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 ma.s.s 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.

LETTER XIX.

THE ACTION OF THE LUNGS.

I hope I have told you enough, my dear child, to enable you fully to estimate the force with which air presses upon everything on the surface of the earth, and consequently upon our own bodies among the rest.

If you understand this, nothing is easier than to understand how air comes and goes in our lungs.

When the cook wants to light her fire with two or three hot coals, what does she do?

She takes the bellows and blows it, does she not?

But if she has no bellows at hand, what does she do? You answer at once, she blows it herself with all the strength of her lungs.

By which it would seem--does it not?--that we are a sort of living bellows, being able, in case of necessity, to act as a subst.i.tute for the wood and leather ones of common use. And if we really possess the power of doing the work of a bellows, may not this be because we have within us some little machine of the nature of a bellows?

Exactly; and this fact gives me the opportunity of making you understand the action of the lungs by explaining that of the bellows, which is in everybody's hands, but which three-fourths of the people use, without troubling themselves to inquire how it is made or acts.

"A bellows, as you know, is composed of two pieces of board, capable of being separated and brought together again at will, and united by a piece of leather so shaped and arranged that it doubles up when the boards close, the intermediate s.p.a.ce forming a firmly-closed box, the size of which increases or diminishes at every movement of the boards.

"We take the bellows down to use it, and there are the boards, lying flat upon each other, the box between them quite small. Is there anything inside, do you think?

"Nothing," you answer; "the bellows is empty."

Do you think so really, my child? Do you think a tumbler is empty, then, when you have drunk out its contents; and that jelly pots are empty when all the jelly is eaten? There are not so many empty things in the world, I a.s.sure you, as you suppose. You forget the air--that monster who is always wanting to stretch himself out, and pushes against everything he meets. He is an unceremonious gentleman, who takes possession of every vacant place; as fast as you put a spoonful on your plate, he takes up the room of the jelly which has been removed, and at each mouthful you swallow, he slips into the place of the water which goes away. When you think the gla.s.s and pot are empty, they are, in reality, full of air. You cannot see it; but it is there, you may rely upon it.

There is air, then, in the bellows-box, because there is air in every place where there is nothing else to dispute possession with it. The quant.i.ty is small in this case, no doubt, because the box is small and cannot hold much.

But now, look! I separate the boards, and the box, which was small, becomes large. For once, then, here is a box which must be partially empty; for it has just, as if by magic, made a s.p.a.ce in itself in which positively there cannot be anything, since there was nothing there beforehand.

Ay! but look down at the centre of the upper board. You see a little hole there, do you not, and below the little hole a small piece of leather, which seems to close it up? That is a _valve_, one of those doors, such as we noticed before in the heart, and such as are to be found, moreover, in most houses, which let people through on one side but not on the other. This one opens when it is pushed from without, but lets nothing out which has once got in. Now, the air outside, as I said before, is always pushing against everything. He pushes as a matter of course, therefore, against the valve, and as there is nothing behind it to resist the pressure, in proportion as room is made inside the box, he enters and fills it with himself.

But presently some one begins to close the bellows, and he finds himself caught between the boards; on which these invite him to begone, with the same sort of politeness displayed by the police, when the hour of departure comes in a place of public exhibition; when, _i.e._, they spread out on all sides, and force the crowd before them till they have found the road to the door. But the air cannot get back by the way it came in, the door being shut. As, however, it must go out somewhere, whether it likes it or not, it pa.s.ses through the tube at the end of the box (the _nozzle_ of the bellows), and comes out thence with a rush upon the fire. When it is once gone the bellows can be distended again, and the process be repeated as before indefinitely.

And this is just what goes on inside ourselves. Your chest, my child, is a box which expands and contracts alternately; making a place for the air by the first effort, and then driving it out by the second.

It is neither more nor less than a bellows, but of a simpler construction than that used by the cook. The exit pipe serves also for a door of entrance, and there is but one board instead of two.

The _exit pipe_ is the _larynx_, of which we spoke before, when we were talking of swallowing the wrong way, and which communicates with the air outside, through the nose and mouth at the same time, allowing us to breathe through either one or the other as we like.

As to the _board_, I said a few words about it when I was describing the liver. It is the _diaphragm_--that separating part.i.tion--that floor which is placed between the two stories or divisions of the body--the belly and the chest.

But here especially the infinite superiority of the works of G.o.d over the miserable inventions of man comes out in all its grandeur.

A bellows which was to have the honor of keeping up within us that miraculous fire--the pre-eminently sacred fire--which we call Life, required something more than a common board for its foundation. And accordingly this, of which I am now going to give you a detailed history, is as marvellous as it is admirable. I fancy that when you have read my account, you will no longer turn up your nose at the vile word _diaphragm_.

Let us first take a peep at the construction of the bellows.

On each side of the _vertebral column_, from the neck to the loins, spring twelve long bones, one below the other, bent in the form of bows; these are called the _ribs_. The first seven pairs of ribs rest, and as it were, unite, in front, upon a bone called the _sternum_, which you can trace with your finger down to the pit of the stomach, at which point the finger sinks in, for there is no more _sternum_, and the last five ribs on each side no longer unite with those of the opposite one.

For which reason they are called _false ribs_. On the other hand they are joined to each other at the ends by means of a strip or band of a substance sufficiently strong, but at the same time flexible, and somewhat elastic, which is called _cartilage_ or _gristle_. The next time you see a roasting piece of veal on the table, look well at it, and you will see at the end a white substance which crackles under your teeth; that is _gristle_.

This forms the framework of our bellows, which you may picture to yourself as a kind of cage, widening towards the bottom and going to a point at the top, for the arches formed by the upper ribs are smaller than the others. The whole terminates in a sort of ring, through which pa.s.s, together, the _oesophagus_ and the _trachea_.

The s.p.a.ce between the ribs is occupied by muscles which reach from one to the other, and the whole framework or cage is shut in below by the _diaphragm_, that marvellous board whose history I have promised to relate.

The _diaphragm_, as I told you some time ago, is a large muscle, thin and flat, stretched like a cloth between the chest and the _abdomen_. It is fastened by an infinity of little threads called _fibres_, to the lower edge of the cage I have just been describing, and it looks at first sight as if it must be incapable of moving, since it is fixed in one invariable manner all round the body.

It moves nevertheless, but not in the same way as the boards of our bellows.

Ask your brother to hold two corners of your pocket-handkerchief; take hold of the other two yourself, and turn the handkerchief so as to face the wind. The four corners remain in their place, do they not?

but the middle, inflated by the wind, curves and swells out in front like a ship's sail, which itself is only an immense hand kerchief after all. Then draw the handkerchief tightly towards you, each to your own side, and it will recover itself and become flat again. Loosen it a little and it will curve and swell out again in the middle, and this maneuver you can go through as often as you choose.

Which very maneuver the _diaphragm_ is continually performing, of and by itself.

In its natural position it bulges upwards in the middle, like a cloth swollen out by the wind, and thus occupies a portion of the chest at the expense of the lungs. When air has to be admitted, its _fibres_ tighten and bring it flat again, as you and your brother brought the handkerchief flat just now by tightening it.

The whole s.p.a.ce previously occupied by the arch of the _diaphragm_ is thus given up to the lungs, which, being elastic, instantly stretch themselves out to it; while air, running in through the nose and mouth, fills up in proportion the empty place (_vacuum_) created by the extension of the lungs, exactly as in the case of the bellows.

But soon the fibres of the _diaphragm_ relax. It rises up again into its old position, driving back the lungs as it does so; and the air finding there is now no room for it, goes out by the same way the other came in.

I say _the other_, observe, because the air that goes out is no longer the same as when it came in; and this is the secret of _why we breathe_; while the up and down movement of the _diaphragm_ is the explanation of _how we breathe_.