GRAVITATION.
One evening, after tea, when Rollo was a pretty big boy, he came and
began to climb up into his father's lap. When he had climbed up, he took
his place astride of his father's knee, as if he were riding a horse.
His little brother Nathan came up and stood near, wanting to get up too,
only there was not room. His cousin James was there, that evening, on a
visit. He sat upon a cricket before the fire, and his mother was at the
table doing some sort of work.
"O dear me!" said Rollo's father, imitating the tone in which Rollo
sometimes uttered that exclamation.
"What, sir?" said Rollo.
"Why, I should like very well to hold you in my lap," said his father,
"if it was not for the great mighty earth, down below us."
"How?" said Rollo. He did not know what his father meant.
"Why, when you are upon my knee, the earth, the ponderous earth, pulls
you down hard and heavy upon it." So saying, he put his hands upon
Rollo's shoulders, and crowded them down, by way of showing him how the
earth acted upon him. "It pulls," he continued, "with a strong and
steady pull, all the time; and so makes you a very heavy weight."
"Is that what makes weight?" said Rollo.
"Yes," said his father. "So, if I had a monstrous stone to move, and if
I thought the earth would listen to me, and let go its hold, I might
make a speech to it thus:--
"'O earth, thou vast and ponderous ball, please to relax thy hold, for a
few minutes, upon this stone, and leave it free to move; and then Rollo
can tie a string to it, and move it easily along to the place where I
want it to lie; then thou mayst seize it again with thy mighty
attraction, and hold it down as firmly as thou wilt.'"
"O father!" exclaimed Rollo; Nathan and James laughed, and Rollo's
mother looked up from her work to listen to this strange apostrophe.
"It would seem," continued his father, in a pompous tone, as if still
addressing the earth--"it would seem, most mighty planet, a very easy
thing for thee to release this single stone, for a few minutes, from the
grasp with which thou holdest all things down upon thy surface. And by
it I shall gain much, while thou wilt lose nothing; for, if thou wilt
not willingly give up the stone, I must get three or four yoke of strong
oxen, and, by main force, pull it away."
"Is that what makes everything heavy?" said Rollo.
"Yes," said his father, answering now in his natural tone; "the
attraction of the earth is what makes everything heavy, and holds it
down."
"And could we move a monstrous great stone," said Rollo, "as light as a
feather?"
"No," said his father, "it would not move along quick and light, like a
feather. You could not move it quick. Suppose, for instance, you had two
boats, floating upon the water, of the same size; one made very light
indeed, of something very thin, like paper, and empty; and the other
made of wood, and loaded with iron as heavily as it would bear. Now,
they would both be supported upon the water, so that their weight would
be neutralized; and yet they would move very differently. You could push
the light one about easily, anywhere, but the heavy one would move very
slowly. You would not have to push very hard upon it, but you would have
to push _for some time_, to set it in motion; and then it would be hard
to stop it. This is called its _inertia_."
"Yes," said Rollo, "it would go harder against the bank."
"The reason is," continued his father, "that the heavy boat contains a
great many more particles of matter than the light one, and they have
all got to be put in motion. So it requires greater effort, or the same
effort must be continued a longer time.
"For instance, if we suppose that the light boat has one million of
particles of matter, the heavy one would have, perhaps, twenty millions.
Of course the effect of the pushing has to be divided among twenty times
as many particles, and of course will only carry them one twentieth part
as far; so that the bodies that are now large and heavy, would only move
slowly, though they would move _easily_, if the attraction of the earth
were to cease.
"There is another way to illustrate it," he continued. "Suppose there
was a large mass of lead, as big as a load of hay, hanging by a chain;
and also a great puff of feathers, or a balloon of the same size,
hanging in the same way. Now, if they were both suspended freely, they
would both move easily, for their weight would be supported by the
chain; but the heavy one would move very slowly. Nathan could move it,
but he could only move it slowly and a little way."
"I should not think that he could move it but very little," said Rollo.
"No, he could not; because you see that, in that way of suspending
anything, the moment that it begins to move, it begins to swing off and
to rise; so that it cannot be moved at all without being _lifted_ a
little. And the more it is moved, the higher it is lifted, so that it
would take a great force to move it far away from the centre, where it
was hanging. But we can hang it in a way to avoid that difficulty."
"How, sir?" said Rollo.
Rollo seemed to be very much interested in this conversation. He had
dismounted from his father's knee, and stood by his side, listening
eagerly. His mother, too, was paying close attention. As for Nathan, he
sat still; though it is not by any means certain that he understood it
very well.
"Let us suppose," said his father, "that the mass of lead, as big as a
load of hay, is fastened to one end of a stick of timber."
"That would not be strong enough to hold it," said Rollo.
"Well, then, to a beam of iron, as large as a stick of timber," rejoined
his father.
"O," said James, "you could not get such a big bar of iron."
"No," replied his father, "only an imaginary one; and that will be just
as good as any. Now, suppose the great mass of lead is fastened to one
end of this bar, and another one, just like it, to the other end, to
balance it. Now, suppose that the lower end of the great chain is
secured around the middle of the iron beam, and the upper end to be
fastened to some strong support up in the air. Now, we can move the mass
of lead without having to lift it at all; for, if we push against it,
and make it move, it will move round and round, without rising at all,
as it did before, when it was hung up directly by the chain."
Rollo's father then went on to explain to them that, in such a case as
this, the weight of the two masses of lead would not prevent their
moving easily, for they would exactly balance each other. A little child
would be able to move them; but still they would move exceedingly slow
at first, and it would be hard to stop them, when they were in motion.
So, he said, if the earth should cease to attract and draw down any
great, heavy body, like a large stone, for example, the smallest child
could lift it, though it would come up slowly, just as a very heavy body
would move, if it was suspended by a string, or was afloat upon the
water.
"And so," said Rollo, "if the earth should not attract us, could we push
ourselves right up off from the ground?"
"Yes," said his father, "most undoubtedly."
"What, and go about anywhere in the air?"
"Certainly."
Rollo began to laugh aloud at this idea, and looked very much interested
and pleased.
"O, then I wish there was no gravitation," said Rollo; "I do, really."
"But, then," continued his father, "if you should get up into the air,
you could not get down again."
"Why not?" said Nathan, beginning to look a little concerned.
"Unless," said his father, "you had something above you, to push
against, so as to push yourselves down. You would be just like a boy in
a boat, off from the shore, and without any paddle or pole. He could not
get back again."
"We might tie a rope to something," said James, "before we went up, and
so pull ourselves down."
"Yes, that you might do."
"And could not we flap our hands, like a bird, and so fly a little?"
"Perhaps you could," said his father.
Here the children all began to flap their hands, like young birds trying
to fly; and Rollo said again, he wished, with all his heart, there was
no gravitation; "for then," said he, "we should have strength enough to
fly."
"That would lead to serious consequences," said his father.
"What consequences?" said James.
"Much more serious than you would suppose."
"Tell us what they would be, uncle," said James.
"O, I know," said Rollo; "you could not stand up straight without
gravitation."
"O, we could, couldn't we, father?" said Nathan.
"What makes you think, Rollo," said his father, without replying to
James's question--"what makes you think that we could not stand up
straight without gravitation?"
"Why, you see," said Rollo.--Here he paused, and looked confused, and
did not know what to say. He had an indistinct recollection of having
read something about it in some book; but he could not tell what.
"I don't see what should prevent any body's standing up straight, if the
attraction of the earth should cease; in fact, if it made any
difference, it would be rather easier to stand up straight."
Here Rollo looked rather foolish, but he did not reply. The truth is,
like almost all other children, who take an interest in reading, he was
sometimes a little vain of his knowledge; and in this case, instead of
listening attentively, and endeavoring to learn something new from his
father's explanations, he seems to have thought it a good plan for him
to help him elucidate the subject to James and Nathan. He exchanged the
character of learner for teacher too soon.
"Well, uncle," said James, "what would be the consequence if gravitation
should cease?"
"Why, in the first place," said Rollo's father, "all the streams in the
world would stop running."
"The streams!" said Rollo, astonished.
"Yes," said his father, "every river, brook, and rill. The reason why
the streams flow is, that the earth attracts the water from the
mountains and hills, down into the valleys and towards the sea."
"Well, sir, what else?" said Rollo.
"Why, there would never be any more rain."
"No more rain!" exclaimed all the children.
"No," he replied. "The drops of rain fall only because the earth draws
them down by its attraction; and, of course, if this attraction should
cease, they would remain where they are."
The children were musing a minute upon these strange effects, when Rollo
asked if anything else would happen.
"Why, yes," said his father, "worse disasters than these; but I do not
know whether you would understand them, if I should explain them."
"O, try," said Rollo; "I think we shall understand."
"Well, let me think," said his father. "You have noticed how a chaise
wheel, on a muddy road, in a wet day, holds the mud upon it, until when
it is going very swiftly down a hill, and then the mud flies off in all
directions."
"Yes, sir," said all the children.
"And if the mud did not stick to the wheel pretty tight, it would be
thrown off at all times, even when the wheel was going slow. You
understand this."
"Yes, sir."
"Well, now, this whole earth, you all know, is whirling around
through space, and moving on also around the sun. And all the loose
things upon the surface would be thrown off at once, if they were
not held to it by a strong attraction. If this attraction were to
cease suddenly,--whisk!--away we should all go in an instant--rocks,
houses, men, animals, all in confusion."
"O father!" exclaimed Rollo; "where should we go to,--off into the air?"
"Not exactly into the air, for the air would all fly off, and be
dissipated too; we should fly off into the sky somewhere, some in one
direction, and some in another. You'd be a thousand miles off from the
earth, almost before you would know it."
"Would it kill us, father?" said Nathan.
"Yes," said his father. "I don't know that there would be any shock that
would hurt us, but we should have no air to breathe, and it would be
dark and dismal."
"Dark?" said Rollo. "There would be the sun."
"Yes," said his father, "there would be the sun; and the sun would look
bright enough when you looked directly towards it, but there would be no
general light about you, unless there was air."
The children all paused to reflect upon the strange results which their
father had told them would ensue from a suspension of the earth's
attractive force. Rollo began to think that he had been too hasty in his
wish that there was no gravitation.
"But, father," said he, "the houses would not go off, certainly;--only
the loose things would go."
"Very well; houses are loose."
"O father! they are fastened down."
"How are they fastened down?" asked his father.
"O, they are nailed--and,"
"Not nailed to the ground, certainly," said his father.
"No," said Rollo, laughing; "but then they are built with great stones
and mortar."
"Yes, but there is no mortar under the lowest stones. The foundations
are simply laid upon the ground."
"Well," rejoined Rollo, "I thought they were fastened somehow or other."
"No," said his father; "they dig the cellar, and only just lay the
foundations upon the ground, without any fastening. The earth holds them
in place."
"Well, father," said Rollo, "that is what I meant, when I said we could
not stand up straight. I meant the houses. I read in a book that houses
would be blown away, if the gravitation did not hold them down."
Here Rollo's father had a hearty laugh; and he told Rollo that he
thought that was rather wide shooting. Rollo wanted to know what he was
laughing at; and Nathan asked him what he meant by wide shooting.
"Why," said he, "Rollo, you undertook to explain to us, from your stores
of knowledge, what the effects of a suspension of gravitation would be;
and, in attempting to tell that houses would be in danger of being blown
away, you came no nearer than to tell us that boys could not stand up
straight; and that is what I call pretty wide shooting."
So saying, he rose from his seat, and walked away, appearing to be very
much amused. James laughed too, and even Rollo could not help smiling at
the ridiculous figure which his display of his learning made. As for
Nathan, he continued to look grave; and said he did not see that it was
any shooting at all.
After a short pause, Rollo's mother said, "So you see, children, the
cause of all the pressure, both of air and of water, and all the effects
produced by them, are the results of their gravitation towards the
earth."
"Yes," said Rollo, "I believe I understand it now."
After this, Rollo took James and Nathan out into the yard, to see if
some beans had come up, which he had been planting in a sunny corner of
the garden the day before.
QUESTIONS.
What was Mr. Holiday's apostrophe to the earth? What is the
cause of weight? Why did the boys wish that there was no
gravitation? What was the first evil consequence which their
father said would ensue, if there was no gravitation? What was
the second evil consequence? What did their mother say after the
conversation with their father was closed?