"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 ill.u.s.trate it," he continued. "Suppose there was a large ma.s.s 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 ma.s.s 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 ma.s.s 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 ma.s.s 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 ma.s.ses 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 sh.o.r.e, 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?"