"There," said he to Dorothy, "when I put it down into common air, it burns on the same as ever; so it can't be that the wind puts it out." Jonas repeated the experiment a number of times; the effect was always the same. Whenever he put it into the tumbler of common air, it burned on without any change; but whenever he put it into the choke damp, it immediately went out. Even Dorothy was satisfied that there was a difference in the kind of air contained in the two tumblers.
That evening, when Rollo gave his mother a full account of their attempts,-describing particularly their failure at first, and their subsequent successes,-his mother seemed much interested. When he had finished, she said,-
"Well, Rollo, I don't see but that you have learned two lessons in philosophy."
"Two lessons?" said Rollo.
"Yes," replied his mother. "The first is, that fire will not burn in choke damp; and the second is, that it requires nice attention and care to verify philosophical truths by experiment."
"Yes," said Rollo, "we missed the first time, just because we had too big a paper."
QUESTIONS.
Why did Jonas suppose that the stump would not burn? What was
Rollo's first mode of setting it on fire? How did it succeed?
What did Jonas do with his axe, when he came? What was the
object of this? What did he say was necessary to make fires
burn? What did Rollo at first think was the reason why the bark
went out when held over the fire? What did he next think was the
reason? How did Jonas say that different kinds of airs were
prepared? In what places did he say that choke damp was
naturally produced? How did they attempt to prepare some of this
gas? Did they succeed in preparing it? Did they succeed in their
experiment at first? What was the cause of the failure?
CHAPTER X. 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."