Evening at Home 55
WHY THE EARTH MOVES ROUND THE SUN.
_Papa_—_Lucy_.
_Papa._ You remember, Lucy, that I explained to you some time ago what
was the cause that things fell to the ground.
_Lucy._ O yes; it was because the ground drew them to it.
_Pa._ True. That is a consequence of the universal law in nature, that
bodies attract each other in proportion to their bulk. So a very small
thing in the neighbourhood of a very large one, always tends to go to
it, if not prevented by some or other power. Well—you know I told you
that the sun was a ball a vast many times bigger than the ball we
inhabit, called the earth; upon which you properly asked, how then it
happened that the earth did not fall into the sun.
_Lu._ And why does it not?
_Pa._ That I am going to explain to you. You have seen your brother
whirl round an ivory ball tied to the end of a string, which he held in
his hand.
_Lu._ Yes; and I have done it myself, too.
_Pa._ Well, then—you felt that the ball was continually pulling, as if
it tried to make its escape?
_Lu._ Yes; and one my brother was swinging _did_ make its escape, and
flew through the sash.
_Pa._ It did so. That was a lesson, in the _centrifugal_ motion, or that
power by which a body thus whirled, continually endeavours to fly off
from the centre round which it moves. This is owing to the force or
impulse you give it at setting out, as if you were going to throw it
away from you. The string by which you hold it, on the contrary, is the
power which keeps the ball toward the centre, called the _centripetal_
power. Thus you see there are two powers acting upon the ball at the
same time, one to make it fly off, the other to hold it in; and the
consequence is, that it moves directly according to neither, but between
both; that is, round and round. This it continues to do while you swing
it properly; but if the string breaks or slips off, away flies the ball;
on the other hand, if you cease to give it the whirling force, it falls
toward your hand.
_Lu._ I understand all this.
_Pa._ I will give you another instance of this double force acting at
the same time. Do not you remember seeing some curious feats of
horsemanship?
_Lu._ Yes.
_Pa._ One of them was, that a man standing with one leg upon the saddle,
and riding full speed, threw up balls into the air, and catched them as
they fell.
_Lu._ I remember it very well.
_Pa._ Perhaps you would have expected these balls to have fallen behind
him, as he was going at such a rate?
_Lu._ So I did.
_Pa._ But you saw that they fell into his hand as directly as if he had
been standing quite still. That was because at the instant he threw them
up, they received the motion of the horse straight forward as well as
the upright motion that he gave them, so that they made a slanting line
through the air, and came down in the same place they would have reached
if he had held them in his hand all the while.
_Lu._ That is very curious, indeed!
_Pa._ In the same manner you may have observed, in riding in a carriage,
that if you throw anything out of the window, it falls directly
opposite, just as if the carriage was standing still, and is not left
behind you.
_Lu._ I will try that the next time I ride in one.
_Pa._ You are then to imagine the sun to be a mighty mass of matter,
many thousand times bigger than our earth, placed in the centre, quiet
and unmoved. You are to conceive our earth, as soon as created, launched
with vast force in a straight line, as if it were a bowl on a green. It
would have flown off in this line for ever, through the boundless
regions of space, had it not at the same instant received a pull from
the sun by its attraction. By the wonderful skill of the Creator, these
two forces were made exactly to counterbalance each other; so that just
as much as the earth, from the original motion given to it, tends to fly
forward, just so much the sun draws it to the centre; and the
consequence is, that it takes a course between the two, which is a
circle round and round the sun.
_Lu._ But if the earth was set a rolling like a bowl upon a green, I
should think it would stop of itself, as the bowl does.
_Pa._ The bowl stops because it is continually rubbing against the
ground, which checks its motion, but the ball of the earth moves in
empty space, where there is nothing to stop it.
_Lu._ But if I throw a ball through the air, it will not go on for ever,
but it will come down to the ground.
_Pa._ That is because the force with which you can throw it is much less
than the force by which it is drawn to the earth. But there is another
reason, too, which is the resistance of the air. This space all round us
and over us is not empty space; it is quite full of a thin transparent
fluid called air.
_Lu._ Is it?
_Pa._ Yes. If you move your hand quickly through it, you will find
something resisting you, though in a slight degree. And the wind, you
well know, is capable of pressing against anything with almost
irresistible force; and yet wind is nothing but a quantity of air put
into violent motion. Everything, then, that moves through the air is
continually obliged to push some of this fluid out of the way, by which
means it is constantly losing part of its motion.
_Lu._ Then the earth would do the same?
_Pa._ No; for it moves in _empty space_.
_Lu._ What, does it not move through the air?
_Pa._ The earth does not move _through_ the air, but carries the air
along with it. All the air is contained in what is called the
_atmosphere_, which you may compare to a kind of mist or fog clinging
all round to the ball of the earth, and reaching a certain distance
above it, which has been calculated at above forty-five miles.
_Lu._ That is above the clouds, then.
_Pa._ Yes: all the clouds are within the atmosphere, for they are
supported by the air. Well—this atmosphere rolls about along with the
earth, as if it were a part of it, and moves with it through the sky,
which is a vast field of empty space. In this immense space are all the
stars and planets, which have also their several motions. There is
nothing to stop them, and therefore they continually go on, by means of
the force that the Creator has originally impressed upon them.
_Lu._ Do not some of the stars move round the sun, as well as our earth?
_Pa._ Yes; those that are called _planets_. These are all subject to the
same laws of motion with our earth. They are attracted by the sun as
their centre, and form, along with the earth, that assemblage of worlds,
which is called the _solar system_.
_Lu._ Is the moon one of them?
_Pa._ The moon is called a _secondary_ planet, because its immediate
connexion is with our earth, round which it rolls, as we do round the
sun. It, however, accompanies our earth on its journey round the sun.
But I will tell you more about its motion, and about the other planets
and stars another time. It is enough at present, if you thoroughly
understand what I have been describing.
_Lu._ I think I do.
THE UMBELLIFEROUS PLANTS.
_Tutor_—_George_—_Harry_.
_Harry._ What plant is that man gathering under the hedge?
_George._ I don’t know; but the boys call the stalks _hexes_, and blow
through them.
_Har._ I have seen them; but I want to know the plant.
_Geo._ Will you please to tell us, sir, what it is?
_Tutor._ It is hemlock.
_Geo._ Hemlock is poison is it not?
_Tut._ Yes, in some degree; and it is also a medicine; that man is
gathering it for the apothecaries.
_Har._ I should like to know it.
_Tut._ Well then—go and bring one.
[Harry _fetches it_.
_Geo._ I think I have seen a great many of this sort.
_Tut._ Perhaps you may; but there are many other kinds of plants
extremely like it. It is one of a large family called the
_umbelliferous_, which contains both food, physic, and poison. It will
be worth while for you to know something about them, so let us examine
this hemlock closely. You see this tall hollow stalk, which divides into
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