, and set it up like a chimney on the roof of our
cottage, then walk away to a little distance and watch what will happen!
There, little Miss Laugher! have you at last learned to value the
weight of the air, or _atmospheric pressure_ as it is more properly
called; since it is the force with which the atmosphere presses against
rather than weighs upon everything on the surface of the globe? It is
no joke, as you perceive, and it affords plenty of subject
forreflection. I have still to prove to you that I have not been making
fun of you with my calculations, and that the weight of air upon a
square inch is really what I have said--viz., fifteen pounds.
Now, there is a very simple way by which we might get to know your
strength, and tell its amount in figures, if one chose; namely, by
putting a weight on your arms--a heap of books, if you please--and
keep adding and adding to it, until those poor little arms were unable
to bear any more. Then weighing what they had borne, whether we should
find it to be ten or thirty pounds--I cannot guess how much it might
be at this distance--one might safely say, without fear of mistake,
"The strength of this young lady is equal to ten, twenty, or thirty
pounds"--in other words, "she represents a weight of ten, twenty, or
thirty pounds" and by a similar plan people have ascertained the
strength of the air--that is, the weight which it represents. They
have weighed what it is capable of carrying.
I told you lately that the whole surface of the earth was covered by
an immense army of little imps--otherwise called little air-springs,
which, compressed by the giant mass of their comrades above, all of
whom they have to carry on their backs, are always trying to protect
themselves, by pushing back everything which comes across them. Imagine
the bottom of a well. Our imps are permanently installed there as a
matter of course, and face to face with the water they push against
it, each one doing his best, on all points at once. As the pressure
is equal everywhere therefore, and always the same, there are no signs
of it to be seen.
Now insert in the water the end of a tube closed below by a cork which
exactly fits the interior, but which can be moved up and down in the
tube by means of a bar of iron or wood which runs through it. This is
called a _piston_, I may as well tell you as we go on.
When the piston rises in the tube, it drives before it, as it goes,
the air which was already there; and which cannot slip away down the
sides because the piston fits so closely to them all the way along.
The result of this is, that just underneath the piston there is a place
in the water to which the air cannot reach, and at that place the water
has no pressure upon it at all.
Now see what happens. Pressed upon heavily by the air in every other
part and place, like a mouse hunted by a cat, who finds at last a hole
through which to escape, the poor water darts at this and ascends the
tube close after the piston.
So far so good; but if the tube is very long, and the piston rises
rather high;--at thirty-three or thirty-four feet above the level of
the water it has to continue its ascent alone. The water parts company,
stopping quietly behind, half-way up the tube.
"What is the meaning of this?" you will ask.
It means that the force which presses on the well-water all round the
tube, and thus drives it up, has done all it can, and that our little
air-imps refuse to supply any more. The water which rises in the tube
has a weight of its own of course, and with this weight it presses,
as it is fair it should, on the water below. In proportion as the
piston rises, the column of water which follows it gets bigger and
bigger, and naturally its weight increases at the same time. At last
there comes a moment when this weight becomes such that its pressure
on the water below is equal to that with which the air-imps are pressing
on the water in the well. Thenceforth they may push as they please;
no more water will go up. They are in the same position now that they
were before, when their comrades (afterwards driven out by the piston)
were pressing upon the same point, which had only a moment's freedom;
and this water column of thirty-three or thirty-four feet holds them
in check, to exactly the same extent as the gay fellows whose place
it has taken.
Nothing is easier now than to calculate, even to a few grains almost,
the force of the pressure of air. One can get at the weight of water,
thank goodness! and it has been ascertained that our water-column will
weigh fifteen pounds if the tube is a square inch in size. You will
comprehend after this that it might be any size you may please to
imagine, without there being the slightest alteration in the height
of the column. The larger it is, the heavier will be the column of
water on the one hand; but on the other, the greater will be the number
of air-imps turned out; so it comes to the same thing in the end.
If you should feel any doubt about the correctness of this reasoning,
you have only to try the experiment over again, in a well, filled with
mercury for instance. Ask to be shown some pure mercury, which is also
called _Quicksilver_, because one wants to express melted silver,
apt to be constantly on the move; it is often to be met with in houses.
Mercury weighs thirteen and a half times more than water: according
to our calculations, therefore, it would take thirteen and a half times
less of it than of water to bring our little air-imps to reason. And
this is just what you will find happens; you will see the column of
mercury stop short exactly at the moment when it has attained the
orthodox weight of fifteen pounds; that is to say, at a height of
twenty-eight inches.
On the other hand, take some ether. You know that delicate spirit,
which smells so strong, which makes your hand feel cold if it is put
upon it, and which we give to sick people to inhale. Ether weighs
one-quarter less than water. In a well of ether you would therefore
see something quite different, and your column would rise without being
asked, to something like forty-three feet, exactly up to the point of
weighing--like the others--fifteen pounds to every square inch. Air
will not be replaced with less.
That, then, is the measure of its strength, or our scales are deceitful.